Wind

Wind is the horizontal movement of air resulting from differences in atmospheric pressure and air densities. Pressure differences may develop on a local or global scale in response to differences in the distribution of solar energy, which affect the density of air masses and, therefore, the pressure they exert relative to each other.

Factors Affecting Wind Flow

Wind, as defined by meteorologists, is the horizontal movement of air. Differences in heating and internal motion in the atmosphere create differences in atmospheric pressure. When a change in pressure over distance is established, air accelerates down this pressure gradient from higher to lower pressure. The acceleration of this moving air depends on the amount of pressure change over a given distance.

Moving air associated with pressure change over a distance will either spread out over the surface (diverge) or will flow inward (converge). High-pressure areas are regions of divergence, and low-pressure areas are regions of convergence. The force associated with the air moving from high to low pressure is called the pressure gradient force. Pressure gradient force sets the wind into motion. If it were the only force affecting the wind, then winds would blow directly from high to low pressure. However, other forces affect wind direction and velocity.

A second major factor affecting wind flow is Coriolis acceleration, which results from the Earth turning on its axis. An object moving over the surface of the Earth, except at the equator, moves in a curved path when observed from the rotating Earth. In the Northern Hemisphere, there is an acceleration to the right of the path of motion. In the Southern Hemisphere, the acceleration is to the left. Thus, in the Northern Hemisphere, a wind blowing from north to south becomes a northeast wind, and a wind blowing from south to north becomes southwesterly. The reverse occurs in the Southern Hemisphere.

A third force affecting wind flow is centripetal acceleration. Air currents seldom move on a straight path for long but tend to develop a curved pattern as they flow parallel to curved isobars. When this type of flow pattern evolves, centripetal acceleration is directed into the center of the cell or curve, the force acting perpendicularly to the direction of flow. This acceleration is directed outward from both high- and low-pressure cells in the equally opposite sense. Therefore, airflow is affected by high pressure; centripetal acceleration is in the opposite direction from that around low pressure. Thus, air movement about a low-pressure center is cyclonic or counterclockwise in the Northern Hemisphere, and anticyclonic or clockwise is about a high-pressure center. Centripetal acceleration plays a more significant and immediate role in smaller circulations, such as hurricanes and tornadoes than in larger, midlatitude cyclones.

A fourth factor affecting wind velocity and direction is frictional drag, which works in a direction opposite to wind motion. Therefore, friction tends to slow wind velocity. A decrease in wind velocity, however, is accompanied by a decrease in Coriolis acceleration, which causes a slight change in wind direction back toward the direction of the pressure gradient. The effect is inherent in fluid dynamics. A fluid, whether gas or liquid, flowing without restraint of any kind exhibits the property of laminar flow in which every particle of which the fluid is composed moves in unison. The presence of any kind of containing surface exerts a restraining force upon the flowing particles nearest to it as they move, thus disrupting their unity of movement. This, coupled with interactions between the particles themselves, results in the condition of turbulent flow, in which particles at different distances from the surface move at different rates. Air moving over the surface of the planet is constrained at the surface by the surface itself, while the density of the fluid decreases with altitude. Horizontally, there are no surface constraints to affect the flow of air, other than minor differences in density and pressure (the pressure gradient force). Frictional drag is thus at a maximum over land where an uneven surface consisting of trees, buildings, and hills provides barriers to the even flow of wind. Also, friction affects the flow of wind only in the first or second kilometers of the atmosphere. Wind direction and velocity in the lowest kilometer of the atmosphere are based on the sum of pressure gradient acceleration, Coriolis acceleration, centripetal acceleration, and frictional drag.

Above one kilometer, winds blow in response to pressure gradient, Coriolis, and centripetal acceleration. Frictional deceleration is negligible or completely absent. Consider the situation in which pressure is distributed in a linear fashion so that lines connecting points of equal pressure are straight. In this situation, pressure gradient acceleration and Coriolis acceleration are the only forces acting on the wind. Here, pressure gradient force is balanced by Coriolis acceleration so that the wind flows in a direction parallel to the isobars. Such winds are called geostrophic winds. Around circular highs and lows above the friction level, pressure gradient acceleration is balanced by both Coriolis and centrifugal acceleration. Thus, winds blow parallel to isobars in a clockwise direction around highs and in a counterclockwise direction around lows. In the Southern Hemisphere, the reverse is true. The winds thus described are called gradient winds.

Monsoon and Pressure Changes

A seasonal wind system, called the monsoon, that changes direction from winter to summer, exists over eastern Asia and the adjacent oceans and less significantly at other locations worldwide. In winter, the air is cooled over Asia's large landmass, and a cold, dense, high-pressure center forms with a clockwise circulation of winds. Generally, these winds flow from land to sea during winter. In summer, a thermal low forms over India, and the airflow pattern reverses, with cyclonic flow bringing air onshore from the ocean. Reinforcing the thermal low is the migration of the Intertropical Convergence Zone northward over India. Moreover, the jet stream breaks down during summer, reinforcing the monsoon flow. With this annual wind-flow reversal, the climate of Asia is greatly affected. The offshore winds in winter bring dry weather to much of eastern Asia. Conversely, summer onshore winds bring copious amounts of precipitation to India and adjacent areas of southeast Asia.

Daily changes in temperature at many places worldwide result in daily pressure changes, which cause distinctive wind patterns. One such system is that of the land and sea breezes. This system develops along coastal areas and the shorelines of large lakes and inland seas. During the day, as the land heats rapidly, the air above heats up, expands, and becomes less dense, forming a thermal low. The warm, buoyant air rises, and cooler air from the water surface flows in to replace the rising air. In this fashion, a sea breeze develops during the day and usually peaks in mid-afternoon, when the daily high temperature is attained. At night, conditions are reversed. The land cools more rapidly than water. In this way, the pressure relationship between land and water is reversed day to night. At night, pressure is higher over land and lower over water, so air flows from land to water, producing a land breeze. The land breeze is usually not as well developed as the sea breeze because the temperature contrast between land and water is not as great at night as during the day. Another wind system with a day-to-night change in wind direction is that of the mountain and valley breezes. During the day, the mountain slopes warm the air, and it expands. The warm, less dense air rises and is called a valley breeze after its place of origin. At night, the slopes cool, and the air’s density increases. The cool, dense air flows downslope in response to gravity and is called a mountain breeze.

Local Winds

Several local winds occur in response to topographic peculiarities and are difficult to explain based on pressure patterns as they might appear on a weather map. The “chinook” in the Rockies and the “foehn” in the Alps of Europe result from a combination of topographic effects and large-scale atmospheric systems. In response to these systems, winds flow down the lee side and are heated adiabatically by compression. The warming brought by these winds is often rapid.

The “sirocco” (“khamsin” in Egypt and “sharov” in Israel) and the “haboob” are hot, dusty winds that occur on flat terrain. The sirocco precedes a low-pressure system moving across the Sahara Desert. As it crosses the Mediterranean Sea, it picks up moisture and becomes a hot, humid wind when it reaches the coast of Europe. The haboob is created by air spilling out of the base of a thunderstorm and attains high speeds and picks up small soil particles, creating a sand storm extending upward as high as one kilometer or higher.

A katabatic wind is a cold wind flowing downslope from an ice field or glacier. Wind velocities range from as little as ten knots up to hurricane speeds. One such wind is the “bora,” which originates in Russia and blows out across the Adriatic coast of Yugoslavia with speeds sometimes in excess of 100 knots. In France, a wind known as the “mistral” blows out of the French Alps and through the Rhone Valley to chill the Riviera along the Mediterranean Sea.

Study of Wind

Several instruments are used to collect data about wind direction and velocity at the surface or in the upper troposphere. The wind vane is commonly used to determine the direction of surface wind. Most wind vanes are simple, relatively long planar structures that will self-align to the direction of movement of the wind, such as an arrow with a tail. The arrow, or other type of vane, is attached to a vertical pole about which it can move freely and always points in the direction from which the wind is blowing.

The anemometer is an instrument used to record wind velocity. It normally consists of three hemispherical cups attached to crossbars, which are, in turn, attached to a vertical shaft about which it can spin freely. The cups are pushed by the wind preferentially on their open side, causing the shaft to turn, and a counting device records the wind speed at the base of the shaft. An instrument used for recording both wind direction and velocity is the aerovane. It consists of a three-bladed propeller mounted on the end of a streamlined rod, with a vertical fin at the opposite end. The propeller rotates at a rate proportional to the wind speed. The fin and aerodynamic shape keep the propeller blades facing into the wind, so wind direction is easily determined. When a recorder, often remote, is connected, a continuous record of wind velocity and direction can be obtained.

A series of instruments also have been developed to determine wind directions and velocities at higher levels. One is the pilot balloon, a small balloon released at the surface that rises at a known rate. The balloon is tracked using a small telescope called a theodolite, and periodic measurements of the balloon’s horizontal and vertical angles are taken, giving the speed and direction of the winds carrying it. The pilot balloon principle can also be applied to a radiosonde ascent. Measurements of the vertical and horizontal angles tracking the radiosonde’s ascent, taken periodically along with its distance from the observing station, can supply information on wind direction and speed.

A radiosonde can be tracked using radar, so wind speed and direction can be obtained. Radar can also be used in conjunction with rockets to collect wind data at a distance above thirty kilometers. One type of rocket ejects a parachute carrying an instrument package, which can be tracked by radar. Another type of rocket ejects metallic strips at predetermined levels that can also be tracked by radar. Doppler radar can be used to determine the direction and speed of wind by using the Doppler effect on reflected signals. Doppler radar measures the speeds of objects moving toward or away from the antenna. When a signal is sent out and reflected from a raindrop or ice crystal, the returning signal will have a higher frequency if the particle moves toward the radar and a lower frequency if it moves away. One drawback of Doppler radar is that velocities of objects at right angles to the unit cannot be determined, so to achieve a three-dimensional effect, two or more units must be used.

Wind directions and speeds are plotted on charts using the wind arrow symbol. The shaft of an arrow shows wind direction, while barbs on the end of the arrow indicate speed. A barb represents ten knots, one-half barb five knots, and a flag (a triangle-shaped symbol) represents fifty knots. These symbols may be used singularly or in combination to show any wind speed. On a surface chart, the wind arrows point out from a station in the direction from which the wind is coming. In the upper air above the friction level, the wind arrow points in the direction the wind is moving.

Winds above the friction level are plotted on constant pressure charts. A constant pressure chart is drawn using contour lines to show the elevation above the Earth’s surface of a constant pressure level, such as the 500-millibar level. When pressure is particularly high in an area concerning surrounding areas, the height of a constant-pressure surface is higher than surrounding areas, and the heights of a low-pressure region are lower than surrounding regions. The average elevation of the 500-millibar level is 5.5 kilometers but can vary from less than five to more than six kilometers.

Various constant pressure charts are used, ranging from just above the surface, such as the 850-millibar level, to the tropopause at roughly the 200-millibar level. These charts allow meteorologists to gain a sense of the three-dimensional wind-flow profile from the surface up to the tropopause.

Significance

In conjunction with temperature and humidity, winds can greatly affect human comfort and safety. The effects of wind influence the exchange of heat between the human body and the atmosphere. The body, particularly the skin's surface, is continually exchanging heat with the environment. On a cold, windy day, air molecules impact the skin, then move away, taking body heat with them. Clothing provides insulation, creating a shallow layer of warm air molecules, which form a shield that protects the skin from heat loss. The “wind chill factor” relates the rate of heat loss due to wind action to the temperature, which has the equivalent rate of heat loss in the absence of wind.

Humankind’s use of wind power may stem from using winds to propel sailboats or ships. Sails have been used as power sources on ships and boats for thousands of years and were the chief power source for water transportation until the use of steam in the latter part of the nineteenth century. The next step in the use of wind for power was windmills. The first known windmill appeared in Europe in 1105, and by the following century, thousands of windmills were in use in Europe. As burning coal as a power source became less expensive, it and other energy sources replaced windmills. In the early twentieth century, windmills became popular as an inexpensive means of pumping water for agricultural uses on farms and ranches.

Modern wind power is a partial solution to growing global energy needs. The advantages of using wind power are that windmills are nonpolluting and not limited to daylight hours like solar cells. Nearly all wind power in the twenty-first century is produced in wind farms by wind turbines. The liabilities of using wind power are numerous. Windmills can only be used in windy areas where wind flow is steady and neither too weak nor too strong. A weak wind will not turn the blades, and a strong wind might damage the machine. Windmills detract from the aesthetics of the landscape, and their cost can be quite high. Finally, the amount of modern energy needs that could be satisfied by wind power is quite low, around 2 percent.

Principal Terms

constant pressure chart: a chart that shows the altitude of a constant pressure, such as 500 millibars

convergence: the movement of different air masses flowing toward a common point

divergence: a net outflow of air in different directions from a specified region

geostrophic wind: an upper-level wind that flows in a straight path in response to a balance between pressure gradient and Coriolis acceleration

hurricane-force wind: a wind with a speed of 64 knots (118 kilometers per hour) or higher

isobar: a line on a meteorological chart delineating points of equal pressure,

local winds: winds that, over a small area, differ from the general pressure pattern owing to local thermal or orographic effects

pressure gradient: the rate of change of pressure with distance at a given time

rawinsonde: a radiosonde tracked by radar to collect wind data in addition to temperature, pressure, and humidity

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