Planetary (Prevailing) Winds

Definition

The permanent, large-scale winds that blow steadily across the globe from the high-pressure belts toward the low-pressure belts, deflected by the Earth's rotation; the trade winds, the westerlies, and the polar easterlies.

Key points

• Trade winds blow from the subtropical highs toward the equatorial low; they are the north-east trades in the Northern Hemisphere and the south-east trades in the Southern Hemisphere, deflected by the Coriolis force.
• Westerlies blow from the subtropical highs toward the subpolar lows; in the Southern Hemisphere, with little land to slow them, they are very strong, giving the Roaring Forties, Furious Fifties, and Shrieking Sixties.
• Polar easterlies blow from the polar highs toward the subpolar lows.
• The doldrums (equatorial calm) and the horse latitudes (subtropical calm) are belts of light, variable winds between the planetary wind zones.
• These contrast with periodic winds (such as the monsoon and land and sea breezes) and local winds (such as the Loo of north India and the Chinook of North America).

Why it matters for CAPF

The three planetary wind names with their direction, the Coriolis deflection, the Roaring Forties, and the planetary-versus-periodic-versus-local classification are recurring climatology facts.

Common confusion

Trade winds (toward the equator, north-east and south-east) versus westerlies (poleward, south-west and north-west); planetary winds (permanent) versus periodic (monsoon, breezes) versus local (Loo, Chinook); the Coriolis force deflects right in the north, left in the south.

Parent note

climatology atmosphere and winds