The term dew point is applied to the critical temperature at which the air is fully saturated, and below which condensation normally occurs. An excellent illustration of condensation due to cooling is seen in summertime when beads of moisture form on the outside surface of a pitcher or glass filled with ice water. Air immediately adjacent to the cold surface is sufficiently chilled to fall below the dew point temperature, causing moisture to condense on the surface of the glass.
Absolute humidity
Although relative humidity is an important indicator the state of water vapour in the air, it is a statement only of the relative quantity with respect to a saturation quantity. The quantity of moisture present is denoted by absolute humidity, defined as the weight of water vapour contained in a given volume of air. Weight is stated in grams, volume in cubic meters. For any specified air temperature, there is a maximum weight of water vapour that a cubic metre of air can hold (the saturation quantity).
In a sense, the absolute humidity is a geographer's yardstick of a basic natural resource-water-to be applied from equatorial to Polar Regions. It is a measure of the quantity of water that can be extracted from the atmosphere as precipitation. Cold air can supply only a small quantity of rain or snow; warm air is capable of supplying huge quantities.
One disadvantage of using absolute humidity in the study of atmospheric moisture is that when air rises or sinks in elevation, it undergoes corresponding volume changes of expansion or compression. Thus the absolute humidity cannot remain a constant figure for the same body of air. Modem meteorology therefore makes use of another measure of moisture content, specific humidity, which is the ratio of weight of water vapour to weight of moist air (including the water vapour). This ratio is stated in units of grams of water vapour per kilogram of moist air. When a given parcel of air is lifted to higher elevations without gain or loss of moisture the specific humidity remains constant, despite volume in-crease.
Specific humiditys often used to describe the moisture characteristics of a large mass of air. For example, extremely cold, dry air over arctic regions in winter may have a specific humidity of as low as 0.2 grams per kilogram, whereas extremely warm moist air of tropical regions may hold as 18 grams per kilogram. The total natural range on a i-wide basis is such that the largest values of specific humidity are from 100 to 200 times as great as the least.
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