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Deserts


Introduction

Deserts cover more than one-fifth of the Earth's land surface and can be found on all continents. A desert is defined as a location that receives fewer than 10 inches (25 cm) of rain each year. Deserts are part of a larger category of landforms known as drylands. These places have a "moisture deficit," which implies they lose more moisture through evaporation than they receive from annual precipitation.



Vegetation

Both hot and mid-latitude deserts are dominated by xerophytic or drought-resistant plants. This includes the cacti, thorny bushes, long-rooted wiry grasses and scattered dwarf acacias. Except when there is adequate ground water to maintain clusters of date palms, trees are uncommon.


Along western coastal deserts that are affected by cold currents, such as the Atacama Desert, a sparse layer of vegetation exists.


Excessive evaporation increases the salinity of the soil, resulting in the accumulation of dissolved salts on the surface, generating hard pans [Bajada, Palaya].


Moisture deficiency slows decomposition, hence dry soils are extremely lacking in humus.


The majority of desert shrubs have lengthy roots that are widely distributed in order to collect moisture and seek out ground water. Plants with few or no leaves contain waxy, leathery, hairy, or needle-shaped foliage to minimise water loss by transpiration. Numerous grasses and herbs have thick, resistant coats that protect their seeds when they are dormant.




Hot desert (Subtropical)


In the scorching deserts, there is no winter season, and the average summer temperature is around 30°C. In 1922, the maximum temperature recorded was 57.77° C in A1 Azizia, Libya. The reasons for the high temperatures are self-evident: bright, cloudless sky, extreme insolation, dry air, and a high rate of evaporation.

Coastal deserts have substantially lower temperatures due to the ocean influence and the cooling effect of cold currents.


The diurnal temperature range in the deserts is enormous. The temperature rises with the sun during the day due to intense insolation in a location with dry air and no clouds. However, as soon as the sun sets, the ground loses heat quickly due to radiation, and the mercury levels fall. Hot deserts are characterised by a wide diurnal temperature range. The average diurnal temperature ranges from 14 to 25° Celsius. Frosts can occur at night throughout the winter. 



Mid-Latitude Desert Climatic Conditions


These interior basins are hundreds of kilometres from the sea and are surrounded by steep mountains. As a result, the rain-bearing winds have cut them off. Occasionally depressions may penetrate the Asiatic continental mass and bring light rainfall in winter. Snow falls in winter due to their coolness and elevation.

The annual temperature variation is substantially wider than in the scorching deserts.

Winters are frequently harsh, with lakes and rivers freezing and strong cold winds blowing all the time. Flooding occurs in numerous places when the ice thaws in early June.



Why deserts are located along horse attitudes ?


The hot deserts are located along the Horse Latitudes or Sub-Tropical High Pressure Belts, where the air is sinking, making precipitation of any type impossible.

The Rain-Bearer Trade Winds blow off-shore, and on-shore Westerlies blow outside the desert boundaries. Whatever breezes arrive in the deserts move from cooler to warmer locations, lowering relative humidity and making condensation nearly impossible. The relative humidity is exceptionally low, ranging from 60% in coastal areas to less than 30% in the arid inland. Under such conditions, every drop of moisture evaporates, resulting in deserts that are perpetually dry. Precipitation is limited as well as unreliable. The presence of cold currents on the western coast causes mists and fogs by cooling the approaching air. The chilly Peruvian Current's desiccating influence along the Chilean coast is so strong that the average annual rainfall for the Atacama Desert is less than 1.3 cm.

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