Thin Cirrus and Multi-layered Clouds
Radiation from below passes through thin cirrus clouds, making the satellite-measured IR temperatures warmer than the actual cloud top temperatures. This effect is more evident at
3.9 than at 10.7 um because of the stronger response at 3.9 um to the warm radiation from below. In addition, thin cirrus is often patchy and only partially fills an FOV, further enhancing response at 3.9 um. As a result, in regions of thin cirrus, 3.9 um images often reveal lower
cloud layers. At night, the underlying clouds may have different motions, leading to their
detection with animated imagery. During daytime, water clouds, with their higher reflectivity,
can be detected at 3.9 um, while they are obscured, or very difficult to observe, in the VIS and 10.7 um imagery.
In this daytime example, a small water cloud, just south of Georgian Bay (encircled in
light-blue), is moving southward and appears as a bright spot in the 3.9 um image, while it is obscured by thin cirrus at 10.7 um.
Use caution when interpreting 3.9 um images in the presence of thin cirrus. Energy from
below will increase the detected radiance and may result in image shades very close to other cloud scenes, and even clear regions. Corresponding 6.7 um imagery is useful in isolating this effect; see the section on Winter Storms for another example.
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