Convection

Convection is the transfer of heat by movement of a fluid such as air. Convection happens in the atmosphere when air near the Earth's surface is heated. This heating causes the air to expand, become less dense than the surrounding air, and rise. The heated air is buoyant and transports heat energy upward.

Convection in the atmosphere. When an air parcel is warmer than its environment, it will rise. Rising air always cools, but as long as the rising air parcel cools slowly enough that it continues to be warmer than its surrounding environment, it will continue to rise, just like a hot air balloon. Free convection can lead to (or contribute to) cloud and thunderstorm development. Copyright Richard Yablonsky, URI/GSO.

As an air parcel rises, it cools and expands until it becomes saturated with water vapor. At this point, the water vapor in the air parcel condenses to form cloud droplets. Condensation releases the heat energy (latent heat) stored in the water vapor, increasing the buoyancy of the air. The air parcel will continue to rise until it cools to the point that it is colder than the surrounding environment. Convection can form large clouds and thunderstorms, if there is enough initial heat and water vapor to allow the air parcels near the Earth's surface to rise all the way to the tropopause.