The air across the planet moves in a specific way. This movement starts at the equator, the hottest part of the Earth. Due to high temperatures at the equator, the air rises up into the atmosphere. This creates low pressure (as the air is rising it puts less pressure on the Earth’s surface). As the air rises it becomes colder, causing condensation (forming clouds) that leads to rainfall. This is why tropical rainforests are found along the equator!
Global atmospheric circulation model
When the air reaches the top of the atmosphere it needs somewhere to go. Some of the air travels north and some south of the equator. The air cools and then sinks at around 30° north and south of the equator. As the air is sinking this creates high pressure. As moisture in the sinking air fell at the equator it is dry so few clouds form here. This is why deserts are found along 30° north and south of the equator.
The movement of air between the equator and 30° north and south is known as the Hadley Cell. Air rises again at around 60° north and south and descends again around 90° north and south forming the Ferrel and Polar Cell.
Global atmospheric circulation creates winds across the planet as air moves from areas of high pressure to areas of low pressure. It also leads to areas of high rainfall, like the tropical rainforests, and areas of dry air, like deserts.