The rain shadow results when moisture-laden air is forced to ascend the encircling mountains and thus drops most of that moisture on the windward side of the range summits. As the air descends from the ridge, it is drier and warmer. Not only does the compression of descent warm the air, it also evaporates much of the liquid water, causing clouds to disappear. The downward motion of the air masse further inhibits cloud formation on the lee of these ridges. A measure of the fraction of incident radiation falling on a surface that is turned back from it by reflection.
Reflectivity also refers to the degree by which precipitation is able to reflect a radar beam.
The ratio of the actual water vapour content of a given mass of air to that it would hold at saturation, usually expressed as a percentage. Also, the ratio of the water vapour pressure to the its saturation vapour pressure. The length of time that some substance remains in a reservoir. For example, the typical residence time of carbon dioxide in the atmosphere is the average length of time that a molecule of the gas will remain in the air before chemically transforming or moving into the hydrosphere, biosphere, or lithosphere. When an extensive area of high pressure is elongated, it is called a high pressure ridge , or simply a ridge.