David R. Brooks
Principal Investigator, The GLOBE Program’s Water Vapor Monitoring Project
Research Professor, Department of Mechanical Engineering and Mechanics
Drexel University, Philadelphia, Pennsylvania
|National Aeronautics and|
NASA Publication EG-2003-12-06-LARC
This publication is in the public domain and is not protected by copyright. Permission is not required for duplication.
An instrument and experiment protocol for measuring total atmospheric water vapor from a ground observing site has been developed in support of NASA Langley Research Center's Geosynchronous Imaging Fourier Spectrophotometer (GIFTS) instrument, in cooperation with GLOBE, an international science and education program for K-12 students. The GLOBE/GIFTS water vapor instrument is a battery-powered, handheld device suitable for use by middle and secondary school students. It uses the ratio of the outputs from two narrowband detectors in the near-infrared to determine total atmospheric water vapor.
The measurement protocol described in this report has been adopted by the GLOBE Program, which provides a worldwide infrastructure for implementing student-based environmental measurements. Science background, materials for educators and students, alignment to science education standards, detailed field guides, and data entry forms are provided in this comprehensive guide to using the GLOBE/GIFTS water vapor instrument.
Supplementary materials include discussions of geosynchronous orbits, analog-to-digital conversion (for automated collection of water vapor data), a simple model of Earth's greenhouse effect, and an experiment for determining dewpoint temperature.
This Project has been developed in support of NASA Langley Research Center's Geosynchronous Imaging Fourier Spectrophotometer (GIFTS) instrument, in cooperation with the GLOBE Program, an international environmental science and education program. (See www.globe.gov and asd-www.larc.nasa.gov/GIFTS.)
One of the goals of GIFTS is to map global distributions of total precipitable atmospheric water vapor (PW). Ground-based measurements are essential to assess the performance of instruments and algorithms used to extract water vapor values from space-based measurements. The challenge for this project was to develop an inexpensive instrument that can be used by students to measure water vapor. The solution was to adapt a battery-powered handheld sun photometer previously developed for GLOBE by the two lead authors of this report [Brooks and Mims, 2002]. This sun photometer uses two light emitting diodes (LEDs) as spectrally selective detectors to measure the transmission of direct sunlight through the atmosphere at red and green wavelengths. The new GLOBE/GIFTS instrument is identical in appearance to the GLOBE sun photometer, with the visible-light detectors replaced by two near-infrared detectors - one inside the 940-nanometer water vapor absorption band, and one just outside this band. The ratio of the detector responses at these two wavelengths can be calibrated to provide a measure of PW. As part of the data collection protocol, students report meteorological and sky conditions based on qualitative observations, direct measurements, and online sources. These metadata relate the measurements to a broader view of the atmosphere and enhance the value of the PW data.
The GLOBE/GIFTS water vapor protocol is included by GLOBE as part of its Advanced Atmosphere training. There is a Water Vapor Protocol in the GLOBE User's Guide (in both its printed and online versions), along with an online data entry form.
Water vapor measurements are best made as part of a more comprehensive series of atmospheric measurements, using several GLOBE atmosphere protocols. For example, they can be (but don't have to be) made at the same time as GLOBE aerosol measurements (see the GLOBE Aerosols Protocol), which use a similar handheld instrument. The water vapor instrument data can be collected either manually, or by using a computer interface to record data automatically. Data submitted to GLOBE are processed by the GLOBE science team. The results are reported back to students and made available to GIFTS and other scientists. Through GLOBE, extensive online data entry and visualization tools are already available.
It is possible for students and educators to build their own water vapor instrument. Construction details and instructions for obtaining parts are available at asd-www.larc.nasa.gov/GIFTS. NOTE: The NASA link is no longer available. For more information about building a water vapor instrument, contact David Brooks at the Institute for Earth Science Research and Education. This is a project that requires considerable experience and should not be undertaken as a first electronics construction project. Completed instruments must be calibrated in cooperation with the GIFTS team before data can be reported to the GLOBE Program's online database.
This section is based on the GLOBE Program's Water Vapor Protocol and is consistent in its format with other GLOBE science protocols. The material presented here stands on its own but, at the same time, minimizes duplication of other readily available material, such as GLOBE protocols for supporting measurements and observations. Although it is possible to collect water vapor data without participating in the GLOBE Program, these measurements will make more sense when they are related to other GLOBE-supported atmospheric measurements. For information about the GLOBE Program and its protocols,see www.globe.gov. For information about the GIFTS project, see asd-www.larc.nasa.gov/GIFTS.
To measure the total precipitable water vapor in the atmosphere above an observer's site.
Students provide ground measurements of total precipitable water vapor (PW) by using a battery-powered handheld sun photometer that measures the transmission of sunlight through the atmosphere at two near-infrared wavelengths. Students also report meteorological and sky conditions based on qualitative observations, direct measurements, and online sources.
Students learn about the solar spectrum and that energy from visible light from the sun is only a small part of the total energy received from the sun. They learn how specialized sun photometer measurements are related to atmospheric water vapor, how water vapor measurements relate to the hydrologic cycle, and how greenhouse gases such as water vapor play an important role in weather and climate. They learn how satellite-based instruments are measuring global water vapor as part of overall monitoring of Earth's atmosphere, and they learn how these measurements help to improve scientific understanding of Earth's weather and climate.
Scientific Inquiry Abilities
A more detailed alignment of activities associated with the water vapor measurement to science education standards is given in Section 10.
Water vapor is an important atmospheric constituent related to both weather and climate. Water vapor is critical to cloud formation (and, therefore, precipitation), and it is the primary greenhouse gas that helps control temperatures in the lower atmosphere and on Earth's surface. The interactions of water vapor with other constituents of the atmosphere, weather, and climate, are complex and global in scope. Although the presence of liquid water and water vapor near Earth's surface is easily discernible in the form of clouds and relative humidity, atmospheric s cientists still have many questions. How much water vapor is in the atmosphere? How is it distributed around the globe? How does it change in time and space? How does it affect cloud formation and precipitation? How will changes in cloud type and cover affect future weather and climate ( or the other way around)?
|Figure 1. EOS spacecraft (artist's conception).|
Despite its importance, the global distribution and temporal variability of water vapor is not well known. Whenever global measurements are required, scientists turn to satellite-based instruments. A primary motivation for developing this project is to provide measurements in support of the GIFTS (Geosynchronous Imaging Fourier Transform Spectrometer) instrument, part of NASA's Earth Observing 3 (EO3) mission. An artist's conception of the spacecraft is shown in Figure 1. The GIFTS instrument is the irregularly shaped box to the left of the large circular antenna. The main spacecraft is a cube about 2 m on a side. GIFTS will observe weather patterns, atmospheric temperature, water vapor content and distribution, and the concentration of certain other atmospheric gases. From its orbit high above Earth's surface, GIFTS will provide unprecedented detail about the spatial and temporal variability of these quantities. For an introduction to GIFTS, see asd-www.larc.nasa. gov/GIFTS.
As important as satellite-based measurements are to an improved understanding of the global distribution of water vapor, ground-based measurements continue to play a vital role. When GIFTS views the Earth/atmosphere system from space, its geometric resolution (one "pixel") is about 4 km x 4 km. This enables scientists to track storm systems, for example, as large systems can have dimensions on the order of hundreds of kilometers. However, an individual GIFTS pixel can be larger than the size of some individual cumulus clouds, for example. So, ground-based measurements can provide local detail at "sub-pixel" spatial resolution and they can help scientists determine whether the algorithms they have developed for analyzing GIFTS data are working properly.
Investigating Water Vapor (back to the Table of Contents)
|Figure 2. Geosynchronous/geostationary Earth orbits.|
By reporting water vapor measurements regularly, students can provide scientists with some of the data they need to better understand the global distribution of water vapor, and they can start to understand how water vapor is distributed over their own observing site. Water vapor data are especially valuable if they can be compared directly with satellite-based measurements. Even if satellite instruments do not measure water vapor directly, ground-based measurements will be helpful in interpreting other measurements. In some cases, ground measurements need to be timed to coincide with the passage of earth-observing satellites over a site. This is true for spacecraft in NASA's EOS (Earth Observing Satellites) project, for example, as they are in near-polar orbits that pass over or near sites on Earth's surface every day at predictable times.
Spacecraft such as those that will carry the GIFTS instrument are in near-equatorial circular orbits at an altitude of about 36,000 km above Earth's surface, as shown schematically in Figure 2. At this altitude, a spacecraft's orbital period is equal to one day. These are called geosynchronous or geostationary orbits, because they maintain a fixed relationship to sites on Earth's surface, thereby allowing continuous monitoring of large areas of the Earth. A more detailed description of geosynchronous orbits is given in Section 7.
Some space-based measurements can be made regardless of whether it is night or day on Earth's surface under the satellite. The GLOBE/GIFTS water vapor instrument measures the transmission of direct sunlight through the atmosphere so, of course, it must be used during the day.
Whenever an observing site is within view of a water vapor instrument on board a geosynchronous satellite that makes continuous measurements, it will be useful to make ground-based measurements any time during the day, as long as other observing conditions are suitable. Because of the seasonal variability of water vapor, it is important to build a water vapor data record that extends across several seasons. Long-term records are more valuable for scientists, as they provide a better understanding of local environments.
Understanding Measurements of Water Vapor (back to the Table of Contents)
|Figure 3. "Collecting" water vapor.|
Imagine a column of atmosphere perpendicular to a horizontal (level) observing site at Earth's surface, as shown in Figure 3. This imaginary column contains all the local atmospheric constituents, including water vapor, above the observing site. Now imagine collecting all the water vapor in the column, bringing it down to the observing site, and transforming it into liquid form. The thickness of this layer of water, typically a few centimeters, is the total water vapor, or total precipitable water (PW). The unit for expressing PW is cm (of water). The PW includes water vapor near the surface, related to a surface measure of relative humidity, but also all the water in the rest of the atmosphere above the site. Most PW, all but 0.1 cm or so, is in the troposphere, and most of the tropospheric PW is within a few kilometers of the surface.
One way of detecting water vapor is to examine how it affects the transmission of sunlight through the atmosphere. Water vapor (molecules of H2O) absorbs sunlight at specific wavelengths and these absorption bands decrease the amount of sunlight reaching Earth's surface at particular wavelengths. Two absorption bands, shown in Figure 4, are in the near-infrared part of the solar spectrum.
Figure 4 shows three sets of data, based on computer models of the atmosphere. One is the distribution of solar energy as a function of wavelength just outside the atmosphere. The second is the distribution of incoming solar radiation on Earth's surface for a so-called "standard atmosphere" with an average amount of water vapor. The third is for an imaginary atmosphere with no water vapor. The difference between the second and third sets of data show how absorption by water vapor decreases the amount of sunlight reaching Earth's surface.
Now suppose that two detectors respond to sunlight at different wavelengths - one at a wavelength within a water vapor absorption band and one just outside this band. Such detectors are illustrated in Figure 5, in which the normalized detector responses are superimposed on the surface irradiance data from Figure 4. Assuming the position of the sun relative to the observer doesn't change, the amount of light seen by the detector outside the band, centered around 870 nm, will not change if the amount of atmospheric water vapor changes. The detector inside the band, centered around 940 nm, will respond to such changes. Hence, the ratio of the response of two such detectors will change and will therefore be a measure of the water vapor concentration.
|Figure 4. Absorption of solar radiation by water molecules.|
|Figure 5. Detector responses for determining water vapor using a major absorption band around 940 nm.|
In principle, you could use a single detector inside the water vapor absorption band. However, this single detector would be too sensitive to sun lighting conditions and other changes in the atmosphere, such as clouds and haze, that affect the amount of sunlight reaching Earth. So, the ratio of two detectors is used in order to minimize these kinds of problems.
PW is related to other properties of the atmosphere, including those described in other GLOBE atmosphere protocols. It varies diurnally, seasonally, and geographically. Hence, it is helpful to consider water vapor as part of a broader discussion of the atmosphere and its properties. Ideally, water vapor measurements should be made over an extended period of time to observe seasonal effects. The measurements will make more sense if they are combined with other GLOBE atmosphere protocols, including the basic meteorological protocols and aerosols. Some of these other protocols are required in any case to provide the metadata (qualitative observations or quantitative measurements that supplement the actual data) that are reported along with the water vapor instrument data.
The GLOBE/GIFTS Water Vapor Instrument (back to the Table of Conetents)
A drawing of the GLOBE/GIFTS water vapor instrument is shown in Figure 6. This instrument is based on the same principles as the GLOBE sun photometer for monitoring aerosols in the atmosphere, which uses light emitting diodes (LEDs) as spectrally selective sunlight detectors. This concept was first described by Forrest M. Mims III, the co-PI for GLOBE's Water Vapor and Aerosol Monitoring Projects [Mims, 1992]. For the GLOBE sun photometer used for measuring atmospheric aerosols, LEDs detect visible light in the green and red part of the spectrum. The water vapor instrument is also a sun photometer, but its detectors respond to infrared rather than visible light. In some versions of the GLOBE/GIFTS water vapor instrument, one or both LED detectors are replaced by filtered photodetectors to improve the sensitivity of the instrument to small changes in water vapor.
|Figure 6. The GLOBE/Gifts water vapor instrument.|
Measurements taken with the GLOBE/GIFTS water vapor instrument are in units of volts. These values must be converted into PW using calibration data that have been determined for each instrument. The calibrations require access to highly specialized equipment and data and they cannot be duplicated by students in the lab or in the field. When measurements are reported to GLOBE, the PW calculations are done by GLOBE computers and the calculated values are then made available to students - instantly if data are entered through GLOBE's online data entry system.
|Figure 7. Definition of slant path and relative air mass.|
The standard unit for reporting water vapor is centimeters of water in a vertical column of atmosphere directly above the observer. In almost all cases (never at any latitude farther than about 23.5° from the equator), the sun will not be directly overhead when water vapor measurements are collected. In general, the instrument will view the sun through a longer atmospheric path, as illustrated in Figure 7. This path is called the slant path or relative air mass. The smaller the solar elevation angle 2, the longer the slant path and the larger the relative air mass. An approximate relationship between elevation angle and relative mass is m=1/sin(2). (When the sun is near the horizon, the relationship between solar elevation angle and relative air mass is more complicated.) To compensate for this longer path, and assuming that the distribution of water vapor is the same in the atmosphere between the observer and the sun as it is in the atmosphere directly overhead, the water vapor detected by the instrument (the slant path water vapor) is divided by the relative air mass.
PW = PWalong slant path/m
Where and When to Make Water Vapor Measurements (back to the Table of Contents)
For GLOBE schools and other locations where an atmospheric study site has been established, the logical place to take water vapor measurements is in the same place where other atmospheric observations are made. In general, any site that allows an unobstructed view of the sun (weather permitting) is usable. The basic meteorological conditions for using the GLOBE/GIFTS water vapor instrument is that the observer must have an unobstructed direct view of the sun not blocked by clouds. Some atmosphere measurements required for metadata to be reported with water vapor instrument readings may impose additional restrictions on sites. Air temperature thermometers must be mounted in appropriate shelters and be sufficiently far from buildings, trees, and other obstacles. The site should provide a view of the sky that permits reasonable observations of total cloud cover and type, sky color, and clarity.
The decision about what time of day to take water vapor measurements depends primarily on whether the measurements will be associated with a particular satellite instrument and, if so, what kind of orbit that satellite has. For most orbits, including the near-polar orbits occupied by many earth-observing satellites, measurements need to be timed to coincide with overflights of a site. NASA's current major Earth-observing satellites view most of Earth in mid morning or early afternoon. The precise times at which they fly over an observing site are readily available online. (See earthobservatory.nasa.gov/MissionControl/overpass.html . ) For instruments in synchronous orbits (such as GIFTS), or if measurements are not being associated with satellite measurements, measurements can be made at any time during the day. For developing a long-term record of water vapor over an observing site, it is helpful to make measurements at about the same local solar time every day. If possible, solar noon is the best choice for measurements made once a day. Note that solar noon is not the same as clock noon, and varies by location and time of year. Solar noon is the time at which the sun crosses your local meridian. The value can be found online (see, for example, http://www.srrb.noaa.gov/highlights/sunrise/gen.html). If your newspaper publishes times of sunrise and sunset, solar noon is (to a good approximation) the time halfway between sunrise and sunset.
The final decision about whether to take water vapor measurements rests on determining whether sky conditions permit an unobstructed view of the sun. Sun photometer measurements are meaningless if direct sunlight is blocked by clouds. It is not always easy to determine when clouds are blocking the sun. Looking directly at the sun is never an option! Thin cirrus clouds pose a potential problem, as they can be difficult to see. However, PW measurements made through thin and reasonably uniform cirrus clouds are probably OK, as most water vapor is in the atmosphere beneath cirrus clouds. In any case, in order to ensure high-quality PW measurements, it is important to practice cloud identification skills as described in GLOBE's Cloud Cover, Cloud Type, and Contrails protocols.
The GLOBE/GIFTS water vapor instrument is simple and rugged, with no easily breakable parts. However, you must take care of it in order to make accurate measurements. Here are some things you should do (and not do) to make sure your water vapor instrument performs reliably over long periods of time.
Every three months or so, or right away if you accidentally leave your instrument turned on for an extended period of time, check the voltage from the battery in your water vapor instrument and replace the battery if necessary. See the Checking and Changing Your GLOBE Sun Photometer Battery Lab Guide in the GLOBE Aerosols Protocol for instructions. Replacing the battery will not change the calibration of your instrument and measurements made with the old battery will be OK as long as you replace the old battery before its voltage falls below roughly 7.5 V. If the "low battery" symbol appears on the panel meter display, replace the battery immediately.
This measurement is an excellent hands-on activity for any course that addresses the atmosphere, weather and climate, the hydrologic cycle, or Earth as a system. Prior to implementing this protocol, it will be helpful to provide an introduction to electromagnetic radiation and the solar spectrum, including ultraviolet, visible, and infrared energy from the sun. It is important for students to understand that light visible to the human eye spans only a very small portion of the solar spectrum and that light at other wavelengths can have significant effects on humans and other living organisms.
If you have access in the classroom to an electronic device that is controlled by a remote IR controller, you can experiment with this device. How do we know IR light (radiation) is really there? Does it appear to behave like "light" even though we can't see it? What will block the IR signal from the controller? What will allow its passage?
You should spend some classroom time familiarizing your students with the water vapor instrument, including reading the panel meter. In the classroom, the voltages displayed on the voltmeter will be small - only a few millivolts. If you point the instrument at the sun, even through a closed window, you will get much higher values. If you are going to use automated data logging to record the signal from your water vapor instrument, you will need a laptop or palmtop computer that students can take into the field and they will need to practice setting up and using the hardware and software.
Students should also have a chance to study a sample data logged file, by using a spreadsheet to explore and graph the contents. If you are using automated data logging, it may be worthwhile for secondary school students to discuss the mathematics of analog-to-digital conversion. (See Section 6.)
Auxiliary data and metadata for the water vapor protocol are identical to those for the GLOBE Aerosols Protocol. Some of these are based on qualitative observations:
Depending on which GLOBE protocols you are already doing, you will need to organize sources for some or all of these observations and measurements. The requirements are described in detail in the Classroom Guide to Field Work. In some cases, GLOBE protocols are available. Note that, although GLOBE protocols specify the performance of instruments for collecting some of the quantitative metadata, online data from nearby weather stations are reliable and sufficiently accurate for water vapor protocol metadata. In the U.S., it will invariably be necessary to convert data to GLOBE (and internationally accepted) units - for example, Centigrade (Celsius) rather than Fahrenheit degrees.
A Classroom Guide to Field Work is provided to help you prepare for implementing this protocol. It describes in detail the steps involved in recording a complete set of measurements, along with some discussion for each step. It parallels the Field Guide that simply lists the steps in order with no further explanation. As part of their preparation for this protocol, students and teachers should study the Classroom Guide to Field Work to make sure they understand each step.
The purpose of this section is to provide some questions that can lead to student investigations. The fact that some answers are given to the questions does not mean that students should not be encouraged to conduct their own investigations based on these and similar questions.
Dewpoint temperature, which can be calculated from air temperature and relative humidity, is the surface-level meteorological value that has, in general, the highest correlation with water vapor.
Yes, in principle. However, the fact that the response of one or both of the detectors may be affected directly by the amount of water vapor in the atmosphere presents a problem for interpreting the data. In any event, your water vapor instrument must be calibrated specifically for this purpose, in addition to its water vapor calibration. If you are interested in this measurement, contact the senior author of this report.
In temperate climates, PW is typically higher in the summer than in the winter. Rapid changes in PW are especially noticeable when strong weather fronts pass through an area. Over time, you should be able to associate PW ranges with particular kinds of weather systems. Cold fronts, for example, should lead to lower PW values. Even though the association of PW with dewpoint temperature is only an approximate one, it is nonetheless true that cold dry air has a lower dewpoint temperature than warm moist air and, hence, will produce lower PW values.
This section includes a detailed step-by-step description of how to collect water vapor data, with information about and explanations for each step. The steps are keyed to the Field Guide, in which the same steps are listed with a minimum of explanation.
What You Need (back to the Table of Contents)
In order for scientists to interpret measurements made with your water vapor instrument, you must provide the longitude, latitude, and elevation of your observing site. Longitude and latitude can be accurately determined by using a GPS receiver. Make sure that your unit is set to display angles in decimal degrees (the GLOBE standard for reporting longitude and latitude coordinates). GPS values of site elevation may not be very accurate. If possible, you should check the elevation by referring to a reliable topographical map of your area. The site description needs to be done only once unless, of course, you change the location of the site or add an additional site. If you are already a GLOBE school, you should already have defined your observing site. For PW measurements, the primary relevant site characteristics are its elevation and longitude-latitude coordinates. A new site definition is necessary only if it is more than several hundred meters from the original site, or if it is at a significantly different elevation (sites at ground level and at the top of a tall building, for example).
The basic site condition for making water vapor measurements is that it must provide an unobstructed view of the sun and a view of the sky that allows you to make reasonable cloud cover and type estimates. These measurements can be done in an urban setting, even if there may not be enough open space to set up an air temperature thermometer enclosure that meets GLOBE standards.
Metadata are qualitative observations or quantitative data that supplement your actual data; they are essentially "data about data." Metadata are extremely important because they help scientists interpret your measurements. Some of the metadata can be collected in the classroom just before or after your measurements, rather than outdoors at the same time as the measurements.
|Figure 8. Sample weather page from National Weather Service.|
In the U.S., weather data are available online from http://iwin.nws.noaa.gov/iwin/[two-letter state code]/hourly.html is updated once per hour. A sample screen from this site is shown in Figure 8. The values are updated once per hour. Select the weather station closest to your observing site and use the report that is nearest the time of your measurements. For example, if you will be collecting data around quarter past the hour, record online data before data collection. If you are collecting data just before the hour, collect online data after data collection, after the online data are updated.
Accurate barometric pressure values are important. Sources for barometric pressure are, in order of preference:
Note that a classroom barometer is the last choice for reporting barometric pressure data. This is because relatively inexpensive aneroid barometers are difficult to calibrate unless you are near an official weather station at sea level. The factory calibrations for these instruments are rarely accurate and should not be relied upon. However, if you are near an official weather station, then you can use online data to calibrate your classroom barometer.
In many parts of the world, accurate barometric pressure values are readily available online, and are therefore preferable. In the U.S., data found at http://iwin.nws.noaa.gov/iwin/[two-letter state code]/hourly.html.
Some U.S. newspapers publish a daily weather almanac that gives weather information for the previous day, including pressure, relative humidity, and barometric pressure. Use the values closest to the time of your data collection. The example shown in Figure 9 is from the Philadelphia Inquirer, 29 October, 2003. The values given are for the previous day, 28 October, 2003. The barometric pressure is given at noon, and this
|Figure 9. Sample newspaper "weather almanac" listing.|
In the U.S., barometric pressure will need to be converted from inches of mercury (Hg) to millibars (hectopascals), which is the international scientific standard:
The electronics in your GLOBE water vapor instrument, and especially its detectors, are temperature-sensitive. This means that the output of your instrument will change under the same conditions as the instrument warms or cools. Therefore, it is important to maintain your instrument at approximately room temperature. There are some steps you can take to minimize temperature sensitivity problems. Keep your water vapor instrument inside and bring it outside only when you are ready to make measurements. In the winter, transport it to the observing site under your coat. In the summer, you can transport it in a small picnic cooler. In very hot or cold weather, you can wrap the instrument in an insulating material such as an insulated paper sandwich bag, a towel, or pieces of foam plastic. You can construct a shell for your instrument from rigid foam plastic sheets (Styrofoam) held together with aluminum tape. Especially in the summer, keep your instrument shielded from direct sunlight whenever you are not actually making a measurement.
To alert scientists to potential problems with temperature, you should report air temperature along with your water vapor measurements. If you are making water vapor measurements at the same time you record temperature from your GLOBE weather station, then you can report that current temperature for the water vapor protocol.
The temperature accuracy requirements for the water vapor protocol are not as strict as they are for the GLOBE air temperature protocol. So, you can use just about any available thermometer, including digital thermometers that may not meet GLOBE accuracy standards of "±0.5º. With any thermometer, it is essential to keep it away from direct sunlight. Exposing the sensing element of a thermometer to direct sunlight can quickly raise its temperature by several degrees Centigrade.
If you are near a weather station (practically anywhere within the U.S.), you can get air temperature values online. Or, you can use published values, as in the above example from the Philadelphia Inquirer newspaper. Although this is not the same as actually measuring the temperature at your observing site, it is adequate for the purposes of this protocol.
Temperatures from U.S. stations are invariably given in Fahrenheit, so you will have to convert them to Centigrade:
|TC = (5/9)(TF - 32)|
Your water vapor instrument is fitted with an electronic temperature sensor that is located right next to the sunlight sensors. You can display this temperature by selecting the "T" position for the rotary switch on the top of your instrument. The output from the sensor is 10 mV per degree C. So, the temperature is 100 times the "T" voltage reading. For example, if the reading is 0.224, then the temperature inside the case is 22.4ºC. If you use manual data collection, you should record this value once at the beginning of a set of measurements and again at the end. If you are using automated data collection, the temperature will automatically be recorded along with the detector outputs.
Ideally, you should make measurements when the temperature inside the case is near room temperature - in the low to mid 20's Centigrade - regardless of the air temperature outside the case. Meeting this goal requires development of a strategy appropriate to your weather conditions, but it is not difficult to do.
It is important to report accurately the time at which you make measurements because calculations of solar position at your site depend critically on time. The GLOBE standard for reporting time is always Universal Time (UT), which can be calculated from local clock time, your time zone, and time of year (which determines whether you are using standard time or daylight savings or summer time). It is absolutely essential to convert local time to UT correctly. Be especially careful when you switch from standard to summer time, or vice versa. For example, you must add 5 hours to convert Eastern Standard Time (EST) to UT, but only 4 hours to convert Eastern Daylight Time (EDT) to UT. A one-hour error can produce results that look OK but which are, in fact, wrong. If you have a GPS receiver, you can get UT from that.
Time should be reported to an accuracy of no less than the nearest 30 seconds. A digital watch or clock is easier to use than an analog one, but in either case you must set your timepiece against a reliable standard. Even an analog wristwatch can be read to the nearest quarter-minute if it has one-minute marks on its dial.
|Figure 10. www.time.gov Web site.|
It is not difficult to set your clock or watch accurately enough to meet the standards required for this protocol. You can get time online at www.time.gov. A screen from this Web site is shown in Figure 10. For sites outside the United States, use the UTC Time Zone. In many parts of the world, you can buy a clock that sets itself automatically by detecting a radio signal from an institution that maintains a reference clock. These are sometimes referred to as “atomic clocks” because the time standard depends on the decay of radioactive atomic particles. The official US clock, maintained by the National Institute of Standards and Technology, broadcasts official time from station WWV in Fort Collins, Colorado. (See www.boulder.nist.gov/timefreq/stations/wwv.html)
It may be tempting to use the clock maintained by your computer as a standard. However, this is not a good idea, as (perhaps surprisingly) computer clocks are often not very accurate, and they must be set periodically according to a reliable standard. Note that modern computer operating systems will automatically switch your computer clock back and forth between standard and summer time. You should be aware of when this change occurs if you need to manually convert time from your local clock time to UT.
Water vapor measurements can be taken any time during the day. Indeed, it is an interesting project to study the variation of water vapor during the day. However, the water vapor instrument will give the most reliable readings when you make measurements between mid-morning and mid-afternoon. In northern or southern hemisphere temperate and higher latitudes, with low maximum solar elevation angles at least during the winter, you should take measurements as near solar noon as possible, especially in the winter.
If you are making measurements that correspond to satellite overflights, then, of course, the times of those overflights determine when measurements should be made, (see earthobservatory.nasa.gov/MissionControl/overpass.html. How closely must your measurements match the time of the overflight to be useful? This is a question that should be discussed with scientists working with the space-based instruments. In general, the times should match within just a few minutes.
If you are doing the optional GLOBE relative humidity protocol, using either a digital hygrometer or a sling psychrometer, you can use that value. Otherwise, you can use online values. (In the U.S., use http://iwin.nws.noaa.gov/iwin/[two-letter state code]/hourly.html. Relative humidity is reported as a whole number, in percent. Relative humidity and temperature are used to calculate the dewpoint temperature, which is somewhat correlated with PW. (See Looking at the Data at the end of this Section.)
If you use an online weather report, you may also be able to report wind speed and direction. On the http://iwin.nws.noaa.gov/iwin/[two-letter state code]/hourly.html site, this information is given in a string of text, such as NW15G23. This means "wind from the northwest at 15 miles per hour, gusting to 23 miles per hour." Convert the miles per hour to kilometers per hour before reporting. (All GLOBE measurements must be reported in metric units.) You may round off the conversion to the nearest kilometer per hour:
Note that the data shown above from the Philadelphia Inquirer include relative humidity, but not wind speed and direction. If you do not have access to wind speed and direction information, you do not need to report it.
Water vapor measurements can be interpreted properly only when the sun is not obscured by clouds. This does not mean that the sky must be completely clear, but only that there should be no clouds in the vicinity of the sun. This is not necessarily a simple decision. It is easy to see whether low- and mid-altitude clouds are near the sun, but cirrus clouds pose a potential problem. Remember that the water vapor instrument detects light in the infrared part of the solar spectrum, so the fact that cirrus clouds may be only faintly visible to humans does not necessarily mean they are not important. Even though thin and uniform cirrus clouds can block a significant amount of sunlight, they probably do not adversely impact PW measurements. The presence of cirrus clouds should always be noted as part of the metadata report.
Another difficult situation occurs in typical summer weather, especially near large urban areas. In this environment, polluted skies and humid conditions may make it difficult to distinguish cloud boundaries. It is important to describe such conditions whenever you report measurements. Observing the sky (away from the sun!) through orange or red sunglasses or a plastic filter will make cloud boundaries easier to see.
Whenever you try to determine cloud conditions in the vicinity of the sun, you must block the sun itself with a book, a sheet of paper, a building or tree, or some other object. A sensible rule is that if you can see even faint shadows on the ground, you should never try to look directly at the sun. If in doubt, or if you believe you cannot determine sky conditions near the sun, then do not make a measurement. It is recommended that you wear sunglasses during these measurements, except when you are trying to determine sky color.
Safety Reminder: Never look directly at the sun, even through colored sunglasses or plastic filters. This can seriously damage your eyes!
Cloud conditions reports should follow the GLOBE protocols for cloud cover and cloud type, as shown in Table 1.
Contrails should be reported separately, as described in the Contrails Protocol.
|Scale Value||Sky Color||Sky Clarity|
|1||Deep blue||Unusually clear|
|3||Light blue||Somewhat hazy|
With practice, however, you can learn to be consistent about your own interpretations. For example, you can easily learn to recognize the deep blue clear sky that is associated with clean skies and low relative humidity. With higher humidity and increasing aerosols, the sky color changes to a lighter blue. It may appear milky rather than clear. In some places, especially in and near urban areas, the sky can have a brownish or yellowish tint due to air pollution (primarily particulates and NO2).
To determine sky color, look at the sky in a direction away from the sun, as shown in Figure 11. That is, you should be looking out over your shadow. Sky color is generally lighter near the horizon. For this reason, you should be consistent about observing the sky at an elevation angle of about 45º above the horizon. If this part of the sky is cloudy, use the nearest part of the sky for which you can determine the color.
|Figure 11. Observing sky color.|
You can determine sky haziness by using a distant object – a tall building or mountain range, for example – as a reference. When this object appears sharply defined in its natural colors, then the sky is clear. As the object becomes less distinct, then there is probably more water vapor and particles in the atmosphere. Note that this method of determining haziness is related to horizontal visibility, which may not always be an accurate indicator of the condition of the atmosphere above your site.
When there are obvious reasons for unusual sky conditions, you need to report them. Air pollution, dust, and smoke are examples of conditions that need to be reported in the “comments” section.
The water vapor instrument should be at room temperature – about 20-25ºC – before collecting data. In hot or cold weather, place the instrument in an insulated container before you take it outside.
Make sure you have all required materials and that, if you are working as a team, each team member understands her/his role. This is especially important if several different students participate in these measurements on a rotating basis.You can practice data collection in your classroom by pointing your instrument at the sun through a window – even a closed window. If you can open a classroom window and see the sun, you can collect actual data from your classroom, too.
It is easier for two people to collect these data than for one person working alone. If you are not familiar with this protocol, divide up the tasks and go through several practice runs outside before you start recording real data with your water vapor instrument. Remember that these practice runs may result in your instrument being exposed for a relatively long time to hot or cold weather. Before you make “real” measurements, you must be sure your instrument has returned to room temperature, as described in item 3 in the Metadata section of Getting Ready to Take Measurements.
The steps described below may seem complicated at first. However, you can master them with only a little experience in the field.
It is helpful to be able to brace the instrument against your knees, a chair back, railing, or some other fixed object.
This reading is related to the air temperature near the detectors in your instrument. Record the voltage times 100. For example, a reading of 0.224 V should be recorded as 22.4ºC.
The Data Entry Form asks for measurements in IR1/IR2 order. You should develop the habit of always making measurements in this order.
During the next 10-15 seconds, observe the voltage displayed on the panel meter, WHILE KEEPING THE SPOT OF SUNLIGHT CENTERED OVER THE ALIGNMENT DOT, and record the maximum voltage in the “sunlight voltage” column of your Water Vapor Data Sheet.
The voltages will fluctuate by a few millivolts even when you hold your instrument perfectly steady. This is due to real fluctuations in the atmosphere. Do not try to “average” these fluctuating voltages. Be sure to record all the digits displayed on the panel meter: 1.732 rather than 1.73, for example.
An accuracy of 15-30 seconds is required. This is possible even with an analog watch that has been set to a reliable standard.
After you are familiar with your instrument, you do not need to actually observe the dark voltage every time you take a measurement, as it should not change. It the dark voltage does change significantly, it means your instrument is much too hot or too cold.
Measurements should always be reported as IR1/IR2 pairs.
This will give three to five pairs of IR1/IR2 measurements. Remember that it is important to be consistent about the order in which you collect these data: IR1/IR2/IR1/IR2/IR1/IR2... The time between measurements is not critical as long as you record the time accurately. However, especially in hot or cold weather, it is important to minimize the total measurement time, in order to minimize temperature fluctuations inside your instrument case. A set of from three to five pairs of measurements should not take longer than two or three minutes (20-30 seconds per voltage value).
The only difference between the manual and automated protocols concerns how the water vapor instrument is used. The metadata reporting requirements are exactly the same. A brief introduction to using data logging hardware and software is given in Section 3.
In addition to the general In the Classroom instructions given above, additional steps are required to prepare the data logging software.
It is helpful to be able to brace the instrument against your knees, a chair back, railing, or some other fixed object.
The position of the rotary switch does not matter, as all three channels (IR1, IR2, and T) are always recorded simultaneously. It may be helpful to use the "T" position so you can monitor the temperature inside your instrument's case.
If you activate the data collection software before turning on your instrument, you may see voltage spikes in your data file. These will not affect the quality of your data, but they can be annoying when you are trying to interpret those data.
This provides a brief dark voltage sample.
4. Center the sunlight spot coming through the hole in the front bracket on the circular dot on the rear bracket and remove your finger from the sunlight apertures. Hold your instrument in this position for two minutes.
It is important to keep the sun spot centered over the dot on the rear bracket and to hold this position as steadily as possible. You may have to adjust the pointing of your instrument slightly to keep it centered as the sun moves across the sky.
If you turn off the instrument first, you may see voltage spikes in your data file.
If you have a classroom barometer, read it now. If you are using online sources for meteorological information, as described above, do that now. If you cannot conveniently access a computer right before or after your measurements, you may still be able to get meteorological information the following day, using weather almanacs published in some newspapers. Remember that the purpose for reporting these meteorological values is to help scientists interpret your data. Your goal should always be to report values that accurately represent conditions at the time and location where the measurements were made.
Complete the Water Vapor Data Sheet. If you are using automated data collection, record the name of the file in which your results are stored. Be sure that the date on the sheet is the date on which the measurements were actually made. Include the names of the individual(s) making the measurement. Store the original data sheet in a notebook reserved just for this purpose.
This instrument takes advantage of the fact that water vapor molecules in the atmosphere absorb sunlight that would otherwise reach Earth’s surface. The instrument has two detectors that respond to radiation in the near-infrared part of the solar spectrum. One of these detectors overlaps a major water vapor absorption band and one is outside this band. What the instrument measures directly is the amplified current (in the form of a voltage output from two amplifiers) from each of these detectors. When the instrument is properly calibrated, the ratio of the output voltages from the two channels can be related to the amount of water vapor in the atmosphere.
A light-emitting diode is a semiconductor device that emits light when an electrical current flows through it. The actual device is a tiny chip only a fraction of a millimeter in diameter. The chip is housed in either a small metal case with a flat glass cover about 5 mm in diameter, or an epoxy cylinder about 5 mm in diameter.
The physical process that causes LEDs to emit light also works the other way around: if light shines on an LED, it produces a very small electrical current. The electronics in your water vapor instrument amplify this current and convert it to a voltage.
LEDs are found in a wide range of electronic instruments and consumer products. The most familiar LEDs emit visible light – red, yellow, green, or blue. The LED in your water vapor instrument emits (and responds to) infrared light. This radiation is invisible to the human eye. Infrared LED transmitters and detectors are commonly used in the familiar handheld remote control devices included with consumer electronics devices such as TVs and audio equipment.
The other detector in the GLOBE/GIFTS instrument is a photodiode with a built-in filter. This is a broadband detector whose response is limited by a piece of coated glass placed over the detector. The coated glass is called an interference filter. Some versions of the GLOBE/GIFTS water instrument may use two filtered photodiodes, with no LED.
The water vapor instrument is a specialized application of what is generally known as a sun photometer. The equation that describes theoretically how to interpret sun photometer measurements requires that the instrument should see only direct light from the sun – that is, light that follows a straight-line path from the sun to the light detector. This requirement can be met only approximately because all sun photometers will see some scattered light and direct light from the sky around the sun.
The cone of light a sun photometer’s detector sees is called its field of view, and it is desirable to have this cone as narrow as possible. The GLOBE water vapor instrument’s field of view is about 2.5°, which is a reasonable compromise between desires for accuracy and practical considerations in building a handheld instrument. The basic tradeoff is that the smaller the field of view, the harder the instrument is to point accurately at the sun. Very expensive sun photometers, with motors and electronics to align the detector with the sun, can have fields of view of 1° or less. Studies have shown that the error introduced by somewhat larger fields of view is negligible for the conditions under which a GLOBE sun photometer should be used. Scattered light is even less of a problem for the infrared wavelengths used in the GLOBE/GIFTS water vapor instrument.
The detectors in your instrument are temperature-sensitive, so their output is slightly influenced by their temperature. Therefore, it is very important to protect your instrument from getting too hot in the summer or too cold in the winter. If you follow the suggestions given here and take your measurements as quickly as possible once you're outside, then you can minimize undesirable temperature effects.
In the summer, it is essential to keep your instrument case out of direct sunlight when you are not actually taking a measurement. In the winter, it is essential to keep the instrument warm - you can tuck it under your coat between measurements. Also, you can transport your instrument to and from the observing sight in an insulated container such as a small picnic cooler. Never leave your instrument outside or in any other extreme temperature environment for extended periods of time.
The instrument case itself provides some protection from temperature changes that can affect the electronics inside. In extreme conditions (winter or summer), you should consider making an insulating housing for your instrument, using Styrofoam or other rigid foam plastic. Cut holes for the on/off switch, panel meter, and sunlight aperture, and a channel for sunlight to pass from the front alignment bracket to the target on the back bracket. The hole for the sunlight aperture should be no smaller in diameter than the thickness of the insulating material itself, and in no case should it be smaller than about 1 cm.
The "T" switch position on the top of your water vapor instrument monitors temperature inside the case. The temperature in degrees C is 100 times the voltage displayed on the panel meter.
Fortunately, the components inside your sun photometer are virtually indestructible, so they should survive being dropped. If you have made an insulated housing for your instrument, as suggested in the previous question, then it will be well protected. Check the case for cracks. Even if the case is cracked, it may still be OK. Just tape over the cracks using something opaque, such as duct tape or aluminum tape. Open the case and make sure that everything looks OK. In particular, make sure that the battery is still firmly attached to its connector.
Check the alignment of the sunlight spot on the colored dot on the rear alignment bracket. The maximum voltage should be displayed when the sunlight spot is centered around this dot. If this is not true, or if the alignment brackets have obviously moved or are loose as a result of the fall, your instrument should be returned to Langley Research Center or the primary author of this report for recalibration.
When you turn your water vapor instrument on without pointing it at the sun, you should measure a small DC voltage no larger than a few millivolts. When you point your instrument directly at the sun, the voltage should increase to a value in the range of about 0.5 to 2 V. If you do not observe these kinds of voltage changes when you point your instrument at the sun, then it is not working.
The most likely reason for a water vapor instrument to stop working is that the battery is too weak to power the electronics. As indicated in the procedure for changing the battery (see the GLOBE Aerosols Protocol), you should replace the battery if its voltage with the instrument turned on is less than 7.5 V. You should check the battery three or four times per year unless you know your instrument has inadvertently been left turned on for an extended period of time. If the "low battery" indicator appears on the panel meter, replace the battery immediately.
Changing the battery will not affect the calibration of your instrument. If you replace the battery and your instrument still appears not to work, contact the primary author of this report.
As noted in question 1, above, the water vapor instrument works by examining the ratio of the response of two near-IR detectors - one inside and one outside a water vapor absorption band. A practical calibration method is to establish an empirical relationship between this ratio and the total precipitable water vapor measured in some other way. PW can be measured from the ground with sun photometers, instrumented balloons, and various ground-based optical and microwave instruments. A newer method makes use of the microwave signals from Global Positioning System (GPS) satellites. Various methods are used to measure water vapor from satellite-based instruments, but these require ground-based references for validation of the required calculations.
The protocol asks that you take measurements every day, weather permitting. In some parts of the world, it is possible to go many days without having weather suitable for taking these measurements. It is highly desirable to have a plan for taking measurements on weekends and during holiday breaks (especially during extended summer holidays.) Occasionally, it is worth taking many measurements during the day to study the diurnal cycle of water vapor.
The basic rule is that the sun must not be blocked by thick and/or variable clouds during a measurement. It is OK to have clouds near the sun. Thin and uniform cirrus clouds are not a problem as long as sunlight still makes a clear shadow and measurements for the two channels are taken in quick succession.
It can be difficult to decide conditions around the sun, because you should never look directly at the sun. You can look at the sky near the sun by blocking the sun with a book or notebook. An even better idea is to use the corner of a building to block the sun. It is a good idea to wear sunglasses when you make these decisions because they protect your eyes from UV radiation. Orange-tinted sunglasses make cloud/sky boundaries more clear and they will help you see faint clouds that might otherwise be invisible.
If you have concerns about a measurement, indicate them in your qualitative description of sky conditions when you report the measurement. Thin cirrus clouds are notoriously difficult to detect, but they can affect water vapor measurements. If you see cirrus clouds in the hours before or after a measurement, be sure to include that in your comments.
Water vapor is simply water (H2O) in gaseous (molecular) form. Water vapor is unevenly dispersed throughout the atmosphere around the earth. It is measured in terms of the thickness of layer of water that would result if all water vapor in an imaginary column of atmosphere directly above an observer were brought to Earth's surface and condensed into a liquid. Typically, this amounts to a few centimeters of water. All but about 0.1 cm or so of PW is in the troposphere, and most of the tropospheric PW is within a few kilometers of the surface.
This is a difficult question whose answer is the subject of ongoing research. There is no accepted reference standard against which GLOBE water vapor measurements can be compared. All measurements of total atmospheric water vapor content are subject to errors and uncertainties. Calibration of the GLOBE/GIFTS water vapor instrument depends on measurements made with other techniques. Therefore, its accuracy depends on the accuracy of those techniques. Other sun-photometer based measurements of water vapor do not claim accuracies better than about 10%. Although this may seem like a large error, it is sufficient to be useful for improved understanding of the distribution and transport of water vapor.
Practically by definition, it is not possible to infer total atmospheric precipitable water vapor (PW) directly and accurately from other measurements made on the ground. If that were possible, there would be no need for a water vapor instrument! However, atmospheric scientists have long understood that there is an approximate relationship between PW and the surface dewpoint temperature. More than 40 years ago, C. H. Reitan [Reitan, 1963] proposed an empirical relationship:
In the U.S., dewpoint temperature (in degrees Fahrenheit) is included in the hourly meteorological reports at the National Weather Service weather site: http://iwin.nws.noaa.gov/iwin/[two-letter state code]/hourly.html. Equations for calculating dewpoint temperature from air temperature and relative humidity are given in Section 4.
Because the relationship between PW and dewpoint temperature is only approximate, it cannot substitute for an actual calibration of a water vapor instrument.
Water vapor in the atmosphere is the result of the evaporation and transpiration phase of the hydrologic cycle. For on-line introductions to the hydrologic cycle, see
Yes. In many parts of the world, there are no ground-based sources of water vapor measurements. Satellite-based measurements of water vapor are extremely important, but they depend on ground-based measurements to ensure their accuracy and stability.
In temperate climates, the dominant feature of total precipitable water vapor (PW) is its strong seasonal cycle. This can be seen in Figure 12, a 10-year record of PW measurements made with an LED-based instrument similar to the GLOBE PW instrument by Forrest Mims, co-PI for the GLOBE/GIFTS Water Vapor Project, at his observatory in Seguin, Texas, USA [Mims, 2002]. It is clear from Figure 12 that PW values are higher in the summer than in the winter.
PW measurements made by students in temperate climates should exhibit this seasonal cycle. Measurements made in other kinds of climates, such as tropical regions that have wet and dry seasons, should have PW cycles that are related to these seasons. PW values at high-elevation observing sites will be smaller than those for sites nearer to sea level. (Unlike barometric pressure, for example, PW values are not usually normalized to sea level; they represent the actual amount of water vapor in the atmosphere above the observing site.)
|Figure 12. 10-year record of water vapor in Seguin, Texas, USA.|
It might seem reasonable to expect PW to be related to relative humidity. However, the correlation between the amount of water vapor in the entire atmosphere and relative humidity - a measurement made near Earth's surface - is not particularly strong. However, under some conditions, PW is related with another surface meteorological parameter: dewpoint temperature. This quantity, calculated from relative humidity and air temperature, is the temperature close to Earth's surface at which the relative humidity of the atmosphere would be 100%. (That is, when the relative humidity is less than 100%, the dewpoint temperature is less than the air temperature.) Dewpoint temperature is included along with air temperature and relative humidity in the hourly weather updates on the National Weather Service web site:http://iwin.nws.noaa.gov/iwin/[two-letter state code]/hourly.html. (Note that these NWS sites always give temperature in degrees Fahrenheit.)
Figure 13 shows water vapor as a function of dewpoint temperature. These data are from the same set of measurements presented in Figure 12. There are various methods for calculating dewpoint temperature from air temperature and relative humidity, none of which are particularly intuitive or simple. See Section 4 for one widely used method. Recall that an empirical relationship between PW and dewpoint temperature (Reitan's formula) was given in question 12 from Section 2.10. Note that Figure 13 shows PW versus dewpoint temperature, while Reitan's formula indicates that the logarithm of PW is a linear function of dewpoint temperature.
|Figure 13. Relationship of water vapor to dewpoint temperature.|
It would be an interesting student exercise to compare the data in Figure 13 to the predictions of Reitan's formula. Although this relationship is interesting, clearly it cannot be used as a primary calibration for a water vapor instrument. (If this were possible, there would be no need for a water vapor instrument!) At its best, the correlation is not very good and the relationship can break down when the weather is changing rapidly - when a cold front is passing, for example.
Your water vapor instrument is equipped with a standard 25-pin connector (a db25 plug). The purpose of the plug is to connect the instrument to a portable computer, through a device that converts the output voltages from the instrument to digital values that can be stored in your computer.
There are three sources of output voltages in your water vapor instrument: the two IR detector channels (IR1 and IR2) and the output from a temperature sensor. When you use this instrument in the manual mode, you can choose which of these three channels to display on the panel meter.
When you collect data through the hardware interface, all three voltage outputs are available simultaneously. This is true regardless of which value is being displayed on the panel meter. The advantage of this arrangement is that the calculations needed to interpret your data can be done with measurements that are simultaneous, rather than one after the other, as is necessary in the manual mode.
The db25 plug on the GLOBE/GIFTS water vapor instrument is intended for use with a particular product - a 232SDA12 analog-to-digital converter manufactured by B&B Electronics. (See www.bb-elec.com) This device must be connected to a Windows-based computer. It is possible to use other computer interfaces, although these will require modifications of the connections to the db25 plug. If you have questions, please contact the principal author of this report.
Dewpoint temperature is defined as the temperature to which a parcel of air would need to be cooled at constant pressure and constant moisture content in order for the vapor pressure and saturation vapor pressure of the parcel to be equal. In other words, it is the temperature at which a parcel of air would be saturated (i.e., 100% relative humidity). (See, for example, Huschke, 1959.)
Precise calculations of dewpoint temperature are complicated, but a good approximation can be obtained by using relative humidity and temperature to calculate vapor pressure and saturation vapor pressure. The U.S.-government sponsored Advanced Weather Interactive Processing System (AWIPS) includes equations for calculating dewpoint. (See http://126.96.36.199/AWIPS_home for more details.)
The AWIPS formulation for calculating saturation vapor pressure is:
Use a glass of water, pieces of ice, and a thermometer to determine the dewpoint temperature.
How do we know that the atmosphere contains water, in the form of water vapor? Reports of relative humidity provide us with information about water vapor in the air. The higher the relative humidity, the more moisture there is in the lower atmosphere. If you live in a temperate or tropical climate, you know that hot weather is more uncomfortable when the relative humidity is high. The reason is that the degree to which human skin can evaporate sweat (a process that has a cooling effect on the skin) depends inversely on the relative humidity - the higher the relative humidity, the less evaporation can take place.
We can get more direct evidence of moisture in the air whenever condensation occurs on a cool surface in warm weather. The moisture that forms on a cold drink bottle comes not from the contents of the bottle (hopefully!), but from moisture in the air.
Students may be reluctant to believe that water comes from the air, or perhaps they may be unfamiliar with (or not accept) the concept that water vapor in the air is just liquid water in another form. Therefore, it may be helpful to pose the question, "Where does this moisture come from?" and let students arrive at a conclusion by eliminating other possibilities.
The transition from water vapor to liquid water takes place at a temperature determined by the temperature of the air and the amount of water vapor in the air. These two parameters define the dewpoint temperature - the temperature at which water vapor condenses on a surface. See the discussion in Section 4 for details of how to calculate dewpoint temperature.
Various versions of this experiment have been described in books and online sites that deal with weather science. Two print sources are:
This experiment can be conducted inside or outside. If it is done outside, the air temperature should be well above freezing.
Be sure that water does not overflow or splash onto the outside surfaces of the cup. Initially, the outside of the cup should be completely dry.
Wait for the temperature to stabilize. If the outside of the cup appears "frosty" or damp, the water is already too cold. Try again with warmer water.
You can find this value using an online source such as http://iwin.nws.noaa.gov/iwin/[two-letter state code]/hourly.html. This comparison is valid only if the experiment is conducted outside. Note that the National Weather Service gives temperatures in degrees Fahrenheit rather than Centigrade (Celsius).
The temperature at which moisture first appears on the outside surface of the cup should be very close to the theoretically calculated dewpoint temperature.
The experiment assumes that the temperature on the outside surface of the cup is the same as the temperature of the water inside the cup. The more gradually you add ice and stir the water/ice mixture, the more accurate this assumption will be.
It comes from water vapor in the atmosphere. When the outer surface of the container reaches the dewpoint temperature, the air in contact with this cool surface becomes saturated with water vapor (that is, it reaches a relative humidity of 100%) and some of the water vapor then condenses into a liquid state.
When relative humidity decreases, the dewpoint temperature also decreases.
The dewpoint temperature is well above the temperature of a cold drink, so the outer surface of the drink bottle will quickly fall below the dewpoint temperature, causing water vapor to condense.
The results should be the same if the experiment is done carefully - especially if the ice is added slowly. Metal conducts heat better than plastic, so the temperature of the outside surface of the container should reach the temperature of the water/ice mixture more quickly. However, it may not be possible to read a small thermometer without removing it from the water in a metal container; this may introduce a temperature error.
The basic limitation on accuracy in this experiment is the accuracy of the thermometer itself, typically 1 or 2 degrees Fahrenheit (1 degree Centigrade) or more for inexpensive digital or analog thermometers. You can calibrate a thermometer by placing it in a beaker holding roughly equal amounts of water and ice. The thermometer should read 32ºF or 0ºC. If the thermometer says something different, the difference is the "offset" for the thermometer. You must then make the assumption that the same offset applies to other temperatures readable with that thermometer.
If the experiment is done carefully, then the accuracy of the dewpoint temperature should be no worse than the accuracy of the thermometer itself. If the temperature of the outside of the container is not the same as that of the water/ice mixture, then the experimental error can be several degrees. If you add ice too quickly, and do not stir the mixture thoroughly, then the water/ice mixture could temporarily be colder than the outside of the container. This could lead you to underestimate the dewpoint temperature.
There are various methods of calculating dewpoint temperature, as noted in Section 4. Even approximate methods should agree within a few tenths of a degree.
It is easier to read this kind of thermometer. A remote sensor would be especially helpful if you are using an aluminum beverage can to contain the water/ice mixture (because you can't see inside the container). You will still need something with which to stir the water/ice mixture. Note that even though a digital thermometer may display temperature to the nearest 0.1 degree, this does not imply that the thermometer is actually accurate to the nearest tenth of a degree, either Fahrenheit or Centigrade. In fact, typical classroom thermometers, whether analog or digital, will be accurate to no better than roughly 0.5ºC or 1ºF.
When the relative humidity is low, human skin can more efficiently evaporate sweat. It takes energy to convert liquid water (sweat) into its vapor form. The expenditure of this energy, which comes from your body heat, has a cooling effect on the skin.
When air is heated without changing its absolute moisture content, the relative humidity
decreases. The relative humidity of winter air is typically lower than summer air, so heating
this air in an enclosed volume can produce very low values of relative humidity.
9. Why do heated rooms in the winter sometimes feel cold even when the air temperature is set at what should be a comfortable level?
Because heated air can be very dry (see question 8), the evaporation of moisture from your
skin can make the air appear colder than it actually is. Heating systems often try to counteract
this effect by including a humidifier to add extra moisture to heated air.
10. Outside and inside dewpoint temperatures are usually different. Why?
When indoor air is heated in the winter or cooled in the winter, both the temperature and the relative humidity are different from outside air. See questions 8 and 9 about heating buildings in the winter. In the summer, "air conditioning" not only lowers air temperature, but it also removes moisture from the air.
It is sometimes a good idea to collect a time series of measurements with your water vapor instrument. This is useful for checking the performance of the instrument, and can also be used to examine the amount of "noise" in the water vapor signal itself. To do this, the output of the instrument must be connected to an analog-to-digital (A/D) converter that transmits digital values to a computer for storage and later retrieval. The GLOBE/GIFTS water vapor instrument includes an output connector for this purpose.
When you collect data in this way, you will need to specify the interval at which the output voltages are sampled by the A/D converter. Suppose this rate is one sample per second (a reasonable rate for the water vapor instrument). This means that, once per second, the A/D converter samples the voltage outputs and converts the values to equivalent digital values. This digital number is then transmitted to a computer, where it can be converted back again into the corresponding analog value.
Why is this conversion necessary? When information is sent to your computer, it is sent in the form of a series of on/off values called bits - that is, in a digital form. Any analog value, such as a voltage, must be represented in this way.
How does an A/D converter represent analog values? First of all, the hardware expects to see analog values in a specified range - from 0 to 5 V is typical. The converter assigns an analog value a particular digital value. The available range of digital values depends on how many bits are used to represent each analog value; this is called the resolution of the A/D conversion. A/D conversion is often done at 8-bit and 12-bit resolution, depending on the application.
First, here's a quick introduction to binary arithmetic. Consider a representation consisting of 8 on/off bits. Each bit represents a power of 2. The rightmost bit represents 20=1. The leftmost bit represents 27=128. What range of integer values can be represented by these 8 bits? If all the bits are 0's ("off"), then the corresponding integer value is 0. If all the bits are 1's ("on"), then the corresponding integer value is:
Here are some examples:
When the 8-bit A/D converter sees an analog voltage between 0 and 5 V it assigns it an integer value between 0 and 255, where 255 represents 5 V. Suppose the signal is 3.352 V. Then, (3.352/5)·255=170.952. Ideally, this value will be rounded to 171 and it will be represented in binary arithmetic as 10101011 (128+32+8+2+1). It is not always clear how A/D converters handle rounding. So, it is possible that this value could be represented as 10101010. That is, the conversion could be one bit off.
How accurately does an 8-bit conversion represent voltages? A 5 V signal can be represented as one of 255 possible integer values. Hence, one "unit" is 5/255=0.0196 V. So, the resolution of an 8-bit conversion is about 20 mV. That is, adding one to an integer value in this range corresponds to a 20 mV increase in the analog voltage.
This resolution is inadequate for some purposes, including the GLOBE/GIFTS water vapor instrument. The panel meter on the GLOBE/GIFTS water vapor instrument displays voltages to the nearest millivolt and a change of just a few millivolts is significant. The next common resolution level is 12 bits. This divides a 5.0 V signal into 4096 possible values, from 0 through 4095. This gives a voltage resolution of 5/4095=0.00122 V, or about 1.2 mV. This resolution is adequate, if just barely, for the GLOBE/GIFTS water vapor instrument. A/D converters with higher resolution are available, but they are much more expensive than 12-bit converters.
|Figure 14. Geosynchronous/geostationary Earth orbits.|
The spacecraft on which GIFTS will fly will be in a geosynchronous orbit. What does this mean?
When scientists need to make measurements continuously over a particular part of Earth, they place spacecraft in circular orbits whose rotation rate around Earth - the orbit period - is equal to exactly one day. These are called geosynchronous or geostationary orbits, shown schematically in Figure 14.
The planes of geosynchronous orbits can be tilted relative to Earth's equator. So, the point directly under a spacecraft in a geosynchronous orbit moves north and south of the equator during a day. Usually, the orbit inclination is small, so instruments in geosynchronous orbits have a continuous view of nearly the same portion of Earth's surface.
Geostationary orbits are a special case of geosynchronous orbits. They lie in the plane of Earth's equator so that the inclination (tilt) of their orbit plane relative to Earth's equator is 0°. Thus, they are always directly above a fixed point on Earth's equator and they can continuously observe the same portion of Earth's surface.
Some geosynchronous/geostationary spacecraft have onboard propulsion systems that can be used to move them to different locations over the equator. (A minimum propulsion system is needed to maintain keep the spacecraft's orbit at a period of exactly 24 hours. Note that satellites in geosynchronous orbits are critical to modern communications. Their fixed (or nearly fixed) position above Earth's surface is critical for sending, receiving, and relaying messages and data around the world..
The period of an Earth-orbiting spacecraft T is given by:
|T = 2πr(r/G)1/2||(7.1)|
|r = R + a||(7.2)|
Many Earth-observing satellites are in orbits a few hundred kilometers above Earth's surface. Suppose a = 300 km:
|r = 6378 + 300 = 6678 km||(7.3)|
|T = (2π)(6678)(6678/398601.2)1/2 = 5431 s = 96.5 min||(7.4)|
|r = [(T)(G1/2)/( 2π)]2/3||(7.5)|
|rgeosynchronous = [(86400)(398601.21/2)/(2π)]2/3 = 42241 km||(7.6)|
|a = 42241 - 6378 = 35863 km||(7.7)|
The phrases "greenhouse gas" and "greenhouse effect" are now closely associated with the idea of global climate change. The phrase "greenhouse effect" comes from the analogy with a glass-enclosed greenhouse. When sun shines on a greenhouse, the inside of the greenhouse rapidly warms to a temperature much higher than that of the outside air. This is because the greenhouse glass allows most solar radiation to enter the greenhouse. This radiation warms the interior of the greenhouse. But, the thermal radiation that is then emitted by objects in the greenhouse (including the air in the greenhouse) is blocked by the glass.
On Earth, it is easy to show that, in a similar fashion, the atmosphere traps radiation from the sun so that the surface of Earth is warmer than it would be without an atmosphere. This is due to greenhouse gases in the atmosphere, of which water vapor is the most important. How does this phenomenon work?
The basic physical principle is the requirement that all objects, including the Earth/atmosphere system, must be in "radiative balance" with their surrounding environment. This means that the Earth/atmosphere system must radiate into space exactly the same amount of energy that it receives from the sun. An object that is in radiative balance has a constant average temperature.
The Sun emits a lot of energy - a total of about 3.9x1026 W! Earth is about 150,000,000 km away from the Sun, so only a very little of this energy reaches our planet. The amount of solar energy falling on a surface at the average earth-sun distance, S, is about 1370 W/m2. This value is often called the solar constant even though it varies a little due to fluctuations in the sun's energy output. The solar constant also varies during the course of a year, as Earth is in a slightly elliptical orbit, but we can ignore that effect in this discussion. Earth intercepts an amount of energy equal to S times its cross-sectional area, πr2, where r is Earth's radius. So,
|incoming energy at Earth = πr2S||(8.1)|
However, the Earth/atmosphere system doesn't absorb all the solar energy. The absorption is controlled by the system's reflectivity, or albedo A, a dimensionless value that can range from 0 to 1. If Earth and its atmosphere were black, its albedo would be equal to 0 and it would absorb all the energy. If it were white, its albedo would be 1 and it would reflect all the energy. (Although the terms "black" and "white" imply "colors" in the visible sense, the concept of albedo is the same for the solar spectrum, which extends far beyond the visible part of the spectrum.) The average albedo of the Earth/atmosphere atmosphere system as viewed from space is about 0.30. The amount of energy absorbed by the Earth/atmosphere system is
|energy absorbed by Earth = πr2S(1 - A)||(8.2)|
All objects radiate energy based on their temperature. The amount of this outgoing energy is described by the well known Stefan-Boltzmann Law, which states that the amount of energy radiated by a perfect radiator (a "blackbody" radiator) is proportional to the fourth power of the temperature T of the radiator, σT4, where T is in Kelvins and σ is the proportionality constant. If T is the average temperature of the Earth/atmosphere system, then the radiation (the thermal emission) from Earth's entire surface is:
|outgoing energy at earth = 4πr2σT4||(8.3)|
So, in order for the Earth/atmosphere system to be in radiative balance,
|S(1 - A) = 4σT4||(8.4)|
|T = [S(1 - A)/(4σ)](1/4) = [1370·(1-0.3)/(4·5.67x10-8)] (1/4) = 255K||(8.5)|
Converting from Kelvins to degrees Celsius (centigrade), 273K = 0ºC. (That is, absolute zero is about -273ºC.) From this we can see that the temperature of the Earth/atmosphere system as viewed from space is -18ºC. However, the actual average temperature of Earth's surface is about 16ºC (289K), about 34ºC warmer than the Earth/atmosphere system as viewed from space.
This is a big difference and, if our planet had evolved at a temperature of -18ºC, it would be a much different place! The difference between the temperature calculated in equation (8.5) and the actual average temperature at Earth's surface is explained by the greenhouse effect. The details of the greenhouse effect are complicated and somewhat uncertain, but their average effect can be incorporated into our model in a conceptually simple way.
In the previous model, we considered the temperature at the top of the atmosphere, as Earth is viewed from space. Now imagine a thin layer of the atmosphere close to Earth's surface, so the temperature of the layer is the same as that of the surface. The incoming solar energy is still πr2S(1 - A), but this layer also absorbs a fraction x of the energy that is being emitted by Earth at temperature T. (We'll call x the single-layer atmosphere absorption coefficient.) To maintain radiative balance, this energy is re-emitted by the layer in all directions. Half this energy is directed toward Earth's surface, and the other half is directed back out toward space. For simplicity, assume that Earth's surface completely absorbs all this re-radiated energy. This assumption isn't too bad because the relatively high albedo of the Earth/atmosphere system as viewed from space (the value of 0.3 used above) is due mainly to clouds. Almost all of Earth's surface (especially oceans) has a much lower albedo than this.
Now, equations (8.2) and (8.3) can be rewritten as:
|incoming energy at Earth's surface = πr2[S(1 - A) + 4(x/2)σT4]||(8.6)|
|outgoing energy from Earth's surface = πr24(1 - x/2)σT4||(8.7)|
|T4 = S(1 - A)/[4σ(1 - x)]||(8.8)|
Note that the T in (8.8) is now assumed to be Earth's surface temperature, not the top-of-the-atmosphere temperature - the temperature of the Earth/atmosphere system as seen from outer space. Some of the radiation from this imaginary layer of atmosphere is re-radiated to other, higher, layers, which also absorb and re-radiate energy. These layers get colder, in general, at higher altitudes. At the highest layer - the top of the atmosphere - there is no absorption and T is once again equal to the value obtained from (8.5).
Figure 15 shows T as a function of x for equation (8.8). The radiative behavior of the atmosphere is extremely complex, so it is important not to attribute too much importance to the numerical values in Figure 15. However, the figure does illustrate the concept of the greenhouse effect.
Note that (8.8) is undefined when x equals 1. As x approaches 1, the surface temperature increases dramatically, producing a "runaway greenhouse effect" such as appears to exist on Venus.
|Figure 15. Earth's surface temperature as a function of single-layer atmospheric absorption coefficient x. (x = 0.394 corresponds to the current average surface temperature of 16°C.)|
The GLOBE/GIFTS water vapor instrument contains detectors that respond to sunlight at two small wavelength ranges in the near-IR part of the solar spectrum. The basic equation describing the transmission of sunlight through a medium such as the atmosphere (or, to put it another way, the extinction of sunlight through processes of scattering and absorption by molecules and particles in the atmosphere) is the Beer-Lambert-Bouguer Law:
|Iλ = Io,λ e-αm||(9.1)|
There are two important features of this equation:
Conceptually, it may be easier to replace the somewhat abstract term "optical thickness" in equation (1) with "percent transmission." This is the percent of sunlight that reaches Earth's surface at a particular wavelength. The relationship between optical thickness and percent transmission is
|% transmission = 100 ·e-α||(9.2)|
For a near-IR detector whose response lies outside a water vapor absorption band, the optical thickness is determined by aerosols and molecular scattering (Rayleigh scattering):
|Iλ,1 = Io,λ,1 exp[(-αa,1 - αR,1) m]||(9.3)|
|Iλ,2 = Io,λ,1 exp[αa,2 - αR,2 - αwv)m]||(9.4)|
Calibration of this kind of water vapor instrument involves determining specifically how the intensity ratio varies with water vapor. In principle, this relationship can be determined theoretically, based on an accurate knowledge of the transmission properties of the atmosphere and the spectral response characteristics of the detectors. As a practical matter, it is a better idea to try to calibrate this kind of instrument by comparing its results to other measurements of atmospheric water vapor. In any case, calibration of water vapor instruments is difficult because there is no direct measurement that can serve as a reference standard.
The correlation of material in this document with science education standards is based upon the essential elements of inquiry learning described in both National Science Education Standards and the AAAS Benchmarks. Using these documents as a foundation, the Council of State Science Supervisors (CSSS) through the CS3/NASA NLIST Initiative has operationally defined Science as Inquiry as consisting of:
Learning set in a broad context (concepts) can enable deeper understanding and enhance the transfer of knowledge to new and different situations.
Content provides building blocks for constructing and comprehending important concepts.
Skill development becomes the means for continuing the generation of new knowledge.
Habits of mind employed by experts and nurtured in learners can insure the integrity of the discipline and provide a valid world-view from the perspective of science.
When these essential elements are brought together in school learning environments, science becomes both relevant for and applicable to all learners.
The correlations in some cases identify specific sections of this report that are directly relevant to a particular conceptual context or content standard. Concepts or content standards to which the topic of this report does not apply are not included. A matrix provides the user with a generalized look across the material as it relates to the essential elements of Science as Inquiry identified from both NSES Standards and AAAS Benchmarks.
The unifying concepts and processes in this standard are a subset of the many unifying ideas in science and technology. Some of the criteria used in the selection and organization of this standard are:
Earth science ties together traditional science disciplines such as physics, chemistry, and biology. The hydrologic cycle is a major component of Earth science, and the water vapor measurement aids in the understanding of the hydrologic cycle.
The water vapor measurement, conducted in conjunction with other atmospheric observations, provide a comprehensive view of Earth's atmosphere.
The report includes material that will help educators understand the water vapor measurement and its relationship to Earth science.
Gathering, reporting, and analyzing data gives students a wide range of appropriate activities.
These four concepts are addressed specifically in:
Related activities are presented in:
Water vapor has a seasonal component that is especially evident in temperate climates.
Understanding the distribution of water vapor helps to understand the hydrologic cycle.
Global satellite images of water vapor help to understand ocean currents.
Measurements of water vapor and associated observations help to understand the hydrologic cycle and its relationship to the atmosphere and meteorology. Section 8. What Is the Greenhouse Effect? deals specifically with the concepts of radiative equilibrium and its role in maintaining an appropriate environment for life on Earth.
The difference between weather and climate helps to explain this concept. For example, the relatively stable global temperature system is maintained by the interaction of weather systems, which produce temperature extremes on regional and temporal scales.
A study of weather patterns, water vapor, and the hydrologic cycle provides insight to these three concepts.
The "runaway greenhouse effect" mentioned in Section 8. What is the Greenhouse Effect? is an example of possible effects of large disturbances.
Sections 2.10. Questions and Answers for the Water Vapor Protocol and 2.11. Looking at the Data suggest ways to depict and analyze changes in water vapor.
Seasonal cycles in water vapor, and their relationship to the apparent motion of the sun around Earth suggest one kind of cycle.
The calculations discussed in Section 8. What is the Greenhouse Effect? illustrate the usefulness of "powers of ten" (scientific) notation.
The discussion of a very simple climate model in Section 8. What is the Greenhouse Effect? suggests that Earth's climate is extremely complex, with many possible interactions among parts of the system.
"Investigating Water Vapor" in Section 2.6. Water Vapor Protocol Introduction indicates that the measurement is an attempt to determine water vapor in the entire column of atmosphere above an observer. Section 2.10. Questions and Answers for the Water Vapor Protocol includes a discussion of the fact that a relationship between surface conditions and total atmospheric water vapor can be only approximate (because the amount of water vapor varies with altitude).
Global satellite images of water vapor show their association with weather systems.
Global satellite images of water vapor show their association with weather systems. Section 2.11. Looking at the Data discusses the seasonal cycle in water vapor.
Water vapor cycles are discussed in Section 2.11. Looking at the Data.
The concept of radiative equilibrium, as discussed in Section 8. What Is the Greenhouse Effect?, as well as the concepts of global weather patterns as shown in global satellite images of water vapor and clouds, help to illustrate this concept.
Evaporation, condensation, and transpiration are processes involving the exchange of heat energy by radiation or conduction.
Evaporation, condensation, and transpiration in the hydrologic cycle illustrate how water molecules can exist in different configurations.
These five content statements, describing Earth as a dynamic and interconnected system, can be addressed with water vapor measurements in conjunction with other experiment protocols supported by the GLOBE Program.
Because the water vapor instrument uses near-IR detectors, this provides one way to address these three content statements. Section 2.7 Suggestions for Student and Classroom Preparation contains some specific suggestions for dealing with the concept of "invisible" light.
The water vapor protocol, especially in conjunction with other GLOBE-supported protocols, provide numerous opportunities to develop these skills. See the more detailed references for grades 9-12, below.
Section 2.6 Water Vapor Protocol Introduction helps students identify appropriate questions and concepts.
This report provides a detailed guide to designing and conducting a scientific investigation into the distribution of water vapor.
As described in Sections 2.9 Classroom Guide to Field Work and Section 3. Guide to Data Logger Hardware and Software, and Section 6. How Do Analog-To-Digital Converters Work?, computer technology allows scientists to capture data directly in computers.
Especially when undertaken as part of a broader investigation of the atmosphere, the water vapor measurement will help students develop these skills.
Section 2.6 Water Vapor Protocol Introduction and Section 2.10 Questions and Answers for the Water Vapor Protocol discuss how inexpensive light detector technology has been applied to this measurement.
The equations in Section 2.10 Questions and Answers for the Water Vapor Protocol and Section 8. What is the Greenhouse Effect? show how mathematics are used in this inquiry.
By designing and conducting their own experiments, students will better understand the nature of scientific explanation.
Looking for and analyzing water vapor trends will help to develop these skills. The relationship between dewpoint temperature and water vapor discussed in Section 2.10 Questions and Answers for the Water Vapor Protocol could be used as an example of how temperature and humidity data provide an incomplete picture of the factors influencing atmospheric water vapor.
All of these skills can be developed as a result of collecting and analyzing water vapor data, especially in conjunction with measurements of other atmospheric properties.
|Section Number||Conceptual Theme||Content ES/PS||Skills IN/NIS||Habits of Mind||Mathematics||Technology|
An X in a box for a section number indicates one or more items relating to that particular category
The original GLOBE Water Vapor protocol was prepared with the support of the National Science Foundation, through award 0222380. Any opinions, findings, and conclusions or recommendations expressed in this report are those of the authors and do not necessarily reflect the views of either the GLOBE Program or the National Science Foundation. Some of the material in Section 10 has been prepared by Joseph D. Exline, President Exline Consulting Services, Incorporated. Some figures have been prepared for the GLOBE Program by Schafer-LaCasse Design. The cover design is by Anne Costa, NASA Langley Research Center.
1. Hold the instrument in front of you in a position where you can read the digital panel meter and can comfortably keep the sun spot shining through the front alignment bracket and aligned on the rear alignment dot.
2. Turn on your instrument and activate the data collection software.
3. Cover the sunlight apertures with your finger for a few seconds.
4. Center the sunlight spot coming through the hole in the front bracket on the circular dot on the rear bracket and remove your finger from the sunlight apertures. Hold your instrument in this position for two minutes.
5. At the end of two minutes, cover the sunlight apertures with your finger and hold this position for a few seconds.
6. Stop the logging software and turn off your water vapor instrument.
7. If you are using actual outdoor measurements for air temperature and relative humidity, make these measurements now.
Here are links to data entry sheets for the Water Vapoor Protocol.