1. Hadley Cell Circulation:
The Earth's atmospheric circulation consists of three major circulation cells: the Hadley cells, Ferrel cells, and Polar cells. The Hadley cells are the largest and most influential circulation cells, dominating the tropical regions. They consist of rising air at the equator, flowing poleward at higher altitudes, descending around 30 degrees latitude, and returning toward the equator near the surface.
2. Subtropical Deserts:
The descending air in the subtropical regions around 30 degrees latitude creates high pressure zones and stable atmospheric conditions. This subsidence of air leads to a lack of cloud formation and precipitation, resulting in the formation of deserts. As the descending air warms as it approaches the surface, it further inhibits the formation of clouds and precipitation.
3. Horse Latitudes:
The regions around 30 degrees latitude are often referred to as the "horse latitudes" due to the prevalence of calm winds and sailing ships getting stuck in these regions in the past. The lack of strong winds prevents the accumulation of moisture and the formation of significant rainfall.
4. Rain Shadow Effects:
In addition to the subtropical circulation patterns, some deserts are also influenced by rain shadow effects. When moist air rises over a mountain range, it cools and condenses, releasing precipitation on the windward side of the mountains. As the air descends on the leeward side of the mountains, it becomes warmer and drier, creating a rain shadow desert.
For example, the Atacama Desert in South America is located on the leeward side of the Andes Mountains, which block the moisture-carrying winds from the Pacific Ocean.
It's worth noting that while many major deserts occur around 25 degrees north or south due to these factors, there are exceptions to this rule. Some deserts, such as the Sahara Desert, are located at lower latitudes due to additional influences such as continental size and proximity to large landmasses.
