Have you ever wondered why the hottest and driest parts of the world are not at the equator, where the sun’s energy is the most concentrated, but are instead at around 30 degrees north and south - home to great deserts like the Sahara, Kalahari, Arabian, and Australian?

To understand this, we need to understand how the atmosphere moves on the global scale, and what provides the energy that drives that motion.

The driving force behind the movement of the air is energy from the sun’s radiation. This radiation is absorbed by the ground, and warms it. The warm ground then heats the air above the ground. This heat energy can be transferred into kinetic energy, causing the air to move, through the process of convection - warm air rises while cool air sinks.

The sun’s radiation is most concentrated near the equator, where the sun is directly overhead, so this is where this process is strongest.

Convection is the process behind localised thunderstorm clouds - read more about those in our explainer here. Convective thunderstorms are most common in the tropics, which is why these regions receive so much rainfall, hence the rainforests. 

The same physics also applies on much larger scales. This heat causes air to rise across the tropics as a whole - it rises up to 15 km above the surface of the Earth. At this height, the air stops rising and starts to drift towards the poles. Around 30 degrees North or South, the air starts to sink. This air is very dry - most of the moisture rained out by thunderstorms in tropical latitudes. Hence the deserts.

This study of how heat, work, and energy are linked is known as thermodynamics. These same ideas help physicists understand and predict the large-scale patterns of weather and climate around the world.

Hannah Christensen,
Associate Professor of Physical Climate,
University of Oxford