Understanding Winds The Atmosphere in Motion

Wind may be associated with some of the weathers most complex  storms, but its beginnings couldnt be simpler. Defined as the horizontal movement of air from one location to another, winds are created from differences in air pressure. Because unequal heating of the Earths surface causes these pressure differences, the energy source that generates wind is ultimately the Sun. After winds are started, a combination of three forces are responsible for controlling its movement--the pressure gradient force, the Coriolis force, and friction. The Pressure Gradient Force Its a general rule of meteorology that air flows from areas of higher pressure to areas of lower pressure. As this happens, air molecules at the place of higher pressure build up as they get ready to push toward the lower pressure. This force that pushes air from one location to another is known as the pressure gradient force. It is the force that accelerates air parcels and thus, starts the wind blowing. The strength of the pushing force, or pressure gradient force, depends on (1) how much of a difference there is in air pressures and (2) the amount of distance between the pressure areas. The force will be stronger if the difference in pressure is larger or the distance between them is shorter, and vice versa. The Coriolis Force If Earth didnt rotate, air would flow straight, in a direct path from high to low pressure. But because Earth rotates towards the east, air (and all other free-moving objects) are deflected to the right of their path of motion in the Northern Hemisphere. (Theyre deflected to the left in the Southern Hemisphere). This deviation is known as the Coriolis force. The Coriolis force is directly proportional to wind speed. This means that the stronger the wind blows, the stronger the Coriolis will deflect it rightward. Coriolis is also dependent on latitude. Its strongest at the poles and weakens the closer one travels toward 0 ° latitude (the equator). Once the equator is reached, the Coriolis force is nonexistent. Friction Take your foot and move it across a carpeted floor. The resistance you feel when doing this--moving one object across another--is friction. The same thing happens with wind as it blows over the surface of the ground. Friction from it passing over terrain--trees, mountains, and even soil--interrupts the airs movement and acts to slow it down. Because friction reduces wind, it can be thought of as the force that opposes the pressure gradient force. Its important to note that friction is only present within a few kilometers of Earths surface. Above this height, its effects are too small to take into account. Measuring Wind Wind is a vector quantity. This means it has two components: speed and direction. Wind speed is measured using an anemometer and is given in miles per hour or knots. Its direction is determined from a weather vane or windsock and is expressed in terms of the direction from which it blows. For example, if winds are blowing from the north to the south they are said to be northerly, or from the north. Wind Scales As a way to more easily relate wind speed to observed conditions at land and sea, and expected storm strength and property damage, wind scales are commonly used. Beaufort Wind ScaleInvented in 1805 by Sir Francis Beaufort (a Royal Navy officer and Admiral), the Beaufort scale helped sailors estimate wind speed without using instruments. They did this by taking visual observations of how the sea behaved when winds were present. These observations were then matched to the Beaufort scale chart, and the corresponding wind speed could be estimated. In 1916, the scale was extended to include land.The original scale is comprised of thirteen categories ranging from 0 to 12. In the 1940s, five additional categories (13 to 17) were added. Their use was reserved for tropical cyclones and hurricanes. (These Beaufort numbers are seldom used since the Saffir-Simpson scale serves this same purpose.)Saffir-Simpson Hurricane Wind ScaleThe Saffir-Simpson Scale describes the likely effects and property damage by a landfalling or passing hurricane based on the strength of a storms maximum sustained wind speed. It separates hurricanes into five categories, from 1 to 5, based on winds.Enhanced Fujita ScaleThe Enhanced Fujita (EF) Scale rates the strength of tornadoes based on the amount of damage their winds are capable of causing. It separates tornadoes into six categories, from 0 to 5, based on winds. Wind Terminology These terms are often used in weather forecasts to convey specific wind strength and duration. Terminology Defined as... Light and variable Wind speeds below 7 kts (8 mph) Breeze A gentle wind of 13-22 kts (15-25 mph) Gust A burst of wind that causes wind speed to increase by 10+ kts (12+ mph), then decrease by 10+ kts (12+ mph) Gale An area of sustained surface winds of 34-47 kts (39-54 mph) Squall A strong wind that increases 16+ kts (18+ mph) and maintains an overall speed of 22+ kts (25+ mph) for at least 1 minute