Global Wind Patterns

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Uneven solar heating creates circulation cells: warm air rises at the equator and flows toward the poles, sinks at about 30° latitude, forming trade winds (blowing toward the equator, from the east), westerlies in mid-latitudes and polar easterlies. The Coriolis effect gives their directions.

Jet streams are fast, narrow upper-atmosphere winds that steer weather systems. Monsoons are seasonal reversals of wind. These patterns redistribute heat and moisture, and guided old sailing routes.

Picture three belts in each hemisphere: the Hadley cell between the equator and 30 degrees, the Ferrel cell between 30 and 60, and the polar cell between 60 and 90. Where air rises at the equator, the Intertropical Convergence Zone has heavy rain and light winds, which sailors call the doldrums. Sinking dry air at about 30 degrees creates the great deserts, such as the Sahara, Arabian and Australian deserts. The trade winds blew sailing ships across the Atlantic, and the westerlies carried them home, which is why Columbus's route out and back differed.

Jet streams, up to 400 km/h at about 10 km height, form where large temperature contrasts exist, and airliners use them to save time and fuel flying east. The South Asian monsoon happens when the land heats more than the ocean in summer, drawing moist air onto the land and delivering most of India's annual rain, which is crucial for agriculture. A misconception is that winds blow straight from the pole to the equator; the Coriolis effect turns them. Another is that deserts are only caused by heat; they are caused by dry, sinking air or rain shadows.