In strong contrast to the wet equatorial climate are the very dry climates controlled by subsiding outwardly moving air of the continental high-pressure cells which dominate much of the earth's land areas in the latitude belts 150 to 350, roughly centered on the Tropics of Cancer and Capricorn. Here lie the source regions of the continental tropical (cTs) air masses. The vast deserts of north Africa, Arabia, Iran, and West Pakistan exemplify this climate type, as do also the Sonoran Desert of the southwestern United States and northern Mexico, the interior Kalahari Desert of South Africa, and the Australian Desert. Within this region of general aridity we can recognize the truly arid zones, or deserts (BWh), in which annual rainfall is less than 10 in (4 cm), and the semi-arid zones, or steppes (BSh), in which rainfall is from 10 to 30 in (4 to 12 cm) annually.
In the continental interiors in these tropical regions, far removed from oceanic sources of moisture, extreme aridity prevails. Because these are regions of subsiding air in the general global scheme of circulation, adiabatic heating reduces the relative humidity of the air to low levels much of the time. At the drier stations in the Sahara Desert, relative humidity at 1:00 P.M. averages 25 to 30 per cent for the year, but from 15 to 20 per cent in the hottest months. Although in the summer low pressure due to ground-surface heating develops over the tropical landmasses, this is only a low-level condition, with the permanent highs persisting at higher levels of the atmosphere.
The capacity of tropical desert air for evaporation of exposed water surfaces is enormous. In the Sonoran Desert, annual evaporation from a free water surface exceeds 90 to 100 in (230 to 250 cm) annually, or about twenty times as much as falls in rain. It is obvious that the full amount of evaporation that is possible does not take place in the tropical deserts. Once the stream channels and soil have become dry after a rain, further evaporation is limited to a small amount of moisture slowly brought to the surface locally by capillary movement from moist soil or rock at depth.
Although dryness is the dominant characteristic of the continental tropical air mass source regions, sporadic heavy rainfall does occur from violent conventional storms. Penetration of maritime tropical or equatorial air may be responsible for such storms. During a single cloudburst confined to an area of a few square miles, the major portion of rainfall of one or more year's total may fall, producing debris floods in the stream channels.
The concept of variability in rainfall is an important one in climate study. By variability we mean the degree to which the, rainfall of the individual years differs from the average value computed over a long period of years.
Rainfall variability has been estimated for the world in ,terms of percentage departure from the normal value. As would be expected, the tropical desert areas lying near the tropics of Cancer and Capricorn have the highest variability; the equatorial belt of heavy rainfall, the least variability of the low-latitude part of the globe.
Consider next the temperature conditions of the dry continental tropical air mass source regions. Two points are noteworthy;
(a) temperatures are very high during the period of high sun;
(b) annual range is moderately strong.
Normally the annual range of temperature (hottest months average minus coolest month average) in these climates in 30 to 40°F (17 to 22°C) and is directly related to the height of the sun in the sky.
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