Wind is an important part of our atmosphere and determines much of the weather that we experience here on the surface of the Earth. Wind is caused by differences in air pressure. [Different temperatures cause different air pressure. Warm air creates a rising, low pressure zone and cold air creates a sinking, high pressure zone.] Wind travels from areas of HIGH pressure to LOW pressure. We name winds after the direction they are COMING from. For example, the global wind belt named "The Westerlies" comes from the West. The Polar Easterlies come from...you guessed it...the East!
There are two types of winds that we have talked about...
Local and Global
Global Winds
Global wind patterns: Winds are named by the direction from which they blow. The globe is encircled by six major wind belts, three in each hemisphere. From pole to equator, they are the polar easterlies, the westerlies, and the trade winds. Each belt occupies about 30 degrees of latitude, that is, one third of the way from the pole to the equator.
Polar Easterlies: From 60-90 degrees latitude, the Polar easterlies blow irregularly from the east and north.
Polar Front: Between the polar easterlies and the westerlies is the polar front. It is an area where to Low Pressures meet and it results in very little wind.
Prevailing Westerlies: At about the latitude of Western Europe and the U.S. (30-60 degrees), the Westerlies blow from the west, tending somewhat toward the north. This causes most weather in the United States to move from west to east.
Horse Latitudes: Where the Westerlies meet the trade winds at about 30 degrees (Jacksonville, Florida) is the Horse latitudes. This is a region of high pressure, dry air, and variable winds, and is associated with deserts over land. Sailors use to get caught in this area with its scarce winds and would lighten their load by throwing some of their horses overboard.
Trade Winds: South of about 30 degrees the northern or northeast trade winds blow mostly from the northeast toward the equator. These were the sailor's favorite winds, since the weather was warm, and the winds usually blew steadily in an advantageous direction. Columbus used these to sail to the Caribbean.
Doldrums: At about the equator is Intertropical Convergence Zone or doldrums, a region of light and irregular wind broken by occasional thunderstorms and squalls. Sailing ships are sometimes becalmed here for many days waiting for a proper wind.
Southern hemisphere: In the southern hemisphere the belts are reversed. The southeast trade winds blow from the southeast toward the equator. The southern equivalent of the horse latitudes (or Variables of Cancer) is called the Variables of Capricorn. The southern westerlies start somewhat south of South Africa. They tend to be stronger than the northern westerlies because they are mostly over water (roaring forties). The southern polar easterlies are mostly over Antarctica.
Local Winds
There are 4 types of local winds we have discussed in class. Sea breezes, land breezes, mountain breezes, and valley breezes. The same rule for naming applies to local winds. We name a breeze/wind after the direction it is coming from.
Sea Breeze
Sea breezes are the result of differential heating of the land and the sea. During the day, sand heats up more quickly than the sea water. Through conduction, the land heats up the air touching it. The air is warmed, becomes less dense, and begins to rise. As the warm air rises, cool air from over the sea water moves in to replace it. This creates a breeze that moves in from the sea. See the top of the diagram below.
Land Breeze
Land breezes are the result of differential cooling of the land and sea during the night. At night, the sand cools faster than the sea water. As a result, the air over the sand cools and begins to sink. Air over the water stays warmer and begins to rise. This pulls the cold air from the land out towards the ocean to replace the rising air over the sea water. This creates a breeze that comes from the land. See the bottom of the diagram above.
Valley Breeze
In places such as Orem, where there are mountains and slopes nearby, valley breezes are created during the day. During the day, when the sun is out, the slope of the mountains heats up more quickly than the valley. As the slopes become hotter they heat up the air above them and this air begins to rise. The air in the valley stays cooler and a wind moves from the high pressure (valley) to the low pressure (mountain).
Mountain Breeze
During the night the mountain slopes lose heat more quickly than the valley. The air over the slopes cools quickly and begins to sink. Meanwhile, the valley stays warmer for longer and the heated air begins to rise. This creates a wind moving down the mountains and into the valley. We call this a mountain breeze.