The movement of air through Earth's -- or any planet's -- atmosphere is called wind, and the main cause of Earth's winds is uneven heating by the sun. This uneven heating causes changes of atmospheric pressure, and winds blow from regions with high pressure to those with low pressure. This happens on a local scale to produce storms as well as gentle breezes, and it happens on a planet-wide scale to produce global wind patterns.
Atmospheric Pressure
Winds are a result of the tendency of warm air to rise and, conversely, cool air to sink. When the sun's energy heats up a part of the ground or the sea and the heat radiates into the air, the air rises, creating a low pressure area underneath it. Because the atmosphere constantly flows, like a fluid in a container, air rushes into such low pressure areas from cooler, high-pressure ones, and the movement of the air creates wind. The strength of the wind depends on the pressure difference. A small difference produces a gentle breeze, while a large difference may produce a gale.

The Trade Winds
The sun heats the equator more than it does the poles, so there is a perennial movement of air between these two regions. Hot air at the equator rises into the troposphere at the equator, creating a low pressure area underneath it that is filled by cooler air from higher latitudes. The hot air cools as it moves toward the poles, and at about 30 degrees north and south latitude, it cools, sinks and circulates back to the equator as the west-moving trade winds. This air pattern produces dry conditions at 30 degrees latitude, where many of the world's deserts are located.

The Jet Streams
Because the air at the poles is cooler than the air at lower latitudes, it has a tendency to move toward the equator. It doesn't move in a straight line, however, because the Earth's rotation exerts a tangential force called the Coriolis force. The effect of this force isn't straightforward. It bends winds at the poles toward the west, but between 30 and 60 degrees latitude, it bends them in the opposite direction, creating the westerlies. In the upper troposphere, the westerlies can blow at speeds of 160 kilometers per hour (100 miles per hour). These upper tropospheric winds are known as the jet streams.

Local Winds
Topography and ground composition are two of the factors that can influence local wind patterns. For example, beach sand radiates more of the sun's heat back into the atmosphere than seawater, so people at the sea shore usually enjoy a sea breeze on hot days. In mountainous areas where the tops of the mountains receive more sunlight than the lowlands, there is often an updraft along the mountain slopes. Such drafts are an important factor in the spread of forest fires because air from cooler regions rushes to replace the air displaced by the heat of the fire, creating a wind that helps the fire to spread.