why deserts have a very dry climate?

Question

i came to know that it is not because it receives very little rain. it is due to atmospheric pressure.Could you please explain me the exact ,scientific reason?

Answer 1

If you look at a model of the circulation of the atmosphere, you will see that air rises at the equator and, high in the troposphere, moves towards the poles. Between 30° and 40° N and S, the air sinks back to the ground. This produces a belt of low pressure round the equator and a belt of high pressure at about 30°. The high pressure belt is known as the sub-tropical ridge.

Air in high pressure systems is descending. When it reaches the ground it flows out from the centre. Descending air does not produce clouds and without clouds you have no rain. The high pressure systems extend over the oceans producing dry areas at sea. In the days of sail, these were known as the horse latitudes.

As I said, the sub-tropical ridge extends around the globe in each hemisphere. Follow it on a map. You will find that all the major deserts of the world (except polar desrts) lie under the ridge - no cloud, no rain.

The other major deserts are the polar deserts. Again a large high pressure system is responsible - descending air, no cloud, no rain.

Answer 2

There are a number of factors that come into play when answering why a desert has a dry climate. As you mentioned, it has a great deal to do with atmospheric pressure. Basically, this deals with is the amount of moisture the air can hold at different pressures. This concept somewhat works backwards. Low pressures can't hold much moisture, so the moisture in the air will fall to the Earth in some form of precipitation, like rain or snow.

The way atmospheric pressures work is at a low pressure point the air is actually rising. At a high pressure point the air falls. You can basically relate this to density and how ice floats on water. With a low pressure the air is basically less dense and will rise to the top. When there is a high pressure, there is more density in the air and it will fall to the Earth. Why this is important is because most of the deserts are located in the subtropical region and near the polar regions where low pressures are found. When the sun hits the Earth most directly at the equator it helps create a low pressure area, the tropics. That air rises and drops rain. As it drops rain it moves away from the equator north and south. After the precipitation is gone the air forms a low pressure zone in the subtropical area and drops to the Earth causing a majority of the deserts. This pattern continues north and south forming the different climactic zones.

However, in the real world it's not so clear cut and dry as to where low pressures are and high pressures. Above is a basic and general concept. The Earth's rotation and trade winds also influence where low and high pressures are.

Answer 3

Yeah...lack of rainfall is a reason.

However, lack of rainfall alone can't provide an accurate description of what a desert is. For example, Phoenix, Arizona receives less than 250 millimeters (10 inches) of precipitation per year, and is immediately recognized as being located in a desert. The North Slope of Alaska's Brooks Range also receives less than 250 millimeters of precipitation per year, but is not generally recognized as a desert region.

The difference lies in something termed "potential evapotranspiration." The water budget of an area can be calculated using the formula P-PE+/-S, wherein P is precipitation, PE is potential evapotranspiration rates and S is amount of surface storage of water. Evapotranspiration is the combination of water loss through atmospheric evaporation, coupled with the evaporative loss of water through the life processes of plants. Potential evapotranspiration, then, is the amount of water that could evaporate in any given region. Tucson, Arizona receives about 300 millimeters, (12 inches), of rain per year, however about 2500 millimeters, (100 inches), of water could evaporate over the course of a year. In other words, about 8 times more water could evaporate from the region than actually falls. Rates of evapotranspiration in other regions such as Alaska are much lower, so while these regions receive minimal precipitation, they should be designated as specifically different from the simple definition of a desert: a place where evaporation exceeds precipitation.

Answer 4

Yes, it is due to a lack of water, and also to soils that can retain water for long peroids of time.