Deserts dominated by the eastern portions of subtropical high pressure cells originate in part from the stability produced by these pressure and wind systems.

A deficiency of moisture (especially when resulting from a permanent absence of rainfall) is known as aridity. Aridity is a nature produced permanent imbalance in the water availability consisting of low average annual precipitation with high spatial and temporal variability, resulting in overall low moisture and a low carrying capacity of the ecosystems.

Definition of deserts

It is a common belief that all deserts are hot and sandy places. While this is generally not true, a common factor of deserts is aridity, the temporal and/or spatial scarceness of water. True deserts can be delineated from other biomes based on their aridity. Of the following groups, only the first two are considered true deserts here.

Aridity, or the deficiency of moisture, can be divided into four groups:

  • Extremely arid: less than 60–100 mm of mean annual precipitation;
  • Arid: from 60–100 to 150–250 mm;
  • Semiarid: from 150–250 to 250–500 mm; and
  • Nonarid (= mesic): above 500 mm.

Since evaporation depends largely on temperature, bioclimatic aridity cannot be defined solely by the amount of precipitation. Therefore, the higher limits given above refer to areas with high evaporativity in the growing season (e.g., in subtropical areas with rainfall in warm seasons).

This is taken into consideration in UNESCO’s ‘World Map of Arid Regions’ which defines bioclimatic aridity by P/ET ratios (annual precipitation/mean annual evapotranspiration).

P/ET ratios smaller than 0.03 qualify for hyperarid zones (roughly corresponding to the extreme arid zone above) and a ratio of 0.03–0.20 as an arid zone (thereby corresponding to the arid zone mentioned above).

Another common way of delineating deserts is based on their vegetation pattern and optional land use. Extremely arid zones typically show contracted vegetation restricted to favourable sites or lack vegetation altogether.

Arid zones are characterised by diffuse vegetation. Semiarid zones are mostly characterised by continuous vegetation cover (if edaphic conditions allow for it), and only very locally is dry-land farming (without irrigation) possible. Farming without irrigation becomes a reliable option at larger scales in non arid zones only.

Based on geographic location and a combination of temperature and geographical causes of aridity, deserts can be separated into five classes:

  • Subtropical deserts. They are found in the hot, dry latitudes between 20° and 30°, both north and south. These deserts lie within the subtropical high-pressure belt, where the descending part of Hadley’s cell air circulation causes general aridity.
  • Rain shadow deserts. They are found on the landward side of coastal mountain ranges.
  • Coastal deserts. Found along coasts bordering very cold ocean currents that typically wring moisture as precipitation from the air before it reaches the land, these deserts are often characterised by fog.
  • Continental interior deserts. They are found deep within continents and far from major water sources.
  • Polar deserts. They are found both in the northern and southern cold, dry polar regions.

Causes of aridity:

Aridity, or a deficiency of moisture, arises from general causes acting individually or in combination.

The causes of aridity are the following:

  1. Distance:

One of these causes is the separation of the region from oceanic moisture sources by topography or by distance. Part of the desert area of the United States and the Monte-Patagonian Desert to the leeward of the Andes in South America are a result of the acidifying effect that major mountain barriers have on air masses that move over them.

Wind System:

A second general cause of aridity or deficiency of moisture is the formation of dry, stable air masses that resist convective currents. The Somali-Chalbi desert probably owes its existence to a stable environment produced by large-scale atmospheric motions.

Deserts dominated by the eastern portions of subtropical high pressure cells originate in part from the stability produced by these pressure and wind systems.

Aridity and a deficiency of moisture can also result from a lack of storm systems and the mechanisms that cause convergence, create unstable environments, and provide the upward movement of air that is necessary for precipitation. The paths, frequencies, and degrees of development of mid-latitude cyclones or tropical cyclones are crucial factors in the production of rainfall.

The deserts of the subtropical latitudes are particularly sensitive to the climatology of cyclones. The Arabian and Australian deserts and the Sahara are examples of regions positioned between major wind belts with their associated storm systems. 

  • Rain:

Widespread rains are almost unknown over large parts of the hot deserts, with most of the precipitation coming in violent convectional showers that do not cover extensive areas. The wadis, entirely without water during most of the year, may become torrents of muddy water filled with much debris after one of these flooding rains.

Because of the violence of tropical desert rains and the sparseness of the vegetation cover, temporary local runoff is excessive, and consequently less of the total fall becomes effective for vegetation or for the crops of the oasis farmer. Much of the precipitation that reaches the earth is quickly evaporated by the hot, dry desert air. Rainfall is always meagre.

In addition, it is extremely variable from year to year. The dependability of precipitation usually decreases with decreasing amounts. No part of the earth is known for certain to be absolutely rainless, although in Africa and in northern Chile, over a period of 17 years, the rainfall was only 0.5 mm. During the whole 17 years, there were only three showers heavy enough to be measured.

  • Temperature:

The skies are normally clear in the low latitude deserts, so sunshine is abundant. Annual ranges of temperature in the low latitude deserts are larger than in any other type of climate within the tropics. It is the excessive summer heat, rather than the winter cold, that leads to the marked differences between the seasons .

During the high-sun period, scorching, desiccating heat prevails. Midday readings of 40 to 45° C are common at this time of year. During the period of low sun, the days are still warm, with the daily maxima usually averaging 15 to 20° C and occasionally reaching 25°C. Nights are distinctly chilly, with the average minimum in the neighbourhood of 10°C.

On the basis of the aridity index, the region can be classified climatically into the following types: 

  1. Hyperdesert (arid) or extreme arid occurs when the aridity index of the region is greater than 80 percent.
  2. Desert (Arid) – A location with an aridity index ranging from 66.7 to 80.0 percent. 
  3. Semi desert (semi-arid): If the aridity index lies between 33.3 and 66.7 percent

The aridity index is useful in understanding the moisture status of a place.

The knowledge of aridity duration and intensity plays a vital role in agricultural and hydrological planning and in preparing contingent strategies to meet the aberrant weather situation in time.

By wajeeh ur Rehman

Research Associate Department of Agronomy, University of Agriculture, Faisalabad.