The first type of satellite imagery, VIS, is based upon the principle of reflected sunlight. The amount of reflected sunlight that the satellite can remotely sense is dependent on the reflectivity, or albedo, of the surface off which the sunlight is reflecting. Table 11.1 presents reflectivities for a few common surfaces, including fresh snow cover (extremely reflective), water (not very reflective), and various cloud types. We can see that cumulonimbus (thunderstorm) clouds have the largest reflectivity among cloud types. This is because the amount of reflected sunlight is generally proportional to the thickness, or depth, of a cloud. On a VIS satellite image, the thickest clouds (e.g., cumulonimbus) appear the brightest white, while thin clouds (e.g., cirrus) appear dim gray. Land surfaces, especially bodies of water, have the smallest reflectivities and therefore appear dark.