# Understanding Winds

The Atmosphere in Motion

Wind may be associated with some of the weather's most complex storms, but its beginnings couldn't 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 Earth's 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.

It's 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 didn't 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. (They're 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. It's 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 air's 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.

It's important to note that friction is only present within a few kilometers of Earth's 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 Scale
Invented 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 Scale
The Saffir-Simpson Scale describes the likely effects and property damage by a landfalling or passing hurricane based on the strength of a storm's maximum sustained wind speed. It separates hurricanes into five categories, from 1 to 5, based on winds.
• Enhanced Fujita Scale
The 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.

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