Convergence

Convergence occurs when air moving from different directions collides. When convergence happens near the surface of the earth, such as when air spirals into the center of a low pressure cell, air is forced upward, a process called ascent. When convergence occurs high in the atmosphere, for example near the top of the troposphere, air is forced downward toward the surface, a process called descent, or subsidence.

Fig. 1. Vertical movements of air associated with convergence and divergence.

Winds flowing along a spiral path toward the eye of a hurricane begin to converge some distance from the center of the eye. The ascent of the moisture laden air leads to cumulus convection and thunderstorm formation in the spiral cloud bands of the hurricane. Convergence closer to the center of the storm causes the remaining inflowing air to turn sharply upward 10-100 km from the center of the eye, creating the eye wall of the hurricane.

Fig. 2. Diagram of air circulation in a hurricane, showing ascent of converging air
in spiral cloud bands and the eye wall.