What Causes Pressure Variations and Winds?
THE MOVEMENT OF AIR IN THE ATMOSPHERE produces wind, or movement of air relative to Earth's surface. Circulation in the atmosphere is caused by pressure differences generated primarily by uneven insolation. Air flows from areas of higher pressure, where air sinks, to areas of lower pressure, where air rises.
How Do We Measure the Strength and Direction of Wind?
Wind speed and direction are among the most important measurements in the study of weather and climate. On short time scales, wind can indicate which way a weather system is moving and the strength of a storm. When considered over longer time scales, winds indicate general atmospheric circulation patterns, a key aspect of climate.
1. Wind directions can be assessed as easily as throwing something light into the air and tracking which way it goes, but it is best done with a specially designed measuring device that can measure the wind speed and direction. Wind speed is expressed in units of distance per time (km/hr) or as knots, which is a unit expressing nautical miles per hour. One knot is equal to 1.15 miles/hr or 1.85 km/hr.
2. Wind direction is conveyed as the direction from which the wind is blowing. Wind direction is commonly expressed with words, such as a northerly wind (blowing from the north). It can instead be described as an azimuth in degrees clockwise from north. In this scheme, north is 0°, east is 090°, south is 180°, and west is 270°.
3. The atmosphere also has vertical motion, such as convection due to heating of the surface by insolation. A local, upward flow is an updraft and a downward one is a downdraft.
What Causes Air to Move?
Air moves because there are variations in air pressures, in density of the air, or in both (recall that pressure and density are related via the Ideal Gas Law). Such pressure and density variations are mostly caused by differential heating of the air (due to differences in insolation) or by air currents that converge or diverge. The atmosphere is not a closed container, so changes in volume (i.e., air being compressed or expanded) come into play. These volume changes can make air pile up or spread out, resulting in variations in air pressure.
1. Movement of air occurs to equalize a difference in air pressure between two adjacent areas, that is, a pressure gradient. Air molecules in high-pressure zones are packed more closely together than in low-pressure zones, so gas molecules in high-pressure zones tend to spread out toward low- pressure zones. As a result, air moves from higher to lower pressure, in the simplest case (as shown here) perpendicular to isobars.
2. High-pressure zones and low-pressure zones can be formed by atmospheric currents that converge or diverge. Converging air currents compress ore air into a smaller space, increasing the air pressure. Diverging air currents move air away from an area, causing low pressure. Forces associated with converging and diverging air are called dynamic forcing.
3. Most variations in air pressure and most winds, however, are caused by thermal effects, specifically differences in insolation from place to place. This cross section shows a high-pressure zone caused by the sinking of cold, high-altitude air toward the surface. In the adjacent low-pressure zone, warmer near-surface temperatures have caused air to expand, become less dense, and rise, causing low pressure. Near the surface, air would flow away from the high pressure and toward the low pressure. Different air currents would form higher, in the upper troposphere, to accommodate the sinking and rising of the air. In an atmosphere that doesn't have strong updrafts or downdrafts, at any altitude the upward pressure gradient will approximately equal the downward force of gravity.
What Forces Result from Differences in Air Pressure?
Differences in air pressure, whether caused by thermal effects or dynamic forcing, result in a pressure gradient between adjacent areas of high and low pressure. Associated with this pressure gradient are forces that cause air to flow. Pressure gradients can exist vertically in the atmosphere or laterally from one region to another.
1. Elevation differences cause the largest differences in air pressure. At high elevations, there is less atmospheric mass overhead to exert a downward force on the atmosphere. As a result, density decreases with elevation, and air pressure does too.
2. These vertical variations in air pressure cause a pressure gradient in the atmosphere, with higher pressures at low elevations and lower pressures in the upper atmosphere. This pressure gradient can be thought of as a force directed from high pressures to lower ones. This pressure-gradient force is opposed by the downward-directed force of gravity, which is strongest closer to Earth's surface.
3. Lateral variations in air pressure also set up horizontal pressure gradients, and a pressure-gradient force directed from zones of higher pressure to zones of lower pressure. On the map below, the pressure-gradient force acts to cause air to flow from high pressure toward lower pressures, as illustrated by the blue arrows on the map.
4. Places where isobars are close have a steep pressure gradient, and a strong pressure-gradient force, so movement of the atmosphere (i.e., winds) will generally be strong in these areas.
5. Places where isobars are farther apart have a more gentle pressure gradient, a weak pressure-gradient force, and generally lighter winds. Although winds tend to blow from high to low pressure, other factors, such as Earth's rotation, complicate this otherwise simple picture, causing winds patterns to be more complex and interesting.
How Does Friction Disrupt Air Flow?
As is typical for nature, some forces act to cause movement and other forces act to resist movement. For air movements, the pressure-gradient force acts to cause air movement and friction acts to resist movement.
1. Friction occurs when flowing air interacts with Earth's surface.
2. As represented in this figure by the shorter blue arrows low in the atmosphere, wind is slowed near the surface, because of friction along the air-Earth interface. As the air slows, it loses momentum (which is mass times velocity). Some momentum from the moving air can be transferred to the land, such as when strong winds pick up and move dust or cause trees to sway in the wind. It is also transferred to surface waters, causing some currents in oceans and lakes and forming surface waves.
3. Friction with Earth's surface, whether land or water, also causes the wind patterns near the surface to become more complicated. On land, air is forced to move over hills and mountains, through valleys, around trees and other plants, and over and around buildings and other constructed features. As a result, the flow patterns become more curved and complex, or turbulent, near the surface, with local flow paths that may double back against the regional flow, like an eddy in a flowing river. Friction from the surface is mostly restricted to the lower 1 km of the atmosphere, called the friction layer.
4. Stronger winds occur aloft, in part because these areas are further removed from the frictional effects of Earth's surface. Some friction occurs internally to the air, even at these heights, because adjacent masses of air can move at different rates or in different directions. Friction can also accompany vertical movements in updrafts and downdrafts.