Global atmospheric circulation
Atmospheric circulation is the movement of air across the planet, and this occurs in a pattern. The Equator is the hottest part of the Earth. When air rises at the Equator, this leads to low pressure and rainfall. When the air reaches the edge of the atmosphere, it is unable to go any further and so it travels to the north and south. The air becomes cold and then falls, creating high pressure and dry conditions at around 30° north and south of the Equator. Large cells of air are created in this way. Air rises again at around 60° north and south and descends again around 90° north and south.
Global atmospheric circulation creates winds across the planet and leads to areas of high rainfall, like tropical rainforests, and areas of dry air, like deserts.
What is a tropical cyclone?
A tropical cyclone is a very powerful low-pressure weather system which results in strong winds (over 120 km/h) and heavy rainfall (up to 250 mm in one day). Tropical cyclones have different names depending on where they occur in the world. In the US and the Caribbean they are known as hurricanes, in South Asia - cyclones, in East Asia - typhoons and in Australia they are known as willy-willies. They all develop in the same way and have the same characteristics.