Wind is literally air in motion. Because air is free to move, it very readily reacts to forces exerted upon it. The primary driving force for the wind is a difference in atmospheric pressure along a horizontal surface.
Atmospheric pressure is the weight of the column of air above an area on the surface. Though invisible (except when there are clouds), air has weight. Differences of the atmospheric weight or pressure exist because of variances in density of the air (cold, dry air weighs more than warm, moist air for example) and in depth of the column (certain upper level flows pile up air in one place, others remove air). The result is that on the surface there are areas of high and low pressure.
For example in Illustration A, a column of air which is cold and dense results in higher pressure at the surface than where a column of the same depth is filled with warmer and thus less dense air. These cold high pressure systems develop, not surprisingly in the polar or arctic regions and are called polar highs or arctic highs.
Higher pressure can also result if the column is higher in one region than another. This piling on of air as shown in Illustration B, occurs in the sub-tropics in both hemisphere. This is a zone of high pressure known as the sub-tropical high. The so-called Bermuda High which brings hazy, hot, and humid conditions to the eastern United States in summer is an example of this kind of high pressure system.
When you have a high adjacent to a low pressure at the surface, air will move from the high to the low. This is an attempt on the part of the atmosphere to restore an equilibrium (eliminate the pressure differences). The rotation of the earth on its axis adds an apparent deflection (to the right in the Northern Hemisphere, to the left in the Southern Hemisphere). This is a force we in meteorology call the coriolis force.
The surface of the earth, especially where the surface is rough due to terrain, trees or buildings, interferes with the movement of air (a force called friction). The net result of all these forces is that air tends to blow clockwise and out from high pressure and counter-clockwise and in towards low pressure.
|The wind at the surface around both low and high pressure systems are shown in this actual map analysis. The wind (green streamlines) can be seen to spiral out in a clockwise rotation around a high pressure centered over Minnesota while the wind blows spirals in a counterclockwise way into a low pressure seen just northwest of Maine.
Winds are usually strongest in the winter when high and low pressure systems are strongest. When strong winds occur in summer, they are usually associated with tropical storms or are localized near thunderstorms. Winds tend to be stronger and gustier during the day than at night because the suns heating during the day causes vertical currents of air that can mix down higher speed air from aloft. During clear nights, the surface cools and the air becomes more stable. The air becomes more sluggish and steady near the ground. When winds at night are strong and gusty, it usually means cold air moving in over warmer ground (as behind a strong cold front).
THE IMPORTANCE OF WIND
Aside from the damage that strong winds can do, the winds play a number of important roles in our weather. Most importantly, the winds transport air from one location to another. Air, as it travels, tends to maintain many of the properties it had at the source in transit. That is why a southerly wind from the Gulf of Mexico is warm when it blows through the arch in St. Louis and a northerly wind is cold when it blows down Michigan Avenue in Chicago. However, air is slowly modified enroute. The cold air is warmed by contact with the warm surface and is less cold by the time it reaches Jacksonville, Florida. The warm air loses some of its warmth and moisture through radiational cooling and condensation before it reaches Boston.
Winds change direction and speed constantly as weather features move and change strength.
Winds also tend to increase with height. Winds outside of hurricanes, tornadoes and strong winter storms usually average less than 20 mph at the surface. High up, where the jet airplanes fly (at 30,000 feet or above) the winds sometimes reach 200 mph. This is called the jet stream. The winds here are strongest in regions where the temperature contrast in lower levels is strongest. That is usually along what is known as the polar front, the boundary between cold polar air and warm tropical air.
These high level winds steer low level features (highs and lows) and help bring about the day-to-day changes in our weather.
SOME LOCAL WINDS
THE SEA OR LAKE BREEZE
The sea breeze or lake breeze is an important climate modifier in many shore-line locations. It occurs during the warmer months on sunny days. The sun warms the land much faster than the water bodies.
Since cooler air is denser and heavier than warm air, high pressure tends to form over the water and low pressure over the land. Air begins to flow inland cooling the temperature. At night, the land cools more than the water and the reverse pressure and offshore wind flow tends to occur (called a land breeze).
|This high resolution forecast model shows the afternoon sea breeze developing along the southern and eastern coasts of New England. The background color bands represent wind speeds while the black lines represent instantaneous wind flow directions. Note the relatively light west and northwest wind regime shown in inland areas. Along the south coasts, the winds turn onshore at increased speeds due to the thermal differences from land to sea. This is one of the reasons we can travel to the seashore to "cool-off" on a hot summers day.
The winds tend to increase with elevation. Thus it is not surprising that when you climb up mountains you often find stronger winds. Local funneling effects can create very strong winds in certain locations. Also, in mountain valley regions, a circulation not unlike the sea/land breeze occurs. During the day as the mountain slopes are heated by the sun the air rises and draws air up from the valley beneath. If the valley slopes in elevation, winds may blow up the valley as well in the absence of strong weather systems.
At night as air cools rapidly along the slopes, it becomes heavy and sinks downslope (and down-valley).