The middle-latitude desert climate is also designated as the temperate-boreal desert climate. The basic difference between the sub-tropical deserts and the middle-latitude deserts lies in lower temperatures that characterize the latter. Unlike the hot deserts, the middle-latitude deserts have very cold winters.
All the middle-latitude deserts of the Northern Hemisphere are found in the low basin-like areas in the deep interiors of continents partly surrounded by high-lands. Tarim, the Gobi, Dzungaria, Russian Turkistan and central Iran-all these deserts occupy the interior areas of the great continent of Asia.
These deserts are found between 35° and 50° N and S. In the United States of America such a type of desert occupies the Great Basin area. Extreme aridity of the above-mentioned deserts arises from the rain-shadow effect that they are subjected to.
In the Southern Hemisphere, the only example of a middle- latitude desert is found in Patagonia, South America.
All the middle latitude deserts in the great continents of North America and Asia are flanked by humid climates that are found both on the eastern and western margins of the continents.
As discussed earlier, these deserts show a large seasonal contrast in temperature. Therefore the annual ranges of temperature are very large. Annual range of temperature for Turkul, Russian Turkistan, located at 4£v°N, is 34°C.
Temperature for Urga, Mongolia, at 48°N, ranges from -27°C in January to 17°C in July. Thus the annual range is 44°C. In respect of temperature the middle-latitude deserts have much greater annual ranges and much lower winter temperatures than the low-latitude deserts.
Diurnal ranges of temperature are also large because of the aridity and other related factors. Patagonia occupies a transitional zone between the desert and the steppe and differs from other deserts in being not typically continental.
There is always a certain oceanic influence so that the winters are not very severe and summers not very hot. Daily range of temperature seldom exceeds 8°C in winter and 11°C in summer. Dust raising storms are common in Patagonia.
Middle-latitude deserts get meager precipitation. There are three major factors responsible for their dryness: (1) These deserts are covered by a strong winter anticyclone. (2) The basin-like areas of these deserts are surrounded by highlands. (3) They are situated far away from the oceans.
Even the few rain-bearing winds that reach the basins have to descend the highlands and in the process they are compressed and warmed adiabatically so that they get dried.
Such desert areas as are located towards the Mediterranean climate get their maximum precipitation in winter from the extra-tropical cyclones.
Russian Turkistan and the high plateaux of Persia and Afghanistan are regions of winter precipitation from the passage of occasional cyclonic storms. Here the spring is the wettest part of the year.
Winter precipitation is in the form of rain and snow. Merv, Teheran, and Quetta have 63, 46 and 50 percent of their winter precipitation in February, March and April respectively.
Since in the rainy season the melting of snows of the mountains occurs, the rivers are in floods. Similarly, the southwestern part of the Great Basin in the United States of America also receives its maximum precipitation in winter.
On the contrary, in the more interior and continental desert areas, maximum precipitation is received during summer. Larger part of interior Asia like Mongolia, the Gobi, Tarim and Dzungaria has maximum precipitation in summer.
Urga situated in the Mongolian desert gets 84 per cent of its rainfall in the three summer months. Most of the summer precipitation is of the convectional type.
Like the low-latitude deserts, the amount of precipitation is highly variable so that the annual average precipitation cannot be depended upon.