Fog and stratus layers appear smooth, fairly uniform in area and slightly gray or milky in imagery. In visual pictures, the edges of fog or stratus are often sharp and distinct; these edges terminate abruptly along mountains or coastlines or along low-level wind shear lines. See figure 10-2-7. The edges are not as easily detected over land on nighttime IR pictures, because the temperature contrast between the fog and stratus and its surroundings is less at night than during the day. When the temperature radiated by the top of a fog or stratus layer is nearly the same as that of the surrounding terrain, differentiating between the fog or stratus layer and the surrounding land becomes extremely difficult. You must be familiar with the topographical features in a cloud-free picture to be able to determine the distribution of the fog or stratus during such an occurrence

Figure 10-2-7.Fog and stratus.

If fog and stratus appear lighter (cooler) than the surrounding area at night, the layer is thick and may contain some internal or induced con-vection. Drizzle may be falling at the surface in such a region.

Stratocumulus not in a cellular pattern and areas of cold water that are completely or par-tially surrounded by warmer water appear very similar to fog and stratus in IR imagery. Thin fog may go undetected by a radiometer.

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