Section 8.2:
Factors Affecting Wind

Section Content

Earlier this semester, we examined the upward movement of air and its role in cloud formation. As important as vertical motion is, far more air moves horizontally, the phenomenon we call wind. What causes wind?

Simply stated, wind is the result of horizontal differences in air pressure. Air flows from areas of higher pressure to areas of lower pressure. You may have experienced this when opening something that is vacuum-packed. The noise you hear is caused by air rushing from the higher pressure outside the can or jar to the lower pressure inside. Wind is nature’s attempt to balance such inequalities in air pressure. Because unequal heating of Earth’s surface generates these pressure differences, solar radiation is the ultimate energy source for most wind.

If Earth did not rotate, and if no friction existed between moving air and Earth’s surface, air would flow in a straight line from areas of higher pressure to areas of lower pressure. But because Earth does rotate and friction does exist, wind is controlled by a combination of three factors: pressure gradient force, Coriolis effect, and friction.

Pressure Gradient Force

The force that generates winds results from horizontal pressure differences. When air is subjected to greater pressure on one side than on another, the imbalance produces a force that is directed from the region of higher pressure toward the area of lower pressure. Thus, pressure differences cause the wind to blow, and the greater these differences, the greater the wind speed.

Variations in air pressure over Earth’s surface are determined from barometric readings taken at thousands of weather stations. These pressure measurements are shown on surface weather maps using isobars (iso = equal, bar = pressure), or lines connecting places of equal air pressure (▼). The spacing of isobars indicates the amount of pressure change occurring over a given distance, which is called the pressure gradient force. Pressure gradient is analogous to gravity acting on a ball rolling down a hill. A steep pressure gradient, like a steep hill, causes greater acceleration of a parcel of air than does a weak pressure gradient (a gentle hill). Thus, the relationship between wind speed and the pressure gradient is straightforward: Widely spaced isobars indicate a weak pressure gradient and weak winds; closely spaced isobars indicate a steep pressure gradient and strong winds. The figure below (▼) illustrates the relationship between the spacing of isobars and wind speed. Note also that the pressure gradient force is always directed at right angles to the isobars.

Isobars show the pressure gradientIsobars are lines connecting places of equal air pressure. The spacing of isobars indicates the amount of pressure change occurring over a given distance—called the pressure gradient. Widely spaced isobars indicate a weak pressure gradient and low wind speeds, whereas closely spaced isobars indicate a strong pressure gradient and high wind speeds.

In order to draw isobars on a weather map to show air pressure patterns, meteorologists must compensate for the elevation of each station. Otherwise, high-elevation locations, such as Denver, Colorado, would always be mapped as having low pressure. This compensation is accomplished by converting all pressure measurements to sea-level equivalents.

The figure below (▼) is a surface weather map that shows isobars (representing corrected sea-level air pressure) and winds. Wind direction is shown as wind arrow shafts, and speed is shown as wind bars (refer to the key accompanying the map in the figure). The isobars on such maps are rarely straight or evenly spaced. Consequently, wind generated by the pressure gradient force typically changes speed and direction as it flows.

Isobars on a weather mapIsobars are used to show the distribution of pressure on daily weather maps. Isobars are seldom straight but usually form broad curves. Concentric isobars indicate cells of high and low pressure. The “wind flags” indicate the expected airflow surrounding pressure cells and are plotted as “flying” with the wind (that is, the wind blows toward the station circle). Notice on this map that the isobars are more closely spaced, and the wind speed is faster around the low-pressure center than around the high.

The area of somewhat circular closed isobars in eastern North America, represented by the letter L, is a low-pressure system. In western Canada, a high-pressure system, denoted by the letter H, can also be seen. We will discuss highs and lows in the next section.

In summary, the horizontal pressure gradient is the driving force of wind. The magnitude of the pressure gradient force is shown by the spacing of isobars. The direction of force is always from areas of higher pressure toward areas of lower pressure and at right angles to the isobars.

Coriolis Effect

The previous figure (▲) shows the typical air movements associated with high- and low-pressure systems. As expected, the air moves out of the regions of higher pressure and into the regions of lower pressure. However, the wind does not cross the isobars at right angles, as the pressure gradient force directs it to do. The direction deviates as a result of Earth’s rotation. This has been named the Coriolis effect, after the French scientist who first thoroughly described it.

All free-moving objects or fluids, including the wind, are deflected to the right of their path of motion in the Northern Hemisphere and to the left in the Southern Hemisphere. The reason for this deflection can be illustrated by imagining the path of a rocket launched from the North Pole toward a target located on the equator (▼). If the rocket took an hour to reach its target, during its flight, Earth would have rotated 15 degrees to the east. To someone standing on Earth, it would look as if the rocket had veered off its path and hit Earth 15 degrees west of its target. The true path of the rocket is straight and would appear so to someone out in space looking at Earth. It is Earth turning under the rocket that creates the apparent deflection.

Coriolis effectThe deflection caused by Earth’s rotation is illustrated by a rocket traveling for 1 hour from the North Pole to a location on the equator. A. On a nonrotating Earth, the rocket would fly straight to its target. B. However, Earth rotates 15 degrees each hour. Thus, although the rocket travels in a straight line, when we plot the path of the rocket on Earth’s surface, it follows a curved path that veers to the right of the target.

Note that the rocket is deflected to the right of its path of motion because of the counterclockwise rotation of the Northern Hemisphere. In the Southern Hemisphere, the effect is reversed. Clockwise rotation produces a similar deflection but to the left of the path of motion. The same deflection is experienced by wind regardless of the direction it is moving.

We attribute the apparent shift in wind direction to the Coriolis effect. This deflection (1) is always directed at right angles to the direction of airflow; (2) affects only wind direction, not wind speed; (3) is affected by wind speed (the stronger the wind, the greater the deflection); and (4) is strongest at the poles and weakens equatorward, becoming nonexistent at the equator.

Note that any free-moving object will experience a deflection caused by the Coriolis effect. The U.S. Navy dramatically discovered this fact in World War II. During target practice, long-range guns on battleships continually missed their targets by as much as several hundred yards until ballistic corrections were made for the changing position of a seemingly stationary target. Over a short distance, however, the Coriolis effect is small.

Friction with Earth’s Surface

Near Earth’s surface, in an area called the friction layer, friction acts to slow air movement and, as a consequence, alters wind direction. To illustrate friction’s effect on wind direction, let us look at a situation in which friction has no role. Above the friction layer, the pressure gradient force and Coriolis effect work together to direct the flow of air. Under these conditions, the pressure gradient force causes air to start moving across the isobars. As soon as the air starts to move, the Coriolis effect acts at right angles to this motion. The faster the wind speed, the greater the deflection.

Eventually, the Coriolis effect will balance the pressure gradient force, and the wind will blow parallel to the isobars (▼). Upper-air winds generally take this path and are called geostrophic winds.

Geostrophic windThe only force acting on a stationary parcel of air is the pressure gradient force. Once the air begins to accelerate, the Coriolis effect deflects it to the right in the Northern Hemisphere. Greater wind speeds result in a stronger Coriolis effect (deflection) until the flow is parallel to the isobars. At this point, the pressure gradient force and Coriolis effect are in balance, and the flow is called a geostrophic wind. It is important to note that in the “real” atmosphere, airflow is continually adjusting for variations in the pressure field. As a result, the adjustment to geostrophic equilibrium is much more irregular than shown.

The lack of friction with Earth’s surface allows geostrophic winds to travel at higher speeds than do surface winds. This can be observed in the following figure (▼) by noting the wind flags, many of which indicate winds of 50 to 100 miles per hour.

Simplified upper-air weather chartA. This simplified weather chart shows the direction and speed of the upper-air winds. Note from the flags that the airflow is almost parallel to the contours. Like most other upper-air charts, this one shows variations in the height (in meters) at which a selected pressure (500 millibars) is found, rather than showing variations in pressure at a fixed height, like surface maps. Do not let this confuse you because there is a simple relationship between height contours and pressure. B. Places experiencing 500-millibar pressure at higher altitudes (toward the south on this map) are experiencing higher pressures than places where the height contours indicate lower altitudes. Thus, higher-elevation contours indicate higher surface pressures, and lower-elevation contours indicate lower surface pressures.

The most prominent features of upper-level flow are jet streams. First encountered by high-flying bombers during World War II, these fast-moving “rivers” of air travel between 120 and 240 kilometers (75 and 150 miles) per hour in a west-to-east direction. One such stream is situated over the polar front, which is the zone separating cool polar air from warm subtropical air.

It is below 600 meters (2000 feet) where surface friction complicates the airflow just described. Recall that the Coriolis effect is proportional to wind speed. Friction lowers the wind speed, so it reduces the Coriolis effect. Because the pressure gradient force is not affected by wind speed, it wins the tug of war, as shown in ▼. The result is a movement of air at an angle across the isobars, toward the area of lower pressure.

The effects of friction on windA. Friction has little effect on winds aloft, so airflow is parallel to the isobars. B. In contrast, friction slows surface winds, which weakens the Coriolis effect, causing winds to cross the isobars and move toward the lower pressure.

The roughness of the terrain determines the angle of airflow across the isobars. Over the smooth ocean surface, friction is low, and the angle is small. Over rugged terrain, where friction is higher, the angle that air makes as it flows across the isobars can be as great as 45 degrees.

In summary, upper airflow is nearly parallel to the isobars, whereas the effect of friction causes the surface winds to move more slowly and cross the isobars at an angle.

Section Review

Wind is controlled by a combination of (1) the pressure gradient force, (2) the Coriolis effect, and (3) friction. The pressure gradient force, which results from pressure differences, is the primary force that drives wind. It is depicted by the spacing of isobars on a map. Closely spaced isobars indicate a steep pressure gradient and strong winds; widely spaced isobars indicate a weak pressure gradient and light winds.

The Coriolis effect, which is due to Earth’s rotation, produces deviation in the path of wind due to Earth’s rotation (to the right in the Northern Hemisphere and to the left in the Southern Hemisphere). Friction, which significantly influences airflow near Earth’s surface, is negligible above a height of a few kilometers.

Above a height of a few kilometers, the Coriolis effect is equal to and opposite the pressure gradient force, which results in geostrophic winds. Geostrophic winds follow a path parallel to the isobars, with velocities proportional to the pressure gradient force.

Section Glossary

Coriolis effect: The apparent deflective force of Earth’s rotation on all free-moving objects, including the atmosphere and oceans. Deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

geostrophic winds: Winds, usually above a height of 600 meters (2000 feet), that blow parallel to the isobars as a result of interactions between the Coriolis force and the pressure gradient force.

isobars: Lines drawn on a map that connect points of equal atmospheric pressure, usually corrected to sea level.

jet streams: Fast (120–240 kilometers per hour) narrow bands of high-altitude winds generally blowing from east to west and encircling the globe.

pressure gradient force: The amount of pressure change between high and low pressure centering over a given distance.

wind: Air flowing horizontally with respect to Earth’s surface from areas of higher pressure to areas of lower pressure.

Section Study Questions

Try to answer each of the following on your own. Then click the question to see its answer.

List three factors that combine to direct horizontal airflow (wind).

Pressure gradient force, Coriolis effect, and friction with Earth’s surface are the factors that combine to direct horizontal airflow.

What force is responsible for generating wind?

The Pressure gradient force is responsible for generating the wind.

Write a generalization relating the spacing of isobars to wind speed.

The closer together the isobars are, the faster the wind speed.

Briefly describe how the Coriolis effect influences air movement.

In the Northern Hemisphere, the Coriolis effect causes the deflection of the path of a moving object, including moving air, to the right. In the Southern Hemisphere, this deflection is to the left. The effect is greater with increasing latitude.

Unlike winds aloft, which blow nearly parallel to the isobars, surface winds generally cross the isobars. Explain what causes this difference.

At the surface, friction affects wind and as a result alters its direction. The pressure gradient force and the Coriolis effect at the surface causes air to move across the isobars. Aloft, geostrophic winds lack friction with Earth’s surface and can travel more quickly as the Coriolis effect balances the pressure gradient force.

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Section 8.2:Factors Affecting Wind