An downdraft is the vertical (up and down) movement of air falling down towards earth. A mass of warm air will typically be less dense than the surrounding region, and so will rise until it reaches air that is either warmer or less dense than itself. The converse will occur for a mass of cool air, and is known as subsidence. This movement of large volumes of air can create large clouds and is the main cause of thunderstorms.
The term downdraft can also refer to a type of backdraft that occurs through chimneys located on the lower levels of buildings and involves cold air coming down the chimney due to low air pressure inside the building.