Forces that drive the wind

Horizontal winds near the surface respond to three forces: the pressure gradient force, friction and the Coriolis force (and gravity acts downward).

• Pressure gradient force: the rate of change of pressure over distance. It is strong where isobars are close together and weak where they are far apart. A stronger gradient means a faster wind.
• Frictional force: slows the wind. It is greatest at the surface and reaches up to about 1-3 km; it is least over the sea.
• Coriolis force: the earth's rotation deflects winds to the right in the northern hemisphere and to the left in the southern. It is named after the French scientist who described it. It is stronger for faster winds, maximum at the poles and zero at the equator, and acts at right angles to the pressure gradient force.

Results:

• With these two forces at right angles, winds blow around a low instead of straight into it.
• At the equator, with no Coriolis force, wind blows straight across the isobars, so a low fills up instead of deepening.
• 2-3 km up, free of friction, winds depend on the pressure gradient and Coriolis force alone. Where isobars are straight, the two balance and the wind blows parallel to the isobars: the geostrophic wind.

Cyclones and anticyclones:

• Circulation around a low is cyclonic: anticlockwise in the northern hemisphere, clockwise in the southern.
• Circulation around a high is anticyclonic: clockwise in the north, anticlockwise in the south.
• Over a low, air converges and rises; over a high, air sinks and diverges at the surface. Rising air (from convergence, eddies, convection, mountains or fronts) is essential for clouds and rain.

Example (new): on a weather map, isobars squeezed tightly together over the Bay of Bengal and spread far apart over central India tell a forecaster to expect strong winds over the bay and light winds inland.