The interiors of North America and Asia, in latitudes 35° to 50°, and, to a limited extent, that of southernmost South America have desert and steppe climates of somewhat complex origin. Three basic air mass controls operate.
(1). In summer, when pressure and wind belts are shifted poleward, these regions temporarily become the source regions for continental tropical air masses, developed because of intense heating of the large continental interiors.
(2) In winter, the intense development of the Siberian and Canadian highs, which are the source areas for continental polar air masses, causes frequent invasions of relatively dry continental air.
(3) Mountain ranges separate these deserts from moist maritime polar and maritime tropical air masses, which supply abundant rainfall to the west and southeast coasts.
Through forced ascent over these ranges, followed by adiabatic heating upon descending the lee slopes the maritime air masses are deprived of their moisture and raised in temperature as well. Thus the regions in question are poorly situated for obtaining precipitation. Because of the prevailing westerly winds in these latitudes, maritime tropical air masses cannot easily reach these areas from the east. In southern Asia, the northward flow of moist tropical air from the Indian Ocean is blocked by the great Himalayan chain.
Under the Koppen system, two climate varieties are recognized, the semi-arid steppes, BSk, and the true desert, BWk. The letter k signifies a cool climate, with average annual temperature below 64.4°F (l8°C). Where the letter k' is used, a cold climate is indicated, with the warmest month average below 64.4°F(18°C).
Only a small proportion of the area covered by dry middle-latitude climates is extremely dry: the Turkestan and Gobi deserts of central Asia and parts of the Great Basin in Nevada and Utah. The principal respect in which these deserts differ from the tropical deserts of lower latitudes is that their annual temperature range is much greater and the winter temperatures much lower. The enormous annual range of 58°F (32°C) is almost double that of Aswan, Egypt; the January mean temperature is a severe 20°F (-7°C), as compared with 60°F (16°C) for Aswan; and the July maximum is only 15°F (8°C) less than for Aswan.
Of considerably greater importance geographically than the deserts are the vast semi-arid steppe lands of the middle-latitude dry climates. Partly because of higher elevation, which increases the precipitation, or because of location nearer to maritime air mass invasions, these regions receive from 10 to 20 in (25 to 50 cm) of precipitation annually.
When the semi-arid steppes are followed from subtropical highlands in Mexico in middle latitudes, the annual temperature cycles show not only progressively lower temperatures but also greatly increased annual ranges. A typical steppe climate of intermediate position is that of Pueblo, Colorado. Compared with the Mediterranean climate of Monterey, California, which lies at about the same latitude, the annual temperature range of Pueblo is very much greater, the precipitation cycle is just reversed, with the summer maximum clearly marked. In summer large highs drifting over the central United States produce a northerly return flow of air on their western sides, spreading moist tropical air from the Gulf of Mexico far northward and westward into the continental interior. This air is unstable and readily produces thunderstorms when lifted over a mountain range or along a cold front.
The summer moisture deficiency persists through seven months. Recharge in the winter months is not sufficient to generate a surplus runoff.
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