Night-time Fog, Stratus and Cirrus Determination (2 of 2)
The night-time fog and stratus product is also useful in locating clouds composed of ice
crystals. This is due to the differing optical properties of the 3.9 and 10.7 um channels with respect to water droplets and ice crystals. In the 3.9 um channel fog product example below, two separate cloud decks are apparent, with one moving southeast over MN, IA and WI and the other moving east over NY. The water clouds appear bright and the ice clouds are darker, or noisy, as over MI, OH, WV and VA. (The factors governing the appearance of water and thin cirrus clouds in the "fog product" were discussed on the previous page.) In regions of cirrus, where the cloud is thick, the resulting image product is very noisy due to the noisy nature of the 3.9 um channel at cold temperatures. Thin cirrus often exists around the edges of thicker cirrus, making both easier to detect. As is apparent in this imagery over VA, the cirrus scene changes from noisy (thick) to dark tones (thin).
Click on the image above to download the associated loop images.
Coincident imagery from the 10.7 um channel is available below for comparison purposes. In this imagery enhancement, cloud tops are progressively colder from yellow-red-purple-green. These images also show the thin and thick cirrus over VA.
Click on the image above to download the associated loop images.
Users should be aware of the following two caveats when attempting to use the fog/stratus product:
- water clouds and bands of cumulus downwind from regions such as
the Great Lakes at night-time will have the same appearance as fog
and stratus (refer to the section on Cumulus
Bands at Night ) and
- when surface emissivity varies between the 3.9 and the 10.7 um
regions, the "fog product" will be compromised (as in the Cap
Rock Escarpment region in west TX). Users should become
familiar with such regions, which can be easily found
by comparing early morning visible imagery with the "fog product"
near sunrise.
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