Climatology of snow cover and snow water equivalent

• Chapter II: Physical snow characteristics determined from space
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• Satellite instruments' sensitivity
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• Snow detection in the visible spectrum
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• Snow detection in the infrared spectrum
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• Snow detection in the microwave spectrum
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• Factors which may lead to misidentification of snow

Factors which may lead to misidentification of snow

There are several features of satellite imagery that must be taken into consideration in order to prevent the misidentification of snow and snow-free areas in the final composite.

1. Clouds
Cloud tops exhibit a very bright reflectance in the visible spectrum and are often indistinguishable from snow. Differentiating between clouds and snow is one of the major problems in the use of satellite data for snow mapping. Snow can be distinguished from clouds by using near-infrared and short-wave infrared channels because clouds reflect noticeably more radiation than snow in these regions (Crane and Anderson, 1984; Dozier, 1984).

2. Forest cover
Forested areas can consist of anything from dense conifers to less dense deciduous forests to sparse range-type vegetation. The reflectance of these areas will be considerably lower than non-forested areas even with substantial depths of snow, since the snow signal will be blocked by the forest canopy. Vegetation indexes can be used to identify the forested areas and additional multi-spectral thresholds can be applied to determine if the forest floor is covered with snow.

3. Shadows
During the winter, sun angles are generally low and the resulting northern hemisphere terrain shadows on north-facing slopes with snow cover may be difficult to distinguish from bare south-facing slopes. Topographic maps and summer imagery may help in the interpretation. In shadowy areas, snow may be distinguished from rocks or soil by selecting a threshold brightness temperature (Dozier and Marks, 1987).

4. Rocks
During the melt period, highly reflecting bare rock may be difficult to distinguish from late season snow. Also, the spectral response of cold rocks is similar to snow in the microwave spectrum. Summer imagery and topographic maps, as well as vegetation patterns, can help in the differentiation.