The causes of Aridity
Pressure and winds
The air that rises around the equator eventually descends in the horse lattudes oo the hot
deserts and this descending air is a major cause oo their aridity. Having risen to the
tropopause, the air moves towards the poles and cools, becoming denser as a result. It then
sinks at about 30 degrees North and South, creatng high pressure at the suroace. Sinking air
compresses and causes warming which results in a decrease in its relatve humidity. Afer
reaching the suroace, this dry air moves orom the high pressure area back to the low
pressure area in equatorial lattudes as the trade winds. The circulatons oo air between the
equator and 30 degrees North and South are known as the Hadley Cells.
The trade winds are strong and constant and derive orom sinking air and blow over land to
the deserts and thereoore they do not contain much moisture. This leads to low
precipitaton in the hot areas. Thereoore, the areas are on the west coasts oo the contnents
as opposed to the east coasts.
North east trade winds blow in the Northern hemisphere and south east blow in the
southern hemisphere. This is caused by two oactors: winds blow out oo high pressure
systems into low pressure systems and they are defected as they do so by the earth’s
rotaton.
Summer convectonal rainoall occurs in areas on the margins with the Savannas, such as the
Sahel, on the southern margins oo the Sahara. Winter rainoall due to depressions occurs on
the margins oo the areas oo Mediterranean climate on the poleward margins oo the deserts.
Pressure and winds
The air that rises around the equator eventually descends in the horse lattudes oo the hot
deserts and this descending air is a major cause oo their aridity. Having risen to the
tropopause, the air moves towards the poles and cools, becoming denser as a result. It then
sinks at about 30 degrees North and South, creatng high pressure at the suroace. Sinking air
compresses and causes warming which results in a decrease in its relatve humidity. Afer
reaching the suroace, this dry air moves orom the high pressure area back to the low
pressure area in equatorial lattudes as the trade winds. The circulatons oo air between the
equator and 30 degrees North and South are known as the Hadley Cells.
The trade winds are strong and constant and derive orom sinking air and blow over land to
the deserts and thereoore they do not contain much moisture. This leads to low
precipitaton in the hot areas. Thereoore, the areas are on the west coasts oo the contnents
as opposed to the east coasts.
North east trade winds blow in the Northern hemisphere and south east blow in the
southern hemisphere. This is caused by two oactors: winds blow out oo high pressure
systems into low pressure systems and they are defected as they do so by the earth’s
rotaton.
Summer convectonal rainoall occurs in areas on the margins with the Savannas, such as the
Sahel, on the southern margins oo the Sahara. Winter rainoall due to depressions occurs on
the margins oo the areas oo Mediterranean climate on the poleward margins oo the deserts.
