Temperature inversions are relatively common in the Arctic during the winter months. The image on the right is a radiosonde temperature profile collected at King Salmon, Alaska on Fubruary 15, 1990. The temperature at the surface is ~-18 C, increases to about -5 C at 1000 meters altitude, and remains relatively contstant up to 3000 meters, were it begins to cool. Note that the temperature of the atmosphere isn't colder than the surface until approximately 5000 meters altitude. Thus, any clouds that were between the surface and 5000 meters would be warmer than the surface.
To simulate these atmospheric conditions, the Volcanic Ash Retrieval code was run for a volcanic cloud and a meterological cloud (water) with the following boundary conditions and the results are shown below:
The image on the left shows a meterological cloud and the image in the middle shows a volcanic cloud. Note that the meterological cloud has negative band 4 minus 5 values and that the volcanic cloud has positive band 4 minus 5 values. These results may help to explain the observations shown in the image on the right, a band 4 minus 5 image from February 15, 1990, which shows negative band 4 minus 5 values over the land surface, but not over the ocean. This example illustrates that the meterological conditions of each scene must be taken into account when interpreting the imagery.