WIND

Weather Info Links Temperature Clouds Precipitation Wind Air Masses & Fronts Weather Forecasting Air Pollution Climate Change

an anemometer is used to measure wind speed and direction

giant windmills are used to obtain energy from wind

Where does the wind come from? In general terms, wind is created by the solar cycle through the uneven warming and cooling of the earth's surface. As the sun warms the land, air above the land is also warmed. This air rises and cooler air rushes in to replace it, producing a gentle breeze - or a howling tempest.

Large-scale winds are caused by the fact that the earth's surface is heated to a greater degree at the equator than at the poles. The rotation of the earth also affects these planetary winds. On a smaller scale, winds flow through mountain valleys and spill over high peaks across unobstructed prairies.

The first breath of wind starts with temperature. Notice on a summer day how heated air over a roadway shimmers as it rises. Open a freezer door and the ensuing condensation tracks the descent of cold air as it sinks to the floor. These different characteristics, rising warm air, sinking cold air and uneven heating of the Earth's surface are the principal mechanisms of atmospheric circulation (wind).

Wind strength is dependent upon the pressure field and is highest when pressure differences are the greatest. For example, a deepening low pressure system advancing across the mid latitudes in winter will usually generate strong winds in advance.

The lower the pressure, the greater the likelihood of gale force winds. In the Northern Hemisphere winds blow inward and counterclockwise about a low pressure center. Winds associated with a high pressure system blow slightly outward from the center and in a clockwise fashion. A Dutch meteorologist named Buys Ballot formulated a technique in 1857 to determine the location of pressure centers. With your back to the wind, pressure to your left is lower. In the Southern Hemisphere, the opposite is true.

The sea breeze is a classic example of the thermal-pressure relationship. Sea breezes occur when the land mass next to the water area heats up at a much greater rate. This results in lower pressure over land while pressure over water is higher. The sea breeze is the ensuing flow of air from high to low pressure. The strength of the sea breeze depends upon the difference between the temperature of the water and the adjacent coastal area. At night, the land cools more quickly than the water resulting in a reversal of wind flow from land to water.

Wind is also responsible for altering the water surface of oceans and lakes. Moving air in contact with water causes it to pile up in ridges, much like sand dunes. The height of the ridges -- waves -- depends upon the strength of the wind. The characteristics of a ocean wave as depicted in the graphic include its height and length.

The height of a wave is the vertical distance between the crest and the trough; the length is the distance between successive crests or troughs. The time interval between passage of successive crests at a stationary point is called the wave period.

Small wavelets or ripples will appear as soon as a breeze of 3 km/h flows across the water surface. Whitecaps begin to form on top of waves when winds reach about 25 km/h. If the wind velocity increases to 80 km/h for an extended period it is possible for waves of 15 to 20 feet to develop. Waves whipped up by strong winds impart tremendous energy which can seriously damage beaches, piers, and breakwaters. A moderate storm at sea can send ashore as many as 600 waves an hour. The energy of each wave can be equal to a force of one ton slamming against every foot-length of beach obstruction.

Copyright © Malta Weather Services 2003