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Table of Contents:
To measure UV-A radiation reaching Earth's surface and to measure the optical thickness of the atmosphere at UV-A wavelengths.
Students use a small battery powered instrument to measure full-sky radiation and aerosol optical thickness in the UV-A part of the solar spectrum. They also observe and report cloud and sky conditions and record temperature, relative humidity, and barometric pressure.
Students understand the concept that the atmosphere prevents all of the sun's ultraviolet radiation from reaching Earth's surface and how satellite- and ground-based measurements work together to provide information about Earth's atmosphere.
Sunlight reaching Earth's surface spans a wide range of wavelengths, including ultraviolet radiation (UV, 100-400 nm), light visible to humans (400-700 nm), and infrared radiation (IR, >700 nm). Although humans naturally are most aware of visible light, the entire range of solar radiation from UV to IR is extremely important to the operation of Earth as a dynamic and interconnected system. To cite just two examples, UV radiation influences the breeding patterns of potentially harmful insects such as mosquitoes, and some animals, including insects, can see light that is beyond the response of the human eye.) UV radiation is of special concern to scientists and the general public because UV overexposure is a serious public health problem, especially for light-skinned people. However, the interactions of UV radiation with life on Earth are complex and, in many cases, poorly understood. UV radiation is inherently neither "good" nor "bad." Life on Earth has evolved within a particular radiation environment that includes some UV radiation. Disruptions to this radiation environment, causing some forms of radiation to increase or decrease, can have serious and sometimes unforeseen consequences.
| UV category | Wavelength range, nm |
|---|---|
| UV-A | 315-400 |
| UV-B | 280-315 |
| UV-C | 100-280 |
UV-B is partially absorbed by ozone in the atmosphere. It is considered a destructive form of UV radiation at Earth's surface because it can damage living tissue and manmade materials. Increasing exposure to UV-B radiation from sunlight is widely accepted as the cause of increasing rates of the most serious forms of skin cancer (melanomas). Even though overexposure to the sun is a serious human health problem, "getting a tan" is still considered important by many light-skinned individuals. Because UV-B is only partially absorbed by ozone, even a small decrease in the amount of stratospheric ozone can significantly increase the risk of skin cancer for light-skinned humans. Overexposure to UV-B is also associated with eye diseases such as cataracts, which can affect humans regardless of their skin color.
Low energy UV-A ("black light") sources can cause some materials to emit visible light (fluoresce), which makes them appear to glow in the dark. UV-A radiation does not penetrate as far into human skin as UV-B. Tanning lamps are designed to produce UV-A rather than UV-B radiation because exposure to UV-A will darken light skin without burning. However, UV-A exposure can cause premature skin aging and eye problems. Therefore, health professionals warn against excessive UV exposure, including UV-A, no matter what the source.
The amount of UV-A and UV-B radiation reaching Earth's surface depends on several factors. These include:
1. The stratospheric ozone layer
The stratospheric ozone layer is Earth's basic defense against harmful levels of UV radiation. The amount of ozone varies seasonally, but there is also an overall downward trend in the amount of stratospheric ozone, leading to increased levels of UV-A and UV-B radiation at Earth's surface.
Ozone occupies a thin layer in the stratosphere at an altitude of about 20 km. In the 1980's, satellite measurements showed that stratospheric ozone was being reduced around Earth's South Pole. The reduction was so large and unexpected that the satellite measurements were initially thought to be in error. Later it was found that similar, but smaller, reductions also occurred around the North Pole. Although these seasonal events are popularly known as "ozone holes," it is more accurate to refer to them as seasonal "depressions" in the amount of stratospheric ozone. They are caused by chemical reactions between chlorine and ozone, with the energy from sunlight acting as a catalyst. The primary source of chlorine in the stratosphere is chlorofluorocarbons (CFCs) such as freon, a manmade coolant. When scientists reached an understanding of how CFCs are transported into the stratosphere and broken down by sunlight to produce byproducts such as chlorine, the manufacture and use of freon was banned by most countries (including the U.S.) for most purposes.
It is important not to confuse the stratospheric ozone layer ("good ozone") with ground-level ozone ("bad ozone"). The former is formed by natural and (more recently) human-induced photochemical reactions in the upper atmosphere. Ground level ozone is an air pollutant and irritant formed mostly as a result of photochemical reactions with chemicals produced by fossil fuel combustion.
2. Latitude
Direct sunlight is, on average, more intense at lower latitudes because the
sun reaches higher in the sky. At latitudes less than about 23.4°, south or north, the sun can appear directly
overhead at noon (a solar elevation angle of 90°), but this is never true at more northerly or southerly latitudes. The higher the sun in the sky, the less atmosphere there is for sunlight to penetrate. Also, because the stratospheric ozone layer is thinner near the equator compared to higher latitudes, less UV radiation is absorbed by ozone at lower northern and southern latitudes.
3. Time of day and time of year
The solar elevation angle reaches its maximum value at solar noon -- the time at
which the sun crosses the observer's meridian. Thus, solar radiation is most intense at solar noon. Note that solar noon
is not, in general, the same as "clock" noon. There are two
reasons for this. First, consider that time zones span a longitude of 15 degrees (with some irregularities in populated
land areas). Everyone's clock is set to the same time in the same time zone, so it's not possible for solar noon to
occur at the same instant throughout the time zone. Second, clock time is based on the "mean solar time," a fictitious
time based on the average apparent motion of the sun around Earth. The actual apparent motion of the sun around Earth
varies during the course of a year because Earth's orbit around the sun is slightly elliptical rather than circular.
The apparent motion of the sun is faster when Earth is closer than the average Earth/sun distance.
The noontime solar elevation angle has a seasonal cycle, with the maximum
occurring in the summer. Thus, solar radiation is most intense during the summer months.
There is a seasonal cycle in stratospheric ozone. It increases around the poles
during the winter and early spring and is depleted in the fall. (Thus, the northern and southern hemisphere ozone "holes"
occur six months apart.) UV levels when ozone levels are depleted are higher than they normally would be.
4. Elevation/altitude
The higher the elevation or altitude, the less atmosphere there is to absorb UV radiation. Therefore, exposure to UV radiation increases with elevation or altitude. When humans work or play at high elevations, they need to take extra precautions against overexposure. For human travel in spacecraft beyond Earth's atmosphere, it is essential to block the UV-C radiation that would otherwise be absorbed by the atmosphere. (Fortunately, this is not difficult to do.)
5. Weather and surface conditions
Although it seems obvious that cloudy weather should reduce the amount UV radiation reaching Earth's surface, it is still possible for light-skinned individuals to get sunburned even on cloudy days. Under certain partly cloudy conditions, sunlight reflecting from the sides of clouds can actually increase the total amount of UV radiation compared to a cloud-free day.
Surfaces such as snow, water, sand, and even grass, can reflect a significant fraction of the UV radiation that reaches them. When reflected UV radiation is added to direct radiation, total exposure to radiation can increase. If you work or play around water or other UV-reflective surfaces, you need to be especially careful to protect your eyes from both direct and reflected UV radiation. You cannot conclude that you are adequately protected just because you are standing in the shade.
Because of the human health hazards posed by increased levels of UV radiation, and the fact that humans have disrupted the stratospheric ozone layer, thereby causing UV radiation at Earth's surface to increase, it is tempting to say that this situation is "bad" for the environment. However there are benefits from UV radiation, too. For example, some exposure to sunlight is necessary for humans to synthesize vitamin D, which is essential to human health. Increased UV radiation may kill harmful bacteria in the air and disrupt the breeding patterns of mosquitoes. Indeed, UV radiation is essential for maintaining the delicately balanced ecological forces on our planet.
UV-A measurements are best understood in the context of the other GLOBE atmospheric measurements. There may be observable relationships between UV-A and aerosols, tempearture, cloud cover, relative humidity, and precipitation. Certainly, it is helpful to approach this topic as part of a "big picture" of the atmosphere and its properties.
The UV-A measurement uses a GLOBE/GSFC UV-A instrument developed under the sponsorship of the National Aeronautical and Space Administration's Goddard Space Flight Center. The UV-A Protocol consists of two parts: a sun photometer measurement that is used to calculate aerosol optical thickness at UV-A wavelengths, and a full-sky measurement that determines the total amount of UV-A radiation reaching Earth's surface. Unlike aerosol measurements, as described in the Aerosols Protocol, UV-A measurements can be made in the full-sky mode under any sky conditions as long as it isn't precipitating. Ideally, measurements should coincide with overflights of satellite-based instruments that measure UV radiation.
If you are also doing the Aerosols Protocol, the UV-A sun photometer measurements may be made at the same time. The metadata requirements are the same, so it takes little extra time to do both protocols together. Any sun photometer measurement requires that you must have an unobstructed view of the sun. Full-sky measurements can be made under any sky conditions as long as it isn't actually precipitating. (The GLOBE/GSFC UV-A instrument is not waterproof!)
Not all school sites are suitable for the full-sky UV-A measurement. In its full-sky operational mode, the UV-A instrument must have a relatively unobstructed 360° view of the horizon. A flat rooftop is the ideal location, but ground locations are OK if they are far enough away from buildings and trees. Especially if there are some obstructions on the horizon, it is essential to complete an Atmosphere Site Description to characterize your site. See Choosing a Site for UV-A Measurements. If you are in doubt about the suitability of your site, consult with the Science Team. Photographs of your site (north-east-south-west) will be helpful.If you have a suitable site, you must also provide a permanent level platform on which to place the UV-A instrument when it is measuring full-sky radiation. This is also discussed in Choosing a Site for UV-A Measurements.
When the GLOBE/NASA UV-A instrument is placed on a level surface, the detector measures UV-A radiation from the entire sky. Each instrument also includes a collimating tube that fits over the nylon housing containing the detector. When the tube is in place and the instrument is pointed directly at the sun, the detector measures direct UV-A radiation that can be used to determine aerosol optical thickness (AOT) at about 370 nm. When it is used in its sun photometer mode, the measurements are identical in principle to measurements made with the GLOBE sun photometer as described in the Aerosols Protocol.
In the full-sky mode (without the collimator), the UV-A instrument measures the total amount of UV-A radiation at your site. Together, the full-sky and sun photometer measurements can be used to separate UV-A radiation into its direct and diffuse components. The full-sky measurements are more valuable when they are combined with UV-A AOT values as well as AOT values in the visible part of the solar spectrum, derived from the two-channel GLOBE sun photometer.
The GLOBE/GSFC UV-A instrument is fitted with a db25 connector. This can be used with an appropriate hardware and software interface to record data on a portable computer. There are several combinations of manual and automated data procedures described in this protocol.
1. Manual sun photometer measurement followed by manual full-sky measurement
Manual full-sky measurements should be taken directly after manual sun photometer measurements, when there is an unobstructed view of the sun. The two measurements should be taken as quickly as possible, one after the other.
2. Manual sun photometer measurement followed by automated collection of full-sky data.
Automated full-sky radiation measurements are valuable because they provide information about the short-term temporal variability in UV-A radiation. This information is especially important under partly cloudy sky conditions.
3. Automated sun photometer measurement followed by automated collection of full-sky data.
This is the preferred data collection mode whenever you have an unobstructed view of the sky, because it gathers all data together in a single data file.
4. Automated collection of full-sky data without sun photometer measurements, when you do not have an unobstructed view of the sun.
Full-sky UV-A measurements can (and should) be made even when you do not have an unobstructed view of the sun. This is an important measurement because it is difficult to model the behavior of UV-A radiation at Earth's surface based just on measurements from space when they sky is partly cloudy. Ground-based measurements will help scientists develop better models and to verify the performance of existing models.
Although it is also possible just to take a sun photometer measurement with this instrument, this is not a suggested procedure because there is no reason not to include a full-sky measurement even if the data are collected manually.
If you are collecting these data at a time that coincides with a satellite overflight, the first step in any of the procedures described above is to determine the time of maximum elevation angle for the overflight. Predictions of these times several days into the future are available online at earthobservatory.nasa.gov. Full-sky measurements should begin 10 minutes before the time of maximum elevation angle and continue for 10 minutes after. Typically, satellite overflight times are at roughly the same time of day -- late morning or early afternoon, for example -- but the exact times can vary by as much as an hour and half (one orbit period). Sun photometer measurements should be made before the full-sky measurements begin, so plan your schedule accordingly.
|
| The GLOBE/GSFC UV-A radiometer, with collimating tube. |
 |
| Spectral response of unfiltered UV-A detector. |
An important characteristic of full-sky detectors is their cosine response. In principle, the response of a detector to direct sunlight should be proportional to the cosine of the zenith angle of the sun. The incident energy is maximum when rays of direct sunlight are perpendicular to the detector. If the detector is mounted on a horizontal surface, the response should be maximum when the sun is directly overhead at an elevation angle of 90° and should fall to zero as the sun approaches the horizon. All detectors depart somewhat from a perfect cosine response. In the GLOBE/GSFC UV-A instrument, the detector and filter assembly is covered by a small circular disk of Teflon®. This improves the cosine response by diffusing the incoming light before it strikes the detector.
All GLOBE measurements require the definition of a study site. As a minimum, this definition must include longitude and latitude. As described below, the UV-A instrument can be used both as a sun photometer, measuring light direct from the sun, and as a full-sky instrument that records radiation from the entire sky. Because of the full-sky measurements, it is especially important to have an appropriate observing site.
The sun photometer measurements can be made anywhere you have an unobstructed view of the sun. However, the full sky measurements require a full view of the sky, away from buildings, trees, and other objects that block your view of the horizon. It is very rare to have a site with no obstructions on the horizon in any direction. For a site on the ground, your school building is one potential obstruction. If your school building has a flat roof, this might be a possible site. Whether this is feasible depends on the configuration of your school buildings and on resolving whatever safety concerns might exist. Note that the roof of a building is not a good site for a GLOBE thermometer enclosure.
In any event, you must prepare a horizon obstruction description for your site, using the site description form on the GLOBE web site. This will help the Science Team interpret your full sky measurements. As a general guideline, the Science Team can make corrections for sky obstructions if they do not intrude more than 20E above the horizon and their azimuth (angular position clockwise relative to north) is known. This description needs to be done only once unless there are significant changes, such as a new building.
If your school is surrounded by trees, tall buildings, or other significant obstructions, you cannot make the full sky measurement. In that case, you can still make sun photometer measurements. It may be possible to establish a site somewhere away from your school where horizon obstructions are less of a problem. This is worth doing even if you cannot make everyday measurements at this site.
Once you have selected a site, you need to decide exactly where to place the UV-A instrument. Ideally, it should be at or just below student eye level. The instrument must be level and oriented the same way (relative to north) every time you make measurements. It is important to be consistent about instrument placement, especially with respect to how the instrument is leveled. So, a permanent flat site for the instrument is important. Make a mounting platform about 20 cm square. The exact dimensions are not important as long as it is stable. Use metal, exterior plywood, or solid wood. Plywood and solid wood need to be sealed and/or painted to prevent warping. Attach this board permanently to a very stable stand or mark a place on a horizontal ledge or wide railing. Level the board as carefully as you can, using the bubble level on the top of your UV-A instrument. (If necessary, stand on a sturdy stool or stepladder to do this.) It must be convenient to operate the instrument switches and read the digital display when the instrument is facing north according to the arrow on the top of the case. Once the instrument is leveled and oriented appropriately, trace around the base onto the mounting board, using a permanent marker. Use this tracing as a guide to instrument placement for all measurements in the future.
If you need to raise one or two corners of your UV-A instrument to level it, you can permanently attach some feet to the bottom of the case. Use sheet metal, thin wood or plastic, or even cardboard if you are careful not to get it wet. Attach the feet with epoxy or "super glue" (cyanoacrylate glue).
An important difference between the GLOBE/NASA UV-A instrument and the GLOBE sun photometer, which also uses LED detectors, is that the UV-A detector assembly is exposed on the top of the instrument. The LED is mounted in a nylon housing, under a filter and Teflon diffuser. (The diffuser is the white circle on the top of the detector housing.) The detector itself is protected, but it is still important to protect the surface of the Teflon diffuser. Do not touch it with your fingers. If it becomes dusty or dirty, use a soft clean brush or lens cleaning paper to clean the surface. Do not use any kind of cleaning fluid or detergent on the housing. A cleaning paper can be lightly moistened with clean water (preferably distilled water), but never put water or any other fluid on the detector assembly.
The UV-A instrument is not waterproof, and should not be used when it is precipitating. It should never be left outside when you are not actually collecting data. Store this instrument in a clean plastic bag with the collimating tube in place when it is not in use.
The full-sky UV-A measurement protocol requires that the instrument be turned on for many minutes at a time. Under normal operation, the battery in your UV-A instrument will still last for many weeks. However, depending on the type of battery, outside temperature, and measurement frequency, battery life can vary greatly in this instrument. Battery voltage should be checked, with the instrument turned on, at least once a month. Ideally, the battery should be replaced if the voltage falls below 7.5 V. The panel meter has a "low battery" indicator that will appear as a small "B" on the meter display. If this indicator appears, the battery must be replaced immediately.
If the battery requires frequent replacement, it may be desirable to use an alkaline battery. It is also possible to use a rechargeable 9-V battery in this instrument, but its voltage needs to be checked every week or so.
If possible, access earthobservatory.nasa.gov for satellite overflight times. If you are collecting data at a time that coincides with a satellite overflight, begin classroom preparations far enough in advance so that you can start recording full-sky data at least 10 minutes before the maximum satellite elevation angle of the overflight. Don't forget to allow enough time to take sun photometer measurements before starting full-sky measurements.
Be sure that the longitude and latitude coordinates you have reported for your study site is accurate. Devise a plan for reporting time accurately. This preparation is especially important for sun photometer measurements because GLOBE needs to know the elevation angle of the sun in order to calculate aerosol optical thickness. The astronomical equations used to do this calculation require accurate position and time values as inputs. Of course, time needs to be reported as Universal Time (UT), as for other GLOBE protocols.
Note that computer clocks are often not very accurate and should not be used as a time reference. You can set clocks and watches using the time reported at www.time.gov
Read through the Metadata Reporting Protocol, below. Some of the data can be collected using GLOBE protocols. Other values can be obtained from online or broadcast sources. Devise a plan for collecting these data. It might be worthwhile to collect just metadata for a few days to make sure your plan is working.
Aerosol optical thickness measurements can be taken either by manually recording voltages or by using automated data collection in conjunction with the full-sky measurements described below. You should always collect both sun photometer and full-sky data unless your site is not suitable for full-sky measurements.
1. Attach the collimating tube to the detector assembly.
Use a gentle twisting motion. Make sure the bottom of the tube rests on the top of the instrument case.
2. Turn the instrument on.
3. Select the "T" switch setting and record the temperature.
The temperature is 100 times the voltage reading displayed on the panel meter. For example. 0.225 V is 22.5°C.
4. Select the "V" switch setting.
5. With the panel meter facing you, point the instrument toward the sun so the circular shadow from the top of the collimating tube is symmetrical around the base of the tube. Record the maximum voltage appearing on the panel meter during about 15 seconds. Record the time to the nearest 15 seconds.
6. Repeat step 5 at least 2 and no more than 4 times, recording the maximum voltage and time for each repetition.
7. Select the "T" switch setting and record the temperature.
8. Proceed to one of the full-sky measurement protocols.
If you need time to set up an automated full-sky measurement, you can turn the instrument off until you are ready to proceed. Skip this step only if your site is not suitable for full-sky measurements.
9. Complete the metadata reporting as described in Metadata Reporting Protocol.
Skip this step if you are proceeding to one of the full-sky measurement protocols.
1. Attach the collimating tube to the detector assembly.
Use a twisting motion. Make sure the bottom of the tube rests on the top of the instrument case, so the detector housing is completely covered.
2. Attach a computer interface to the db25 connector on the side of your instrument and set up the software according to the procedure below.
The connector on this instrument is made to work only with B&B Electronics' 232SDA12 analog-to-digital converter. Do not attach any other device to this connector.
3. Select the "V" switch position, turn your instrument on, and activate the data collection software.
When you record data from your instrument, the temperature channel will automatically be recorded at the same time. There is no need to manually record the temperature channel.
4. With the panel meter facing you, point the instrument toward the sun so the circular shadow from the top of the collimating tube is symmetrical around the base of the tube. Find the orientation that gives the maximum voltage and hold this position for approximately 15-60 seconds.
5. Proceed immediately to the Automated Full-Sky Measurement Protocol.
If you need time to set up an automated full-sky measurement, you can turn the instrument off until you are ready to proceed. Skip this step only if your site is not suitable for full-sky measurements.
6. Complete the metadata reporting as described in Metadata Reporting Protocol.
Skip this step if you are proceeding to one of the full-sky measurement protocols.
You should always collect full-sky data if your site is suitable for these measurements.
1. Remove the collimating tube and put it in the instrument storage bag.
2. Place the instrument on its platform. Stand so you can read the panel meter.
In order to take this measurement, your head should be no higher than the UV-A instrument itself. You must not block any sunlight or sky light from reaching the instrument.
3. Select the "V" switch setting.
4. Record the maximum voltage appearing on the panel meter during about 15 seconds. Record the time to the nearest 15 seconds.
5. Repeat step 5 at least 2 and no more than 4 times, recording the maximum voltage and time for each repetition.
6. Select the "T" switch setting and record the temperature.
7. Turn the instrument off. Attach the collimating tube and return the instrument to its protective bag.
8. Proceed to the Metadata Reporting Protocol.
When you make the outdoors observations required for the metadata, be sure to move away from the vicinity of the UV-A station. It is important not to block some of the sky radiation from reaching the detector.
1. Remove the collimating tube and put it in the instrument storage bag.
The collimating tube must be removed in order to take a full-sky measurement.
2. If the computer interface is not already in place, attach it now and set up the software according to the procedure below.
If you have just recorded sun photometer measurements, you will skip this step. The connector on this instrument is made to work only with B&B Electronics' 232SDA12 analog-to-digital converter. Do not attach any other device to this connector.
3. Place the UV-A instrument on its platform, oriented with the arrow on the top of the case pointing north.
It is important always to use the same platform and to orient the instrument case on the platform the same way every time you collect data. If it is windy, the instrument must be secured to the platform so that it will not move during the measurements.
4. Turn your instrument on.
It does not matter whether the panel meter switch is set to V or T. Both channels will be recorded.
5. Start the data collection software and move away from the instrument.
Do not stand in the vicinity of the instrument during data collection. You should be at least 10 meters away, if possible. Make sure your portable computer is in a safe place, out of direct sunlight.
6. Proceed to the Metadata Reporting Protocol.
When you make the outdoors observations required for the metadata, be sure to move away from the vicinity of the UV-A station. This important to ensure that you do not block some of the sky radiation from reaching the detector. You can complete the outdoors observations for metadata while the data are being recorded. Do the classroom metadata reporting when you are done.
7. After 20 minutes of recording, stop the data collection. Turn your instrument off. Attach the collimating tube and return the instrument to its protective bag.
8. Be sure to complete the metadata reporting when you return to the classroom.
If you are using automated data collection for full-sky measurements, you can complete the metadata reporting while your UV-A instrument data are being recorded. Be sure that you move well away from the instrument -- at least 10 meters, if possible -- so that you do not block any light from the sun or sky.
1. Do the Clouds and Contrails Protocols.
2. Do the sky color and horizontal visibility observations as described in the Aerosols Protocol.
3. If you are using the Air Temperature Protocol, do that now. However, if you are using an automated data collection protocol and your thermometer enclosure is close your UV-A instrument, wait until the data collection is completed. If you are not using the Air Temperature Protocol, defer this measurement until you return to the classroom.
4. If you are using the Relative Humidity Protocol, do that now.
5. If you are using online or broadcast sources for air temperature and relative humidity, obtain these values now.
6. Determine barometric pressure either by direct measurement or from an online or broadcast source.
You can use the Optional Barometric Pressure Protocol, but this is not required. On the data entry page, you must specify the source of your barometric pressure value and whether you are reporting sea-level or station pressure. Online or broadcast sources almost without exception give sea-level values.
7. Fill in the Comments section, as appropriate.
These comments should indicate any unusual conditions that you believe could have influenced your data. Record information that may influence the interpretation of your measurements, especially changing cloud or other meteorological conditions during the data collection. If you were using an automated data collection protocol and cloud conditions changed significantly during the measurement period, describe those changes.
If you are using an automated data collection protocol for full-sky UV-A measurements, you may wish to record data for longer than 20 minutes. Up to an hour before and after the overflight will be useful for scientists. No changes to the protocol are required, but there are some additional considerations:
1. The total recording time should always be centered as nearly as possible around the time of the maximum satellite elevation during the overpass.
2. In hot or cold weather, it may be necessary to provide extra protection for your portable computer. It is not a good idea to expose your computer to very hot or cold conditions. Computers can overheat when they are in direct sunlight even when air temperatures are moderate.
3. You may need an electrical outlet to power your computer during a long recording session. This will depend on your computer and whether its batteries are fully charged.
4. If there is even a possibility of precipitation, pay close attention. Your UV-A instrument and, of course, your portable computer, should not be exposed to precipitation.
5. If you extend the measurement times, you should check the battery every week or so. Be sure to use a good-quality alkaline battery. (The battery originally provided with your instrument is not an alkaline batter.) If you have persistent problems with battery life, and you can locate an electrical outlet or extension cord (which you may already need for your computer) near your instrument platform, you might consider replacing the battery with a DC power supply. For help with this project, consult the Science Team.
1. Will scientists really be interested in my measurements?
Yes. This protocol has been designed in cooperation with NASA scientists specifically to provide data that can be used in the processing and interpretation of space-based measurements.
One of the science goals of the ozone monitoring instrument (OMI) on EOS-Aura is to monitor the UV radiation reaching Earth's surface. If this can be done from space, then it will be possible to construct global maps that will help scientists understand the spatial and temporal variation of UV radiation. UV-B radiation is especially important because of its effects, both "good" and "bad," on life at Earth's surface.
However, spacecraft instruments cannot directly measure UV radiation at Earth's surface; scientists must infer this value based on solar energy reflected and scattered from Earth/atmosphere system. In order to improve the performance of the algorithms used to infer UV radiation, scientists need to know the amount of UV-A radiation actually reaching the ground, as well as the optical thickness of the atmosphere at UV-A wavelengths.
One of the advantages of a student-based monitoring network is that many relatively inexpensive instruments can be widely distributed around the globe. This is important because scientists need to study the performance of their data analysis algorithms over different kinds of land surfaces. For example, the energy reflected from a desert surface is much different from that reflected from a tropical rain forest or a heavily urbanized area.
So, measurements made with the GLOBE/NASA UV-A radiometer can play an important role in processing and understanding OMI's measurements. Regardless of where you live, there will be unique properties of Earth's surface at your site that scientists can use to improve their understanding of how to interpret space-based measurements.
2. What is UV radiation?
UV (ultraviolet) radiation is light with wavelengths shorter than the wavelength of blue light --
from about 100 nm to about 400 nm. UV radiation is divided into three categories: UV-A, UV-B, and UV-C. These ranges have
been given in Table 1 earlier in this document.
3. What is a sun photometer?
A sun photometer is a light-detecting instrument that measures direct sunlight at a specific wavelength (or narrow range of wavelengths).
4. What is a radiometer?
A radiometer is an instrument that measures radiation from the entire sky. Some radiometers measure radiation across a broad range of wavelengths. Others, such as the GLOBE/GSFC UV-A instrument, measure radiation at a specific wavelength (or narrow range of wavelengths).
5. What is the difference between direct and diffuse solar radiation?
Direct solar radiation follows a path directly from the sun to a detector. The GLOBE sun photometer measures direct radiation. Diffuse solar radiation is radiation from the sky, resulting from scattered sunlight. The total solar radiation is the sum of its direct and diffuse components. The GLOBE/GSFC UV-A instrument can be used to measure direct and total solar radiation.
6. Why can't we see UV radiation?
All solar radiation is electromagnetic radiation, governed by the same set of physical laws. There is no physical difference between what humans perceive as visible light and radiation above (infrared) or below (ultraviolet) these wavelengths. (If these names seem backwards, it is because they apply to frequency, which is inversely proportional to wavelength.) So, "visible light" is simply that part of the electromagnetic spectrum that human eyes (and the eyes of many other animals) have evolved to perceive as light. Perhaps, as eyes developed, the tradeoff between being able to see ultraviolet light and the cell and DNA damage that UV radiation can cause was settled in favor of sensitivity limited to what we now call the "visible" part of the electromagnetic spectrum. Some insects, such as mosquitoes, can see ultraviolet light. Some animals, such as pit vipers, have receptors that are sensitive to infrared radiation; these "eyes" allow pit vipers to "see" their warmblooded prey. .
1. Attach the collimating tube to the detector assembly.
2. Turn the instrument on.
3. Select the "T" switch setting and record the temperature.
4. Select the "V" switch setting.
5. With the panel meter facing you, point the instrument toward the sun so the circular shadow from the top of the collimating tube is symmetrical around the base of the tube. Record the maximum voltage appearing on the panel meter during about 15 seconds. Record the time to the nearest 15 seconds.
6. Repeat step 5 at least 2 and no more than 4 times, recording the maximum voltage and time for each repetition.
7. Select the "T" switch setting and record the temperature.
8. Proceed to one of the full-sky measurement protocols.
9. Complete the metadata reporting as described in Metadata Reporting Protocol.
1. Attach the collimating tube to the detector assembly.
2. Attach a computer interface to the db25 connector on the side of your instrument and set up the software according to the procedure below.
3. Select the "V" switch position, turn your instrument on, and activate the data collection software.
4. With the panel meter facing you, point the instrument toward the sun so the circular shadow from the top of the collimating tube is symmetrical around the base of the tube. Find the orientation that gives the maximum voltage and hold this position for approximately 15-60 seconds.
5. Proceed immediately to the Automated Full-Sky Measurement Protocol.
6. Complete the metadata reporting as described in Metadata Reporting Protocol.
1. Remove the collimating tube and put it in the instrument storage bag.
2. Place the instrument on its platform. Stand so you can read the panel meter.
3. Select the "V" switch setting.
4. Record the maximum voltage appearing on the panel meter during about 15 seconds. Record the time to the nearest 15 seconds.
5. Repeat step 5 at least 2 and no more than 4 times, recording the maximum voltage and time for each repetition.
6. Select the "T" switch setting and record the temperature.
7. Turn the instrument off. Attach the collimating tube and return the instrument to its protective bag.
8. Proceed to the Metadata Reporting Protocol.
1. Remove the collimating tube and put it in the instrument storage bag.
2. If the computer interface is not already in place, attach it now and set up the software according to the procedure below.
3. Place the UV-A instrument on its platform, oriented with the arrow on the top of the case pointing north.
4. Turn your instrument on.
5. Start the data collection software and move away from the instrument.
6. Proceed to the Metadata Reporting Protocol.
7. After 20 minutes of recording, stop the data collection. Turn your instrument off. Attach the collimating tube and return the instrument to its protective bag.
8. Be sure to complete the metadata reporting when you return to the classroom.
If you are using automated data collection for full-sky measurements, you can complete the metadata reporting while your UV-A instrument data are being recorded. Be sure that you move well away from the instrument -- at least 10 meters, if possible -- so that you do not block any light from the sun or sky.
1. Do the Clouds and Contrails Protocols.
2. Do the sky color and horizontal visibility observations as described in the Aerosols Protocol.
3. If you are using the Air Temperature Protocol, do that now. However, if you are using an automated data collection protocol and your thermometer enclosure is close your UV-A instrument, wait until the data collection is completed. If you are not using the Air Temperature Protocol, defer this measurement until you return to the classroom.
4. If you are using the Relative Humidity Protocol, do that now.
5. If you are using online or broadcast sources for air temperature and relative humidity, obtain these values now.
6. Determine barometric pressure either by direct measurement or from an online or broadcast source.
7. Fill in the Comments section, as appropriate.
School Name: _________________________ Study Site ATM-__________________________
Date: _________
Observer names: ______________________________________________________
For Satellite overflights on date of measurements:
Satellite/instrument name: __________ Time of overflight (UT): ________ Max elevation angle (deg): _____
UV-A Instrument serial number: __________________
Fill in the first five columns of these tables and report your data to GLOBE. GLOBE will provide you with calculated values for precipitable water (PW), which you then record in the sixth column. .
| Case temperature before taking measurements (multiply voltage reading times 100) | ____deg C | Measurement Number(1) | Local Time(2) (hrs:min:sec) |
UT Time(3) (hrs:min:sec) |
Maximum Voltage in Sunlight(4) (volts) |
Dark Voltage(5) (volts) |
AOT(6) (cm) |
|---|---|---|---|---|---|
| 1 | |||||
| 2 | |||||
| 3 | |||||
| 4 | |||||
| 5 | Case temperature after taking measurements (multiply voltage reading times 100) | ____deg C | |||
| Case temperature before taking measurements (multiply voltage reading times 100) | ____deg C | Measurement Number(1) | Local Time(2) (hrs:min:sec) |
UT Time(3) (hrs:min:sec) |
Maximum Voltage in Sunlight(4) (volts) |
Dark Voltage(5) (volts) |
UV-A(6) (cm) |
|---|---|---|---|---|---|
| 1 | |||||
| 2 | |||||
| 3 | |||||
| 4 | |||||
| 5 | Case temperature after taking measurements (multiply voltage reading times 100) | ____deg C | |||
Cloud and contrail conditions (If sky not obscured, check the box for each
cloud or contrail type you observe
and check one box for cloud or contrail cover amount.)
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Sky Conditions (Check one box in each table, as appropriate.
Sky conditions can be checked only if sky not obscured.)
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Current air temperature:_____ °C
Source (You must select one.):
| GLOBE Protocol | Online, broadcast, or print source | other thermometer |
Relative Humidity: __________ %
Source (You must select one.):
| GLOBE Protocol — hygrometer | GLOBE Protocol — sling psychrometer | Online, broadcast, or print source |
Barometric pressure (If you leave this blank, GLOBE will calculate it.): _____________ mbar
Reference (If you have entered a value, you must select one of these.):
| Sea level pressure | Station pressure |
| GLOBE Protocol — Calibrated classroom barometer | Online, broadcast, or print source | Let GLOBE calculate it. |
Comments: Describe conditions that could affect your measurements, such as urban smog, smoke from forest fires, blowing sand, or dust from agricultural activities.
____________________________________________________________________________________________
____________________________________________________________________________________________
Date: _________
Start Time: _________
Finish time: _________
Sampling Interval: _________
File name: _________