Hadley Circulation: according to 
Near the poles, heat lost to space by radiation exceeds the heat gained from sunlight, so air near the poles is losing heat. Conversely, heat gained from sunlight near the equator exceeds heat losses, so air near the equator is gaining heat. The heated air near the equator expands and rises, while the cooled air near the poles contracts and sinks. The combination of these two processes sets up a general circulation pattern: air rises near the equator, flows north and south away from the equator at high altitudes, sinks near the poles, and flows back along the surface from both poles to the equator. This type of flow is called Hadley circulation after George Hadley, an English meteorologist of the eighteenth century, who first described the process.
- definition at Wolfram Research