Sky photography

This photo of the Manhattan skyline was taken with a Canon PowerShot SX120 10X optical zoom camera from the rooftop of the Queens College Science Building on the morning of June 30. The sky is relatively clear. (The Empire State Building is at the left.) ImageJ's "histogram" tool (under the "Analyze" tab) has been used to analyze a rectangle on the dark building at the right and a rectangle of sky of about the same size to the left of that building. The ratio of the mean brightness (on a grayscale of 0-255) is 133.21/170.55 = 0.78. Horizontal visibility is affected by particulates, relative humidity, and smog over the city. The smaller this ratio, the better the horizontal visibility. In the limit for a sky with very poor horizontal visibility, buildings at this distance can no longer be seen against the sky and the the ratio will reach a value of 1.0. Longterm studies of horizontal visibility using this technique should be done using the same object under the same lighting conditions (time of day) and with the same manual camera settings.

This photo was also taken on the morning of June 30. ImageJ's "plot profile" tool (under the "Analyze" tab) is used to graph the grayscale brightness along a vertical line up from the horizon (measured along the x-axis in pixels). The shape of the graph is a function of sky conditions. For a very clear sky, there should be a sharp peak close to the horizon. As the sky becomes less clear, the peak broadens and the maximum moves up from the horizon. This graph indicates a relatively clear sky for New York City, but it is much less clear than a sky might be over a rural area. Longterm studies of sky conditions using this technique should be done using the same scene viewed under the same lighting conditions (time of day) and with the same manual camera settings.