Interpretation of Visible Imageries

Clouds have higher albedo than land (apart from snow cover) and appear white or light grey in a VIS imagery. Their brightness depends on their physical properties. Clouds with high albedo have large depth, high cloud water (ice) content, small cloud-droplet size whereas clouds with low albedo have shallow depth, low cloud water (ice) content, large average cloud-droplet size. The water content and depth of the cloud are the most important. Typical Albedo values are given in Table 1.

Table 1. Albedo values of different surfaces and clouds

Earth surface

(%)

Clouds

(%)

Oceans, Lakes

8

Shallow broken clouds

30

Cu, Ci, Cs, Cc

35

Land Surfaces

14-18

St

40

Sand, Desert

27

Thick clouds (Cs)

74

Ice and Snow

Ac, As, Sc

68

Sea ice

35

Cu

75

Old snow

59

Ns

85

Fresh snow

80

Cb

90

VIS imagery is useful for distinguishing between sea, land and clouds (Figure 1). Seas and lakes have low albedo and hence appear dark. Land appears brighter than sea but darker than clouds. Albedo of land varies with the type of surface. Deserts appear very bright in contrast to the darkness of forests and vegetated areas. When the sun shines obliquely onto clouds the shadow thrown by an upper cloud layer onto a lower layer reveals the vertical structure of the cloud in VIS imagery.

The texture of the cloud in VIS imagery can help in its identification, (for example) its cellular pattern can distinguish stratocumulus clouds (Sc) from stratus (St). No VIS imagery can be obtained at night. To distinguish clouds from snow covered ground a knowledge of the surface topography is essential. Thin clouds have low albedo and do not show up very brightly in VIS imagery so that the cloud cover over dark surfaces may be underestimated. In the same manner thin cloud over a high albedo desert surface may look misleadingly bright and thick. Mesoscale cumulus (Cu) clouds which are smaller than the resolution of the satellite will be depicted in rather lighter grey shades in VIS imagery quite unlike the normal view of convective clouds.

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