Convergence and divergence
Topic: Atmospheric Pressure, Winds and General Circulation · NCERT: Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"
Meaning
Convergence means air flowing together into an area. Divergence means air spreading out from it. Over a low, surface air converges, rises and then diverges high up. The rising air cools and forms cloud and rain. Over a high, air converges high up, sinks and diverges at the surface, giving dry weather. Air can also be lifted by eddies, convection, mountains (orographic uplift) and fronts. Divergence high up helps a low get deeper, which is one condition for tropical cyclones to form.
Example
In July the ITCZ (Inter-Tropical Convergence Zone, the belt where the trade winds meet) forms the monsoon trough over the north Indian plains. Surface winds converge into it and rise, bringing heavy monsoon rain to the Ganga plain.
Don't confuse with
- Divergence aloft and divergence at the surface: they lead to opposite weather. Divergence aloft above a low pulls air upwards and brings rain. Divergence at the surface under a high comes from sinking air and brings dry weather.