On Earth, the density of air depends on its temperature. This means that warm air rises and cold air sinks because warm air is less dense than cold air. This movement of warm air upwards is known as convection. Convection is one of the processes that allows clouds to form. The Sun warms the surface of the Earth. This warmth heats the humid air at the ground and, as a result, the air becomes less dense and begins to rise. As the air rises, it cools. Clouds are formed when the humid air cools below a critical temperature: the water then condenses onto tiny suspended particles and forms water droplets in the air.