The boundaries of the dry type of climatic group are fixed by precipitation values. The characteristic feature of a dry climate is that the loss of moisture through evapotranspiration is far in excess of the annual receipt of water gain from precipitation.
Since no instrument has been developed which can measure the actual loss of moisture through evapotranspiration, and since no actual measurements of evapotranspiration for extensive areas of the earth have been made, the task of locating the boundary between dry and humid climates becomes all the more difficult.
Because of clear and calm weather and the dry atmosphere, the dry climates are severe for their latitudes. Extreme seasonal temperatures cause large annual ranges of temperature.
Daily ranges of temperature are still more pronounced. Annual average precipitation in these climates is always meager and highly variable.
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Thus, the meagerness and undependability are the two distinguishing features of precipitation in this great climatic group. Other salient features of the dry climates are low relative humidity, high potential evaporation, abundant sunshine, and small cloudiness.
On the basis of annual average precipitation the dry climates have been classified into two climatic types:
(1) BW – the arid, or desert type, and
(2) BS – the semi-arid, or steppe type.
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The arid-semiarid boundary is arbitrarily set at one-half the annual precipitation separating dry regions from humid. For example, if the humid-dry boundary happens to be 40 centimeters, then the steppe-desert boundary will be 20 centimeters.
Further, the arid and semi-arid climatic types have been divided into the following subdivisions on the basis of temperature:
(i) BWh – Tropical-subtropical hot deserts
(ii) BWk – Temperate-boreal cold deserts
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(iii) BSh – Tropical-boreal steppes
In Trewartha’s classification, the isotherm of 8 months with a temperature of 10° Celsius or above is taken to be the boundary between hot and cold (h/k) dry climates.
It may be pointed out that the same isotherm forms the boundary separating subtropical and temperate climates. BWh and BWK climates are constantly dry, and are under the influence of subtropical high and dry trades.
BSh climate is characterized by a short moist season. Both in summer and winter this climatic type is also influenced by subtropical high and dry trades. BSk type of climate receives most of its meager annual precipitation during the warmer season.
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Continental winter anticyclones control the weather conditions here. Hot tropical-sub-tropical deserts usually occupy the more stable parts of subtropical anticyclones and trade winds, and all of them are characterized by subsidence and divergence.
According to Trewartha, the middle latitude cold deserts are considered as mere pole-ward extension of the hot deserts. These cold deserts are generally positioned in the leeward of high mountains or in the continental interiors.