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.
Global Patterns
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.
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.
Global Patterns
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.