During the early morning hours of April 12 prior to sunrise, Fog and low clouds developed across N/NE Texas and spread north into Oklahoma. Dense Fog Advisories were issued for portions of the region. At night in the absence of visible imagery, fog and low clouds are difficult to detect in single band satellite imagery. However, by combining multiple bands, such features stand out quite well.
One method of detecting fog and low clouds in GOES-16 imagery at night is to difference the 3.9 um and 11.2 um IR bands. How does this work? For low clouds and fog (water clouds), emissivity at 3.9 um is less than emissivity at 11 um. Therefore, low clouds and fog will appear cooler at 3.9 um than at 11 um (at night). So 3.9 um – 11 um difference will be negative for low clouds and fog at night (usually between -2 and -8 K). For high clouds at night, 3.9 um will be warmer than 11 um due to a greater response from the sub-pixel effect. Therefore, the 3.9 um – 11.2 um difference will be positive for high clouds at night. The difference for clear land will be near 0.
During the day, the sign for water clouds is flipped due to reflectance and scattering of solar radiation at 3.9 um. So 3.9 um will become warmer than 11 um, and the 3.9 um – 11 um difference positive during the day (usually less than 20 K). High clouds will generally have a greater positive 3.9 um – 11 um difference than low clouds during the day due to the sub-pixel effect. Fog detection and monitoring during the day, however, is generally done using visible imagery.
The 3.9 – 11.2 um difference example below uses a gray scale color table. At night, low clouds and fog will appear a dark gray, clear land a medium gray, and high clouds the lightest gray. In this example, there is a transition to visible imagery as the sun rises. The second animation is simply IR imagery, showing that IR imagery alone will not depict low clouds at night.
– Bill Line, NWS
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