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Unit 2: Atmosphere // Section 6: Atmospheric Circulation Patterns

Atmospheric circulation is set up when mass moves in the atmosphere. This motion may be vertical, as when warm air rises and becomes buoyant. It can also be horizontal: wind is created by air moving from high pressure areas, where air is densely compressed, to low-pressure areas, where air is less dense, although horizontal winds follow curved trajectories due to the rotation of the earth (see below). Atmospheric forces cause the air to move, modifying the difference in pressure. On a weather map, pressure differences are demarcated by parallel lines called isobars that show changes in pressure, usually in increments of 2 to 4 millibars.

Sea breezes show how vertical and horizontal movements combine to modify temperature and pressure at a local level. During the day coastal land regions heat up more than the sea because land warms more quickly than water. Air over the land is thus warmed and rises, increasing pressure in the atmosphere above the surface, where it starts to cool and form clouds. It then flows at altitude from the area of high pressure over land to lower pressure over the sea. Because there is then less mass over the land and more over the sea, pressure at the surface is higher at sea, so air flows in from the sea to the land. At night, when land cools more quickly than the ocean, the cycle is reversed (Fig. 7).

Sea breeze

Figure 7. Sea breeze
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Source: Adapted from graphic by National Oceanic Atmospheric Administration, Jet Stream.

The sea breezes in this example flow directly between two points, but many larger weather systems follow less-direct courses. Their paths are not random, however. Winds that move over very long distances appear to curve because of the Coriolis force, an apparent force caused by Earth's rotation. This phenomenon occurs because all points on the planet's surface rotate once around Earth's axis every 24 hours, but different points move at different speeds: air at a point on the equator rotates at 1,700 kilometers per hour, compared to 850 kilometers per hour for a point that lies at 60 degrees latitude, closer to Earth's spin axis.

Because Earth spins, objects on its surface have angular momentum, or energy of motion, which defines how a rotating object moves around a reference point. An object's angular momentum is the product of its mass, its velocity, and its distance from the reference point (its radius). Angular momentum is conserved as an object moves on the Earth, so if its radius of spin decreases (as it moves from low latitude to high latitude), its velocity must increase. This relationship is what makes figure skaters rotate faster when they pull their arms in close to their bodies during spins.

The same process affects a parcel of air moving north from the Equator toward the pole: its radius of spin around Earth decreases as it moves closer to Earth's axis of rotation, so its rate of spin increases. The parcel's angular velocity is greater than the angular velocity of Earth's surface at the higher latitude, so it deflects to the right of its original trajectory relative to the planet's surface (Fig. 8). In the Southern hemisphere, the parcel would appear to deflect to the left.

Coriolis force

Figure 8. Coriolis force
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This effect was discovered by French scientist Gustave-Gaspard Coriolis, who sought to explain why shots fired from long-range cannons were falling wide to the right of their targets. The Coriolis force only affects masses that travel over long distances, so it is not apparent in local weather patterns such as sea breezes. Nor, contrary to an oft-repeated misbelief, does it make water draining from a sink or toilet rotate in one direction in the Northern Hemisphere and the other direction in the Southern Hemisphere. But the Coriolis force makes winds appear to blow almost parallel to isobars, rather than directly across them from high to low pressure.

The Coriolis force makes the winds in low-pressure weather systems such as hurricanes rotate (counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere), curving into spirals. Air initially starts to move through the atmosphere under the influence of pressure gradients that push it from high pressure to low pressure areas. As it travels, the Coriolis force starts to bend its course. The motion tends toward a state called geostrophic flow, where the pressure gradient force and the Coriolis force exactly balance each other. At this point the air parcel is no longer moving from a high-pressure to a low-pressure zone. Instead, it follows a course parallel to the isobars. In Figure 9, the air parcel is in geostrophic flow at point A3.

Geostrophic flow

Figure 9. Geostrophic flow
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When a low-pressure region develops in the Northern Hemisphere, pressure forces direct air from the outside toward the low. Air that moves in as a response to this force is deflected to the right and rotates counter-clockwise around the system. In contrast, a region of high pressure produces a pressure force directed away from the high. Air starting to move in response to this force is deflected to the right (in the Northern Hemisphere), producing a clockwise circulation pattern around a region of high pressure (Fig. 10).

Circulation of air around regions of high and low pressure in the Northern Hemisphere

Figure 10. Circulation of air around regions of high and low pressure in the Northern Hemisphere
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This pattern is modified at altitudes below 1 kilometer as friction with objects on the ground slows winds down. As wind speed declines, so does the Coriolis force, but pressure gradient forces stay constant. As a result, winds near the ground are deflected toward low pressure areas. Air parcels will spiral into low pressure areas near the surface, then rise once they reach the center. As the air rises, it cools, producing condensation, clouds, and rain. In contrast, air parcels will spiral away from high pressure areas near the surface toward low pressure areas. To maintain barometric balance, air will descend from above. In the process, the descending air will warm and its relative humidity will decrease, usually producing sunny weather (Fig. 11).

Winds around highs and lows

Figure 11. Winds around highs and lows
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The first attempt to show how weather patterns combined to produce a general circulation of the atmosphere was offered in 1735 by English meteorologist George Hadley. Hadley pictured global-scale circulation as a large-scale version of the local system pictured above in Figure 7: a vast sea breeze with warm air rising over the equator and sinking over the poles. Hadley wanted to explain why sailors encountered westerly winds at midlatitudes and easterly "trade winds" near the equator. He deduced that this trend was caused by the Earth's rotation.

Hadley's model was accurate in many respects. Due to differential heating of the Earth with more warmth near the equator and cooling by radiation to space, buoyancy develops at low latitudes and mass is moved upward and poleward in the atmosphere, creating pressure gradients. The atmosphere tries to set up a simple circulation, upwelling near the equator and descending in polar regions, but in reality the Hadley circulation terminates at a latitude of about 30°. At this point, air sinks to the ground and flows back to the tropics, deflected by the Coriolis force, which produces easterly winds near the surface at low latitudes ("trade winds") and westerly winds at high latitudes. Farther north and south, this pattern repeats in two more sets of circulation zones, or "cells," between the tropics and the poles (Fig. 12). The strength of the atmospheric circulation is controlled by a dynamic balance between motions caused by differential heating and friction that slows down the winds.

General circulation of the atmosphere

Figure 12. General circulation of the atmosphere
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