Air moves from cooler, high-pressure regions to warmer, low-pressure regions.

If Earth were not rotating, air circulation would be simple, with one pair of convection cells. See the image below, "Global circulation on a nonrotating Earth".

• Warm, low-pressure air would rise at the equator, and cool as it moved northward.
• Cooler, higher pressure surface air would move toward the low-pressure region at the equator.

However, Earth does rotate and atmospheric circulation is affected by theCoriolis effect.See the image below, "Global circulation on a rotating Earth".

• North-south moving air is deflected east-west (to the right in the Northern Hemisphere and to the left in the Southern Hemisphere).
• Warm, low-pressure equatorial air rises and moves away from the equator.
• This air is deflected, creating the trade winds.
• A second pair of convection cells develops at the edge of the equatorial convection cells, and a third pair of polar convection cells develops at the edge of those cells.

Where the three pairs of convection cells found on Earth meet, air moves primarily upward or downward.

• At the equator, air is rising.
• This rising air pulls surrounding air in with it. That means the air is moving in toward the warmer equator and upward.
• Around 30 degrees latitude, however, the cool air is moving downward. This means that air is moving outward at Earth's surface and downward.
• These two air movements, inward and upward air flow and downward and out air flow, help to drive the convection cells.