There are two climatic phenomena that lead to the aridity that characterizes deserts: the progressive decrease in rainfall and the increase in evaporation. These circumstances occur on one side and the other of the Equator, where most of the hot deserts are concentrated. The belt of subtropical anticyclones makes these regions permanently subjected to high pressures that reduce the humidity of the atmosphere and the soil, which prevents the development of the vegetation cover.
However, there are other factors that originate and reinforce desertification, such as continentality, mountain ranges, and cold maritime currents that hit the coast.
About 30% of the Earth’s land surface consists of deserts, which are defined as places of low precipitation, as we indicated at the beginning. While extreme temperatures are often associated with deserts, they certainly don’t define them. Deserts exhibit extreme temperatures due to a lack of moisture in the atmosphere, including low humidity and poor cloud cover. Without a cloud cover, the Earth’s surface absorbs more energy from the Sun during the day and emits more heat at night; hence deserts have this characteristic.
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How Is a Desert Formed?
Most deserts are formed in high-pressure subtropical areas caused by the equatorial wind belts. These are produced with the hot air from Ecuador, which is always increasing; descending into the tropics, it is too dry – the rains have already occurred over the tropics, and the air itself is warmer and less humid. Together with the characteristics of the wind, this lack of clouds means that there is less precipitation.
They can also occur in areas surrounded by mountains since they prevent rain from reaching the flat parts, acting as a kind of barrier. When the ocean air passes over the mountains, it cools and causes precipitation in the windward zone of the elevation, but when the air reaches the shelter of the mountains, it is already dry.
The deserts formed on the southwestern coast of Africa and South America are produced by cold ocean currents, which by cooling the air, cause rain to fall before reaching land.
Another type of desert is snow and ice deserts such as Antarctica, but for that, we would have to know how a glacier is formed, understanding that they are produced by snow that falls over a long period of time and that due to low temperatures, it does not melt.
Deserts, on some occasions, can be formed by the hand of man. A clear example is deforestation. In the absence of trees, wind erosion can act, making it impossible for plants to regenerate.
Why Do They Receive so Little Precipitation
Geographically speaking, most deserts are found on the western side of the continents or, in the case of the Sahara, Arabian, and Gobi deserts and the smaller deserts of Asia, they are found far from the coast in the interior of Eurasia.
The humid air that rises near the Equator cools and condenses into clouds and, later, rain. As the air stream moves toward the pole, the air releases much of its moisture. By the time the current returns to the Equator, the air is descending. It is compressed and heated, and its relative humidity drops further. In these conditions, it is rare for clouds and rain to form. If we add a little wind to this mix to speed up evaporation at the surface, continental regions become extremely arid due to the lack of available moisture. And that’s why deserts are that dry.
What Makes It so Hot in a Desert?
Considering that there is very little humidity in the desert, humidity-free air can heat up and cool down much more quickly than humidity-laden air. Our atmosphere is technically a liquid. Dry air, like desert air, will heat up and cool faster. This explains why desert regions, especially those near the Equator, heat up quickly and for long periods when there is Sun; while at night, dry air removes its heat quickly and can become very cold.
The type of soil is also important in terms of temperature. Rainforests, forests, grasslands, farmlands, and concrete (i.e., cities) absorb heat and take time to release it. Because they keep warm, they don’t get as cool at night. This is due to what is known as “specific heat,” which shows that objects, based on color, density, or reflective qualities, retain or reflect heat differently. And if there is no humidity (or very little), no vegetation can grow. This means that deserts generally lack grass, trees, or even rich (dark) soil.
All that remains is rock, which after being beaten by the wind for thousands of years, turns into sand. Sand has a very low specific heat, which means that it can heat up but cannot retain heat, allowing it to cool quickly (the opposite of concrete).