Geostrophic Wind. Vertical Air Motions

This single cell, thermally driven circulation does not fully explain atmospheric circulation. The Coriolis force is the apparent motion caused by Earth’s rotation, which pushes winds to the right of their intended path in the Northern Hemisphere and to the left in the Southern Hemisphere. The Coriolis force acts at right angles to the wind’s intended direction, influencing its direction but not its speed. The Coriolis force is greater for strong winds than for weak winds and is zero along the equator and greatest at the poles.

The top panel in Figure 5.2 shows how the pressure gradient force and Coriolis force determine wind speed and direction aloft at heights typically greater than 1000 m, where the influence of surface friction is negligible. The pressure gradient force is perpendicular to the isobars in the direction of low pressure. This force accelerates a parcel of air towards the low pressure. As it begins to move, the Coriolis force deflects the air towards the right, curving its path. As the path changes, so does the Coriolis force, which is always directed at right angle to the direction of motion. As the parcel of air increases in speed, the magnitude of the Coriolis force increases.

Fig. 5.2. The difference in winds at the surface and aloft in the northern hemisphere. (a) Geostrophic wind aloft is balanced by the pressure gradient force and Coriolis force with motion parallel to isobars. (b) Surface wind is balanced by the pressure gradient force, the Coriolis force, and friction. Friction slows wind speed and causes wind to cross isobars typically at a 30° angle

Eventually wind speed and direction are such that the pressure gradient force, acting from high to low pressure, is balanced by the Coriolis force acting in the opposite direction. At this point, the net force acting on the air is zero and the wind flows parallel to the isobars at a constant speed. This flow of air is called geostrophic wind. In the Northern Hemisphere, geostrophic wind flows with low pressure to the left and high pressure to the right with a speed directly related to the pressure gradient. In the Southern Hemisphere, the flow is reversed because the Coriolis force deflects winds to the left of their intended path.

Vertical Air Motions. The frictional drag of objects near the surface slows wind speed and in doing so deflects the direction of motion. Surface winds do not flow parallel to the isobars but rather cross them moving from high to low pressure. Figure 5.2 illustrates the difference in wind between the surface, where friction is important, and aloft, where friction is negligible. The upper air wind is geostrophic, flowing parallel to the isobars with the pressure gradient force balanced by the Coriolis force. At the surface, friction, which acts counter to the direction of motion, reduces wind speed. Because of this, the same pressure gradient produces slower winds at the surface than aloft. With reduced speed, the Coriolis force decreases, and the weaker Coriolis force no longer balances the pressure gradient force. The winds do not deflect as much towards the right of their intended path and winds flow across isobars towards low pressure.

The deflection of surface winds by friction creates vertical motions in the atmosphere (Figure 5.3). In the Northern Hemisphere, winds flow in a clockwise direction around high pressure cells and counterclockwise around low pressure cells ) (In the Southern Hemisphere, where the Coriolis force deflects winds to the left, winds flow clockwise around low pressure cells and counterclockwise around high pressures.)

Fig. 5.3. Surface winds and vertical air motions in the northern hemisphere with (a) low pressure and (b) high pressure

Friction deflects surface winds in towards the center of the low pressure. As the surface air moves inward, the converging air slowly rises, typically to a height of several thousand meters, and diverges aloft. So long as the upper-level outflow of air balances the inflow of surface air, surface pressure remains unchanged. However, if the upper-level divergence exceeds the surface convergence, surface pressure decreases, the pressure gradient increases, and surface winds strengthen. In contrast, surface winds flow outward from the center of a high pressure cell. Air from above converges and descends to replace the diverging surface air.