B: Desert Climate
- Deserts are regions where evaporation exceeds precipitation.
- There are mainly two types – hot like the hot deserts of the Saharan type and temperate as are the mid-latitude deserts like the Gobi.
Hot Desert Climate
- The aridity of the hot deserts is mainly due to the effects of off-shore Trade Winds, hence they are also called Trade Wind Deserts.
- The major hot deserts of the world are located on the western coasts of continents between latitudes 15° and 30°N. and S (Question asked in Previous Mains Exam).
- They include the biggest Sahara Desert (3.5 million square miles), Great Australian Desert, Arabian Desert, Iranian Desert, Thar Desert, Kalahari and Namib Deserts.
- In North America, the desert extends from Mexico into U.S.A. and is called by different names at different places, e.g. the Mohave, Sonoran, Californian and Mexican Deserts.
- In South America, the Atacama or Peruvian Desert (rain shadow effect and off-shore trade winds) is the driest of all deserts with less than 2 cm of rainfall annually.
Mid-Latitude Desert Climate
- The temperate deserts are rainless because of either continentiality or rain-shadow effect. [Gobi desert is formed due to continentiality and Patagonian desert due to rain-shadow effect]
- Amongst the mid-latitude deserts, many are found on plateau and are at a considerable distance from the sea. These are Ladakh, The Kyzyl Kum, Turkestan, Taklimakan and Gobi deserts of Central Asia, drier portions of the Great Basin Desert of the western United States and Patagonian Deserts of Argentina etc..
- The Patagonian Desert is more due to its rain-shadow position on the leeward side of the lofty Andes than to continentiality.
Rainfall (Both Hot and Cold deserts)
- Deserts, whether hot or mid-latitude have an annual precipitation of less than 25 cm.
- Atacama (driest place on earth) has practically no rain at all.
- Rain normally occurs as violent thunderstorms of the convectional type.
- It ‘bursts’ suddenly and pours continuously for a few hours over small areas.
- The thunderstorm is so violent, and comes so suddenly that it has disastrous consequences on desert landforms [flash floods].
Major hot deserts in northern hemisphere are located between 20-30 degree north and on the western side of the continents. Why?
- The hot deserts lie along the Horse Latitudes or the Sub-Tropical High Pressure Belts where the air is descending, a condition least favorable for precipitation of any kind to take place.
- The rain-bearing Trade Winds blow off-shore and the Westerlies that are on-shore blow outside the desert limits.
- Whatever winds reach the deserts blow from cooler to warmer regions, and their relative humidity is lowered, making condensation almost impossible.
- There is scarcely any cloud in the continuous blue sky. The relative humidity is extremely low, decreasing from 60 per cent in coastal districts to less than 30 per cent in the desert interiors. Under such conditions, every bit of moisture is evaporated and the deserts are thus regions of permanent drought. Precipitation is both scarce and most unreliable.
- On the western coasts, the presence of cold currents gives rise to mists and fogs by chilling the on-coming air. This air is later warmed by contact with the hot land, and little rain falls. The desiccating effect of the cold Peruvian Current along the Chilean coast is so pronounced that the mean annual rainfall for the Atacama Desert is not more than 1.3 cm.
Temperature of Hot deserts
- There is no cold season in the hot deserts and the average summer temperature is high around 30°C.
- The highest temperature recorded is 57.77° C in 1922 at A1 Azizia, Libya.
- The reasons for the high temperatures are obvious—a clear, cloudless sky, intense insolation, dry air and a rapid rate of evaporation.
- Coastal deserts by virtue of their maritime influence and the cooling effect of the cold currents have much lower temperatures.
- The desert interiors, however, experience much higher summer temperatures and the winter months are rather cold.
- The diurnal range of temperature in the deserts is very great. Intense insolation by day in a region of dry air and no clouds causes the temperature to rise with the sun.
- But as soon as the sun sets, the land loses heat very quickly by radiation and the mercury levels drop.
- High diurnal temperature range is a typical feature of hot deserts. Average diurnal range varies from 14 to 25° Celsius.
- Frosts may occur at night in winter.
Climatic Conditions in the Mid-Latitude deserts
- These inland basins lie hundreds of miles from the sea, and are sheltered by the high mountains all around them. As a result they are cut off from the rain-bearing winds.
- Occasionally depressions may penetrate the Asiatic continental mass and bring light rainfall in winter. Due to their coldness and elevation, snow falls in winter.
- The annual range of temperature is much greater than that of the hot deserts. Continentiality accounts for these extremes in temperature.
- Winters are often severe, freezing lakes and rivers, and strong cold winds blow all the time. When the ice thaws in early summer, floods occur in many places.
- The predominant vegetation of both hot and mid-latitude deserts is xerophytic or drought-resistant.
- This includes the cacti, thorny bushes, long-rooted wiry grasses and scattered dwarf acacias.
- Trees are rare except where there is abundant ground water to support clusters of date palms.
- Along the western coastal deserts washed by cold currents as in the Atacama Desert, support a thin cover of vegetation.
- Intense evaporation increases the salinity of the soil so that the dissolved salts tend to accumulate on the surface forming hard pans [Bajada, Palaya].
- Absence of moisture retards the rate of decomposition and desert soils are very deficient in humus.
- Most desert shrubs have long roots and are well spaced out to gather moisture, and search for ground water. Plants have few or no leaves and the foliage is either waxy, leathery, hairy or needle-shaped to reduce the loss of water through transpiration.
- The seeds of many species of grasses and herbs have thick, tough skins to protect them while they lie dormant.
Life in the Deserts
- Despite its inhospitality, the desert has always been peopled by different groups of inhabitants.
|Bedouin Arabs||Arabia||nomadic herdsmen|
|Gobi Mongols||Gobi||nomadic herdsmen|
|Bushmen||Kalahari||primitive hunters and collectors.|
|Bindibu||Australia||primitive hunters and collectors.|
The settled cultivators
- The life-giving waters of the Nile made it possible for the Egyptians to raise many crops as early as 5,000 years ago.
- Modem concrete dams constructed across the Nile e.g. Aswan and Sennar Dams improved agriculture.
- In the same way, desert cultivators rely on the Indus in Pakistan, the Tigris-Euphrates in Iraq, and the Colorado in the Imperial Valley of California.
- In the deserts, wherever there are oases, some form of settled life is bound to follow. These are depressions of varying sizes, where underground, water reaches the surface.
- Some of them are abnormally large like the Tafilalet Oasis in Morocco which measures 5,000 square miles.
- A wall is usually constructed around the oasis to keep out the violent dust storms called simooms.
- The most important tree is the date palm. The fruit is consumed locally and also exported.
- Other crops cultivated include maize, barley, wheat, cotton, cane sugar, fruits and vegetables.
The mining settlers
- It was gold that brought immigrants scrambling into the Great Australian Desert.
- Some of them like Kalgoorlie and Coolgardie have become towns of considerable size.
- In the Kalahari Desert, the discovery of diamonds and copper has brought many white men to the ‘thirstland’ as it is called.
- Even in the most arid Atacama, in northern Chile, large mining camps have been established for the mining of caliche (cemented gravels) from which sodium nitrate, a valuable fertilizer, is extracted and exported to all parts of the world.
- Besides nitrates, copper is also mined. Chuquicamata is the world’s largest copper town.
- Similarly in the deserts of North America, silver is mined in Mexico, uranium in Utah and copper in Nevada.
- In recent years, the discovery of oil, in many parts of the Saharan and Arabian Deserts has transformed this forgotten part of the globe.
- Saudi Arabia, Iran, Iraq, Kuwait, Algeria, Libya, Lebanon, Nigeria etc. are important oil producing desert countries.
Explain how the aridity of the desert is related to
- off-shore Trade Winds
- the Sub-Tropical High Pressure Belts (the Horse Latitudes)
- cold ocean currents
Bring out any distinct differences between the hot deserts and mid-latitude deserts in
- way of life
Explain any three of the following.
- The hot deserts of the world are located on the western coasts of continents.
- Patagonia is a desert in the rain shadow of the Andes.
- The annual range of temperature is much greater at Kashgar (Gobi) than at Iquique (Atacama).
Write brief notes on any three of these topics.
- Date palm cultivation in an oasis.
- The role of oil in the development of desert economy.