Air in Motion
The energy that drives wind originates with the sun, which heats the Earth unevenly, creating warm spots and cool spots. Two simple examples of this are sea breezes and land breezes.
Sea breezes occur when inland areas heat up on sunny afternoons. That warms the air, causing it to rise. Cooler air rushes in from the ocean to take its place and presto, a wind is born. By late afternoon, a strong breeze can be blowing dozens of miles inland. A similar effect can occur near big lakes, where the wind is referred to as a lake breeze.
Land breezes come at night, when inland temperatures drop enough that the ocean is now warmer than the land, reversing the effect.
Similar forces produce global wind patterns that affect climate. The tropics, for example, are always hot. Air rises here and spreads north and south, high above the land. Lower down, air is pulled in from the north and south. The coriolis effect, an offshoot of the Earth's rotation, makes moving air masses curve, so that the winds converging on the Equator come from the northeast in the Northern Hemisphere and the southeast in the Southern Hemisphere. These winds are called the trade winds.
Farther from the Equator, the surface winds try to blow toward the Poles, but the coriolis effect bends them the opposite direction, creating westerlies. This is why so many weather events in the United States come from the west.
At latitudes higher than about 60°, cold surface winds try to blow toward the Equator, but, like the trade winds, they are bent by the coriolis effect, producing polar easterlies.
Highs and Lows
Within the mid-latitudes, weather effects create high- and low-pressure zones, called highs and lows, respectively. Air moves from areas of high pressure to low pressure. As it moves, however, it spirals due to the coriolis effect, producing the shifting winds we experience from day to day, as highs and lows drift under the influence of the prevailing westerlies.
Winds reaching the center of a low-pressure area have nowhere to go but up. This causes moisture to condense into clouds, producing storms. At the center of high-pressure areas, dry air descends from above, producing fair weather.
On a smaller scale, colliding wind patterns can produce convergence, in which air also has nowhere to go but up. If one of the winds is a humid flow from a warm ocean such as the Gulf of Mexico, the result can be powerful thunderstorms and tornadoes.