Introduction

The drylands of Asia have several common denominators: their cold winters (in all but the extreme south), the prevailing influence of the seasonally shifting Asian monsoon system, the westerly disturbances affecting western and central areas, and the role of topography as a major climatic control. These drylands represent a gradual, east–west transition between summer and winter (or Mediterranean) precipitation regimes. The cold winters distinguish them from the subtropical deserts of most other continents, with extremely low temperatures caused by the persistent presence of the Asiatic High and the relatively high latitude of the region. Insolation is low in winter because the days are short and the sun is low on the horizon, while the high promotes radiative cooling in the clear skies and dry air, and also insulates the region from warmer air masses. Temperatures can plunge lower than −50°C.

Dust is another common denominator. As a result of the combined forces of high winds and dry surfaces, Asia is one of the world’s dustiest regions. Even in relatively humid regions the frequency of dusty conditions can be quite high (Fig. 19.1). In most of the dryland regions of Asia the number of days with dust storms averages 5–20 per year. They are less common in more northern areas of the Former Soviet Union, but in parts of central and western Asia the average frequency is as high as 40–80 days per year.