In this article we will discuss about:- 1. Location of Hot Desert Climate 2. Temperature of Hot Desert Climate 3. Pressure and Winds 4. Rainfall 5. Natural Vegetation.
Location of Hot Desert Climate:
The hot desert or Sahara type of climate is located between the latitudinal belt of 15°-30° (35°) in both the hemispheres on the western parts of the continents.
This climate is found in:
(1) Africa – the Namib and Kalahari deserts of coastal Angola and southwest Africa, interior Botswana and South Africa, and Sahara desert;
(2) Asia – Thar deserts of India and Pakistan, Arabian deserts, Iranian desert;
(3) South America – Acatama desert of coastal Peru and Chile;
(4) Mojave and Arizona deserts of south-western USA;
(5) Australia – Great Sandy Desert, Great Victoria Desert and Tanami Desert (fig. 39.4).
This climate is characterized by annual aridity, and subsiding warm air masses of the subtropical anticyclones.
The following reasons are held responsible for the genesis of perpetual aridity of the tropical-subtropical hot desert climate:
(i) Temperate cyclones do not reach these areas.
(ii) Intertropical Convergence (ITC) also does not influence these areas because of their distant location from the equator.
(iii) The trade winds spend most of their moisture through rainfall in the eastern margins of the continents and as they reach the western margins of the continents they become dry and hence are unable to give rainfall.
(iv) Due to anticyclonic conditions winds descend from above and hence they are warmed adiabatically with the result their moisture retaining capacity increases resulting into marked decrease in relative humidity.
(v) Subtropical high pressure system causes divergence of surface winds which is antagonistic to rainfall.
(vi) The ground temperature is so high that raindrops, if formed at all, are evaporated before they reach the ground surface.
Temperature of Hot Desert Climate:
On the basis of annual distribution of temperature two distinct seasons are recognized e.g. summer season and winter season. Average temperature during summer season ranges between 30°C and 35°C but maximum temperature exceeds 40°C during mid-day.
The temperature of 40°C to 48°C is very common at noon during summer months. The western part of Great Australian Desert records temperature above 40°C for 64 days in continuation and above 32°C for 150 days in continuation but temperature falls at nights giving much relief to the people.
Phoenix of Arizona (USA) records more than 32°C at noon but temperature falls to 24°C in the nights. Azizia has recorded the highest temperature of 58°C (136.4°F) so far. Similarly, exceptionally very high temperature of 56.4°C (134°F) has been recorded in the Death Valley of Californian desert (USA). Day time mean temperature during winter season ranges between 15.5°C and 21°C but sometimes it reaches 27°C but at nights temperature falls to 10°C.
It is, thus, apparent that both annual and diurnal ranges of temperature are high in the tropical-subtropical hot desert climatic areas. Generally, annual range of temperature ranges between 17°C and 22°C while daily range varies from 22°C to 28°C. Sometimes, daily range of temperature exceeds 40°C. Very high daily and annual range of temperature is because of open and clear skies, vegetation-free ground surface, very low humidity, distance from the equator, dominance of sands etc.
It may be pointed out that in the absence of clouds and moisture maximum insolation is received at the ground surface. Loose sands are soon heated and thus ground temperature soon shoots up. Again there is rapid loss of heat from the sandy surface through outgoing long-wave terrestrial radiation at nights due to clear sky and completely dry condition (total absence of moisture in the air) resulting into considerable fall in night temperature.
This mechanism causes very high daily range of temperature. It may be remembered that blankets are needed in the nights in hot desert areas due to very high difference in daytime and night temperatures even during summer seasons. Tripoli recorded highest and lowest temperatures of 91°F and 31°F respectively on December 25, thus registering a daily range of 60°F.
Pressure and Winds of Hot Desert Climate:
Poleward areas of the regions of hot desert climate are affected by divergent air circulation and anticyclonic conditions because they fall in the belt of subtropical high pressure. The winds become stable and dry because they descend from above and are heated and thus there prevails dry condition. The north-east trades (northern hemisphere) become dry when they reach the western parts of the continents in the latitudinal zones of 15°-35°.
Some local low pressure centres are formed during summer season and thus a few local but weak cyclonic storms are produced. The upper air anticyclonic conditions do not allow the winds of these local storms to rise. Heat waves dominate in summer season thus making human life very difficult. The extensive deserts of Sahara and Australia become ideal source regions for the development of tropical continental airmasses.
Rainfall in Hot Desert Climate:
Rainfall in tropical desert climate is so low and variable that it becomes difficult to determine average annual rainfall as it never comes true. The various sources put the annual average of rainfall between 25 cm and 37.5 cm but these figures are highly misleading because there are so many such areas where not even a single drop of rain is received for several years in continuation.
Thus, the annual average rainfall is considered to be less than 12 cm. Most of Sahara receives less than 12 cm of mean annual rainfall. Cairo of UAR (3.0 cm), Lima of Peru (5 cm), William Creek of Australia (13.3 cm), Yuma of Arizona (USA, 8 cm), Toloth in S.W. Africa (5.6 cm) receive very low mean annual rainfall. Northern parts of Chile sometimes do not receive any drop of rains for 5 to 10 years in continuation.
The equatorward relatively more humid areas, however, receive 50 to 75 cm as mean annual rainfall. Most of the rains is of convectional type due to local heating. Sometimes, occasional storms give heavy downpour within few hours causing flash floods. For example, 85 cm of rainfall was recorded within two days in Doorbazi of Rajasthan whereas its mean annual rainfall is only 12.5 cm.
Such occasional catastrophic rainfall causes flash floods resulting into choking of storm drains, destruction of human settlements, silting and choking of canals and destruction and disruption of means of transport and communication. Such heavy rainfall is not useful at all as all of the rainwater are disposed off quickly through surface runoff and the remaining water is evaporated due to high temperature.
Skies are generally free from clouds and thus sun’s rays reach the ground surface without being reflected throughout the year and hence the tropical desert climatic regions receive sufficient bright sunshine all the year round. The Sonoran Deserts of the USA and Mexico receive more than 75 per cent and 90 per cent sunshine during winter and summer seasons respectively. Most parts of Sahara Desert are characterized by 1/10 cloudiness in December and January and 1/30 cloudiness from June to October. It is, thus, apparent that the ground surface is more or less always baked in the sun.
Sometimes, dark cumulo-nimbus clouds are formed, thunderstorms with cloud thunder and lightning are experienced but still there is no rain because raindrops are evaporated before they reach the ground surface. The average relative humidity ranges between 10 to 30 per cent.
Natural Vegetation of Hot Desert Climate:
Hot desert type of climate is not conducive for vegetation growth because of acute scarcity of water. This is why most of the regions under this climate are either devoid of any vegetation such as Lybian and Arabian deserts or if there is any vegetation at all, that is very little, sparse and bushy in character. The vegetation of hot desert climate is of xerophytic type which has special characteristics to withstand harsh climate characterized by extreme aridity, high temperature and very high rate of evaporation.
They have their own moisture conserving devices such as long roots, thick barks, waxy leaves, thorns and little leaves so that they may avoid evapotranspiration and consequent loss of moisture from them. Most of the vegetation are found in the form of bushes. Cactus, acacia, date palm, a few flowering plants etc. form the composition of natural vegetation of hot desert climate.