Deserts are landscapes defined by extreme aridity, receiving very little precipitation. Their defining characteristic is a profound lack of water, which shapes unique ecosystems. Understanding why these regions are so dry involves exploring large-scale atmospheric movements, geographical features, and ocean currents.
Global Atmospheric Patterns
Many of the world’s major deserts owe their dryness to global atmospheric circulation patterns, particularly the Hadley Cells. These cells involve warm, moist air rising near the equator. As this air ascends, it cools and releases its moisture as abundant rainfall over tropical regions. The now-drier air then moves poleward at high altitudes, typically reaching around 30 degrees latitude in both the Northern and Southern Hemispheres.
At these latitudes, the dry air descends back towards the Earth’s surface. As it descends, the air warms and its capacity to hold moisture increases, preventing cloud formation and precipitation. This downward movement creates stable, high-pressure zones that inhibit rainfall, leading to the formation of vast subtropical deserts like the Sahara Desert and the Arabian Desert. These high-pressure belts are often referred to as the “horse latitudes” and are key drivers of aridity.
Geographical and Topographical Factors
Physical landscape features contribute to the formation of deserts through mechanisms like the rain shadow effect. When moisture-laden air encounters a mountain range, it is forced to rise. As the air gains elevation, it cools, and its water vapor condenses, resulting in precipitation on the windward side of the mountains. This process depletes the air of its moisture.
Once over the mountain peaks, the now-dry air descends on the leeward side, warming. This warming increases the air’s capacity to hold moisture, creating a “rain shadow” with minimal rainfall. Notable examples include the Mojave Desert and Death Valley, which lie in the rain shadow of the Sierra Nevada mountains, and the Atacama Desert, affected by the Andes. Additionally, deserts can form in continental interiors, far from large bodies of water, where air masses lose most of their moisture while traveling long distances over land. The Gobi Desert in Asia is an example of a desert influenced by its inland location.
Ocean Currents
Cold ocean currents along continental coastlines contribute to arid conditions, particularly in coastal deserts. When air passes over these cold currents, it cools. This cooling effect stabilizes the air, making it less likely to rise and form rain-producing clouds. As a result, even though the air may contain some moisture, it remains trapped near the surface, often leading to coastal fog rather than rain.
This phenomenon contributes to the dryness of regions like the Atacama Desert in South America, influenced by the cold Humboldt Current, and the Namib Desert in Africa, shaped by the Benguela Current. These coastal deserts receive minimal precipitation because the stable atmospheric conditions prevent the vertical air movement necessary for rainfall. The presence of cold currents on the western coasts of continents in tropical and subtropical regions is a common factor in the aridity of these areas.