Atmospheric Circulation

Cyclones and Anticyclones

Cyclones are areas of low pressure. Cyclones usually exhibit nearly circular isobars. If isobars are oblong or elongate with the lowest pressure near the center we call them troughs. As air enters an area of low pressure from all directions, the Coriolis effect bends the direction of the wind to the right of its path. This creates a counterclockwise rotation around the low and  convergence near the center of the system. As the air collides near the center it is forced aloft where divergence takes air away from the center of the system. The upper-level divergence is necessary for the system to be maintained as an area of low pressure. Without the divergence, the system would fill with air and the horizontal pressure differences would be equalized causing the system to dissipate. Anticyclones are areas of high pressure that exhibit nearly circular isobars. If isobars are oblong or elongate with the highest pressure near the center we call them ridges. For high pressure areas, air descends toward the surface due to convergence aloft. As the air nears  the surface it is forced outward (divergence) from the center. The Coriolis effect bends the air to the right of its path creating a clockwise rotation around the high.

Figure 6.14 Cyclone in the Southern Hemisphere 
(Click image to enlarge)

Because the Coriolis effect works in the opposite direction in the Southern Hemisphere, circulation around lows are clockwise and inward toward the center at the surface and highs exhibit a diverging, counterclockwise rotation. You can see this effect in the cloud pattern created by a cyclone in the Southern Hemisphere.