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. |