Winds

Notes

Wind is the horizontal movement of air from high to low pressure. Three forces act on it: the pressure gradient force, the Coriolis force and friction. High above the surface (about 1-3 km), these balance to give the geostrophic wind, which blows parallel to the isobars.

1. Planetary (primary or permanent) winds

Trade winds: blow from the subtropical highs to the equatorial low; north-east trades in the Northern Hemisphere and south-east trades in the Southern; steady and reliable ("trade" means track).
Westerlies: blow from the subtropical highs towards the subpolar lows (30-60°); south-westerly in the NH and north-westerly in the SH. In the Southern Hemisphere they are the Roaring Forties, Furious Fifties and Screaming Sixties.
Polar easterlies: cold, dry winds from the polar highs to the subpolar lows.

2. Periodic (seasonal) winds

Monsoons (Arabic mausim, season): seasonal reversal of winds. Sea breeze (day, sea to land) and land breeze (night, land to sea); valley breeze (day, up the slope) and mountain breeze (night, down the slope).

3. Local winds

Hot: Loo (hot, dry, dusty afternoon wind of north India in summer), Foehn (Alps), Chinook (the eastern Rockies, a warm, dry wind called the "snow-eater"), Sirocco (Sahara to the Mediterranean), Harmattan (dry, from the Sahara to the Gulf of Guinea, called the "doctor"), Santa Ana (California), Khamsin (Egypt), Zonda (Argentina).
Cold: Mistral (France, the Rhone valley), Bora (the Adriatic), Purga (Russian tundra).
Convectional storms in India: Kalbaisakhi (Nor'westers) in Bengal and Bordoichila in Assam; mango showers in Kerala and Karnataka.

Jet streams: narrow bands of fast westerly winds in the upper troposphere (9-14 km), with speeds of 150-300 km/h or more. The polar front jet and the subtropical jet meander (Rossby waves). The subtropical westerly jet shifts north of the Himalaya in summer, helping the onset of the Indian monsoon; the Tropical Easterly Jet forms in summer over peninsular India.

Related: air masses, fronts, cyclones (tropical: hurricane, typhoon, cyclone, willy-willy; temperate or extra-tropical).

Key Points

Forces: pressure gradient, Coriolis, friction; geostrophic wind above the surface.
Trade winds (NE and SE), Westerlies (Roaring Forties in the SH), Polar easterlies.
Local winds: Loo, Chinook, Foehn, Sirocco, Harmattan, Mistral.
Sea breeze by day; land breeze by night.
Jet streams: upper-troposphere westerlies; important for the monsoon.

MCQs

Q1. The Chinook is a:

(a) Cold wind of France
(b) Warm, dry wind on the eastern slopes of the Rockies
(c) Hot wind of North India
(d) Wind of the Sahara
Ans: (b). Called the "snow-eater".

Q2. The hot, dry summer wind of north India is known as:

(a) Loo
(b) Mistral
(c) Foehn
(d) Sirocco
Ans:
(a).

Q3. The Roaring Forties are:

(a) Trade winds
(b) Westerlies of the Southern Hemisphere
(c) Polar easterlies
(d) Monsoon winds
Ans: (b).