The planetary winds: trades, westerlies and polar easterlies

Detail

Basic idea: what a planetary wind is

• Planetary winds (also called permanent or prevailing winds) blow across the Earth all year in the same general direction. They blow between the big pressure belts.
• Air always moves from high pressure to low pressure. The pressure belts sit in a fixed order, so the winds also form belts.
• Coriolis force: the Earth spins, and this spin makes moving air turn. It turns to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Without it, all winds would blow straight north or south.
• A wind is named after the direction it comes from. A "north-easterly" blows from the north-east.

Wind belt	Blows from → to	Latitude band	Direction in Northern Hemisphere	Direction in Southern Hemisphere	Character
Trade winds	Subtropical high (~30°) → Equatorial low	0°–30°	North-east (NE trades)	South-east (SE trades)	Steadiest winds on Earth
Westerlies	Subtropical high → Subpolar low (~60°)	~35°–60°	South-west	North-west	Stormy and changeable; strongest in the Southern Hemisphere
Polar easterlies	Polar high → Subpolar low	60°–90°	North-east	South-east	Cold and dry

Trade winds

• Trade winds blow from the subtropical highs (high-pressure belts near 30° N and S) to the equatorial low.
• The Coriolis force turns them. They become north-easterlies in the Northern Hemisphere and south-easterlies in the Southern Hemisphere.

• They are the steadiest winds on Earth. Their name comes from the old sailing trade: merchant ships depended on their steady push.

• In the Pacific, the trade winds start from the subtropical highs over the north-east and south-east Pacific, at about 30°–40° N and S. Their average speed is about 24 km/h (15 mph) [2].

• The trades of the two hemispheres meet at the ITCZ (Inter Tropical Convergence Zone: the low-pressure belt near the equator where the two sets of trade winds meet). Here the air rises, forms clouds and gives heavy rain.

Why trade winds wet the east coasts and leave the west coasts dry

• The trades blow from the east. They cross warm oceans and pick up moisture.
• They reach the east coasts of the tropical continents first. They rise over the land and highlands, cool, and drop rain. Examples are eastern Brazil, eastern Madagascar and Queensland (north-east Australia).
• By the time they reach the west coasts, they have lost their moisture. They are now blowing off the land, so they are dry. This is one reason the great hot deserts (Sahara, Kalahari, Atacama, Australian deserts) lie on the western sides of continents.

South-east trades become India's south-west monsoon

• Southeast trade winds: the trade winds of the Southern Hemisphere, blowing from the south-east.
• In the northern summer the ITCZ and the pressure belts move north with the overhead sun.
• The south-east trades from the southern Indian Ocean get pulled across the equator. They cross it at about 40°–60°E, off the East African coast.
• Once they are in the Northern Hemisphere, the Coriolis force turns them to the right.
• They now blow from the south-west. This is India's south-west monsoon (see indian-climate-monsoon).

Trade winds and El Niño / La Niña

• Normal years: the trade winds blow westward along the equator. They push warm surface water from South America towards Asia [3].
• El Niño (a periodic warming of the central and eastern tropical Pacific): the trade winds weaken. Warm water flows back east towards the west coast of the Americas. More heat and moisture rise into the air over the central and eastern Pacific, and this changes weather around the world [3][4].
• La Niña (the opposite, cool phase): the easterly trade winds strengthen. Cold upwelling (cold, deep water rising to the surface) grows stronger along the equator and off the west coast of South America [3][5].

Phase	Trade winds	Eastern Pacific waters	Upwelling off Peru
Normal	Steady easterly	Cool	Normal
El Niño	Weaker	Warmer than normal	Weak
La Niña	Stronger	Colder than normal	Stronger

Westerlies

• Westerlies (or prevailing westerlies) are the winds of the middle latitudes. They blow from the subtropical highs towards the subpolar lows (low-pressure belts near 60° N and S), between about 35° and 60° (NCERT). Britannica gives the wider band as 30°–60° in each hemisphere, also called the temperate zone [6].
• Northern Hemisphere: they blow from the south-west.
• Southern Hemisphere: they blow from the north-west.

Effects on climate

• The westerlies blow from the ocean onto the west coasts of the mid-latitude continents.
• They bring rain all year and a stream of temperate cyclones (travelling low-pressure storms of the middle latitudes).

• Examples: British Columbia (Canada), north-western Europe (UK, Norway, France), southern Chile and New Zealand.

• Mediterranean lands get rain only in winter.

• In winter the pressure belts move south, towards the equator.
• The westerlies then reach the Mediterranean region and bring rain.
• In summer the belts move north. The dry subtropical high sits over the region, so summers are dry.

Roaring Forties, Furious Fifties, Shrieking Sixties

• The Southern Hemisphere is mostly ocean between 40° and 60°S. There is almost no land to slow the westerlies down, so they blow strong and steady.
• Roaring Forties: the belt between 40° and 50°S where strong west winds blow all the time. The winds are often gale-force all year. Sailors who first entered these latitudes gave them the name [7].

• The gales come with the depressions (low-pressure storms). The "roaring forties" is the latitude zone where these storm winds are most frequent [6].

• Going further south, the belts are the Furious Fifties (50°–60°S) [8] and the Shrieking (Screaming) Sixties (around 60°S).

• Uses: clipper ships (fast sailing ships of the 19th century) rode these winds on the Europe–Australia route. Modern round-the-world sailing races use them too.

Polar easterlies

• Polar easterlies are cold, dry winds. They blow from the polar highs (high pressure over the poles) to the subpolar lows.

• The Coriolis force turns them so that they come from the east: north-east in the Northern Hemisphere and south-east in the Southern.

• Polar front: the boundary where the cold polar easterlies meet the warmer, moist westerlies, near 60° latitude.

• The cold, heavy air pushes under the warm, light air.
• The warm air is forced up, and the front bends into waves.
• These waves grow into temperate cyclones (see air-masses-fronts-cyclones).

Beyond NCERT: atmospheric rivers

• Atmospheric river (AR): a long, narrow band of concentrated water vapour in the air. It carries moist air from the tropics to higher latitudes [9].
• Size: about 400–600 km (250–375 miles) wide, and it can be more than 1,600 km (1,000 miles) long [10].

• How it forms: warm tropical seas → a lot of water evaporates → strong low-level winds carry the vapour poleward → the vapour falls as rain or snow over cooler land [10].

• Scale:

• ARs carry more than 90% of the water vapour that moves from the tropics to the mid-latitudes [9].

• A strong AR carries water vapour equal to about 7.5–15 times the average flow of the Mississippi River at its mouth [9].

• When an AR hits mountains: the moist air is pushed up, cools, and drops extreme rain or snow. This causes flash floods and mudslides [9].

• Pineapple Express: an AR that starts near Hawaii and reaches California.
• On the US west coast, about 30–50% of yearly precipitation falls in just a few AR events [9].

• Floods in western Europe and Chile are also linked to ARs.

• India:

• A study of 1985–2020 found that about 65% of ARs reaching land in India were linked to floods [11].
• Seven of India's ten most severe summer-monsoon floods with heavy loss of life were linked to ARs [11].
• More than 95% of AR events in India happen in the summer monsoon (June–September) [11].
• Over the past seven decades, moisture transport has grown stronger and low-level winds have strengthened. This has pushed AR-linked rainfall upward where ARs reach land in India [11]. Research is still going on (verify current).

Prelims Hooks

• Trade winds: blow from the subtropical high to the equatorial low. They are north-easterly in the Northern Hemisphere and south-easterly in the Southern. They are the steadiest winds on Earth.
• Westerlies: south-westerly in the Northern Hemisphere and north-westerly in the Southern. A common trap is to say "south-westerly in both hemispheres".
• Roaring Forties = 40°–50°S. Then the Furious Fifties (50°–60°S), then the Shrieking Sixties. Going poleward, the order is Roaring → Furious → Shrieking.
• The SE trades cross the equator at about 40°–60°E. Once they are turned to the right, they become India's south-west monsoon.
• Trade-wind rain falls on tropical east coasts: eastern Brazil, eastern Madagascar, Queensland. Westerly rain falls on mid-latitude west coasts: British Columbia, NW Europe, southern Chile, New Zealand.
• El Niño: the trade winds weaken. La Niña: the trade winds strengthen and upwelling off Peru grows stronger [3].
• Polar front: where the polar easterlies meet the westerlies. This is where temperate cyclones are born.
• Atmospheric rivers carry more than 90% of the tropics-to-mid-latitudes water vapour [9]. The "Pineapple Express" hits California.
• Mediterranean climate: the westerlies bring rain only in winter, because the pressure belts move towards the equator in that season.

Mains Points

• Wind belts and where rain falls: trade winds wet tropical east coasts and leave the west coasts dry. This, with cold ocean currents, helps explain why the hot deserts lie on western margins. Westerlies wet mid-latitude west coasts, and Mediterranean lands get rain in winter only. The belts move with the sun, and this movement creates seasonal climates such as the Mediterranean type and the Indian monsoon (GS-I).
• Link to India's monsoon: the SE trades cross the equator and turn right to become the south-west monsoon. So changes in the trade winds also affect Indian rainfall. During El Niño the Pacific trade winds weaken, and El Niño years are usually linked with weaker Indian monsoons. This matters for kharif sowing and for drought planning (GS-I/GS-III).
• Atmospheric rivers as a disaster risk: 7 of India's 10 deadliest monsoon floods were linked to ARs, and AR-linked rainfall is rising [11]. India needs AR-aware flood forecasting by IMD and CWC, better warnings for the Western Ghats and the Himalaya, and climate-proof infrastructure (GS-III, disaster management).
• Southern Hemisphere westerlies: there is almost no land at 40°–60°S, so the winds are strong and steady. This made the clipper-ship route to Australia possible. It also drives the Antarctic Circumpolar (West Wind Drift) current and gives a large offshore wind-energy potential (GS-I).