1. Deserts Are Dry for Four Reasons, Not Because Rain Clouds Avoid Them
It seems that deserts are dry not because "rain clouds avoid only deserts." Dictionaries and encyclopedias divide the formation of deserts into four types [1][3]. The first is places in belts where air sinks (subtropical deserts). The second is places on the leeward side of mountain ranges (rain shadow deserts). The third is coasts near cold ocean currents (coastal deserts). The fourth is inland areas far from the sea (inland deserts). Each name describes a situation where rain is unlikely to fall.
A common guideline for a desert is an annual rainfall of 250 mm or less. However, sources vary, citing ranges like 200–250 mm or 250–300 mm, and some definitions depend on whether evaporation exceeds rainfall [1]. Therefore, 250 mm is just a rough estimate. Let us first look at the largest scale: the "belt where air sinks."
2. Why Air Sinking Near Latitude 30 Degrees Makes Rain Unlikely
Air that rises near the equator flows north and south at high altitudes and sinks near latitude 30 degrees. This large air circulation is called the Hadley cell, and the sinking areas become high-pressure belts (subtropical high-pressure zones) [1]. Here, descending air currents are strong, making rain unlikely, which leads to desert formation. The UK Met Office also explains that air rising at the equator sinks in the subtropics to create high-pressure areas, linking this to tropical deserts like the Sahara [2].
The latitude where air sinks is not exactly 30 degrees; sources state it ranges from 20 to 40 degrees depending on the season. Thus, it is accurate to call it a "belt centered around 30 degrees." If air does not sink in a place, does it always get plenty of rain? Next, let us look at the other side of mountain ranges.
3. Why Mountains Leave the Far Side Dry: Rain Shadow Deserts
In places where high mountain ranges block the wind, the wind drops rain or snow as it crosses the mountains. As a result, dry wind blows down to the leeward side, creating dry land [1]. Deserts formed this way are called rain shadow deserts. Dictionaries also explain that the leeward side becomes a zone of descending air currents [3]. This means the wind uses up its moisture before the mountain, and dry wind descends on the other side.
You can think of this like wringing out a wet towel. After wringing, it is hard to get more water out. However, the mountain does not "wring" the wind; rather, the wind turns into rain or snow and falls first as it crosses the mountain.
4. Why Deserts Form Beside Cold Seas: The Namib Desert
Deserts also exist right next to the sea. The Namib Desert on the Atlantic coast of southwestern Africa is one example. The cause of its dryness is said to be the Benguela Current, which brings cold water from deep down and cools the air [6]. Winds blowing over cold currents carry little water vapor, and the atmosphere is stable, making updrafts unlikely, so rain is rare [3]. This is how coastal deserts form.
The annual rainfall in the Namib is often described as averaging about 30 mm. However, figures vary by source and location. Some years have no rain at all, while others see nearly 100 mm [6]. It is interesting that a desert can form even near the sea if that sea is cold. So, what about places far from the sea?
5. Why Land Far from the Sea Is Dry: The Gobi Desert
In continental interiors far from the sea, winds coming from the ocean drop their moisture along the way, causing dryness [1]. This is how inland deserts form. It is similar to carrying water in a bucket with a hole; little remains by the time you arrive. The Gobi Desert, spreading from northern China to Mongolia, is an example. It is a continental climate desert, surrounded by mountain ranges and located inland, which is said to cause strong dryness [7]. Annual rainfall is 50–70 mm in the west and around 200 mm in the north and southeast. This shows that rainfall can differ by several times within the same desert depending on the location.
So far, we have seen four conditions. What happens when these conditions overlap in one place?
6. How Several Causes Combine in the Atacama Desert, With About 15 mm of Rain a Year
The Atacama Desert in northern Chile is sandwiched between two mountain ranges: the Andes and the coastal mountains. Additionally, the cold northward Humboldt Current and the South Pacific High are said to intensify the dryness [4]. Japanese sources also explain that the cold offshore current, along with the Andes and coastal mountains, blocks moist air, so there is no contradiction between the explanations [5].
The average annual rainfall is about 15 mm, with some places receiving only 1–3 mm, and some stations have never recorded rain. It is said to be the driest desert in the world, excluding polar regions [4]. Let us calculate the numbers. The desert guideline of 250 mm is about 17 times 15 mm. Dividing 15 mm by 365 days gives about 0.04 mm per day. Of course, in reality, most days have no rain, and heavy rain may fall once every few years, so this is just an average calculation. The Atacama example shows that rain clouds are not avoiding the area; rather, multiple conditions that make rain unlikely are overlapping.
7. How to Compare Desert Locations and Types on a Map
Look for the Sahara, Namib, Atacama, and Gobi in a school atlas. Once you find them, check three things. First, what is the latitude? Is it near 30 degrees? Second, are there mountains or the sea nearby? Third, is it far from the coast? Write down which of the four types you think applies. Then, check an encyclopedia at the library or the sources in this article to see if your prediction was correct. For deserts like the Gobi, some sources may not state the main cause of dryness clearly. In such cases, noting that uncertainty is also a valid way to investigate.