Night-time Fog, Stratus and Cirrus Determination (1 of 2)
The identification of fog and stratus at night is a new application of the GOES Imager data.
A night-time image product is made by subtracting the 3.9 um scene temperatures from
those at 10.7 um, and scaling the results to show (-/+) differences. This technique, in use
for over a decade with AVHRR , is based on the principle that the emissivity of water cloud
at 3.9 um is less than at 10.7 um. Evolution of night-time fog and low-level stratus clouds
is easily observed by viewing the product in animation. This "fog product" image, out of a sequence from the night of March 14 and the early morning of the 15th, shows coastal fog and stratus spreading southward over the Atlantic Ocean prior to moving inland to cover parts of NJ, DE, MD, VA, NC and the Chesapeake and Delaware Bays.
Click on the image above to download the associated loop images.
In interpreting this image product and its associated look-up table, keep in mind that: 1)
areas of fog and stratus are bright, a positive difference; 2) clear ground has virtually no temperature variability and shows up as a mid-grey shade; and, 3) thin cirrus appears dark,
a negative difference. Note that cirrus appears dark, even though the emissivity of ice
cloud is about the same at these two wavelengths, since much of the sensed energy comes
from the earth's surface and the 3.9 um channel's response to warm sub-pixel temperatures
is greater than it is at 10.7 um. The early morning visible image from March 15 can be viewed for confirmation.
Continue or go back to the previous page.
