Trade winds

Also called: Tropical easterlies · Topic: Atmospheric Pressure, Winds and General Circulation · NCERT: Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"

Meaning

Trade winds (also called tropical easterlies) are steady surface winds. They blow from the subtropical highs (high-pressure belts near 30° N and S) towards the equatorial low. The Coriolis force turns them, so they blow from the north-east in the Northern Hemisphere and from the south-east in the Southern Hemisphere. The two sets meet at the ITCZ.

• They are the steadiest winds on Earth.
• They decide which tropical coasts are wet and which are dry. They feed India's south-west monsoon. Their weakening or strengthening is at the centre of El Niño and La Niña.

Explanation

How trade winds form

• Cause: air always moves from high pressure to low pressure.
• The subtropical highs sit at about 30° N and S.
• The equatorial low sits near the equator.

• So surface air flows from about 30° towards the equator, across the 0°–30° band in each hemisphere.

• Process: the Earth spins, and this makes moving air turn. This turning push is the Coriolis force.

• In the Northern Hemisphere it turns the wind to the right, so the wind comes from the north-east. These are the NE trades.
• In the Southern Hemisphere it turns the wind to the left, so the wind comes from the south-east. These are the SE trades.

• Without the Coriolis force, these winds would blow straight north or south.

• Result: the trades from both 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.

• Naming rule: a wind is named after the direction it comes from. A "north-easterly" blows from the north-east.

Character of the trades

• They are planetary winds (also called permanent or prevailing winds). They blow all year in the same general direction.
• They are the steadiest winds on Earth.
• The name comes from the old sailing trade: merchant ships depended on their steady push.
• In the Pacific:
• They start from the subtropical highs over the north-east and south-east Pacific, at about 30°–40° N and S [2].
• Their average speed is about 24 km/h (15 mph) [2].

Why trade winds wet east coasts and leave west coasts dry

• The trades blow from the east across warm oceans.
• They pick up moisture on the way.
• They reach the east coasts of tropical continents first.

• The moist wind meets land and highlands and is pushed up. Rising air cools, its water vapour condenses, and it rains.

• By the time they cross to the west coasts, they are dry.

• They have already dropped their moisture.
• They are now blowing off the land, so they bring little rain.
• This is one reason the great hot deserts lie on the western sides of continents. Cold ocean currents on those coasts add to the dryness.

Changes with seasons and in ENSO years

• Seasonal shift (the Indian monsoon link):
• In the northern summer, the ITCZ and the pressure belts move north with the overhead sun.
• The SE trades from the southern Indian Ocean are pulled across the equator at about 40°–60°E, off the East African coast.
• 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.

• Normal Pacific year: the trades 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 [3].
• Warm water flows back east towards the west coast of the Americas [3].

• More heat and moisture rise into the air over the central and eastern Pacific. This changes weather around the world [3][4].

• La Niña (the opposite, cool phase):

• The easterly trade winds strengthen [3].
• 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

Where it occurs

• Belt: between 0° and 30° in both hemispheres. They blow from the subtropical highs (~30° N and S) to the equatorial low.
• Wet east coasts (trade-wind rain):
• Eastern Brazil (South America)
• Eastern Madagascar (Indian Ocean, off Africa)

• Queensland (north-east Australia)

• Dry western sides (hot deserts): Sahara (North Africa), Kalahari (southern Africa), Atacama (west coast of South America) and the Australian deserts.

• Pacific Ocean: they start from the highs over the north-east and south-east Pacific (~30°–40° N and S) [2]. They drive the normal westward push of warm water from South America towards Asia [3].
• Indian Ocean and India:
• The SE trades cross the equator at about 40°–60°E, off the East African coast.

• They reach India as the south-west monsoon.

• Meeting zone: the ITCZ, near the equator. It moves north in the northern summer.

Don't confuse with

• Westerlies: these also blow out of the subtropical highs, but poleward to the subpolar lows (~60°). They come from the south-west in the Northern Hemisphere and the north-west in the Southern. They wet mid-latitude west coasts. The trades wet tropical east coasts.
• Polar easterlies: these are also easterly winds, but they are cold, dry winds of 60°–90°. They blow from the polar highs to the subpolar lows. The trades blow between 0° and 30°.
• South-west monsoon: this is not a separate planetary wind. It is the SE trades after they cross the equator (~40°–60°E) and are turned to the right. Direction changes from south-east → south-west.
• NE trades vs SE trades: the NE trades are the Northern Hemisphere trades (turned right). The SE trades are the Southern Hemisphere trades (turned left). Both blow towards the equator, not away from it.

Prelims Hooks

• Trade winds blow subtropical high (~30°) → equatorial low. They are NE trades in the Northern Hemisphere and SE trades in the Southern. They are the steadiest winds on Earth.
• The trades of both hemispheres meet at the ITCZ, where air rises and gives heavy rain.
• Trade-wind rain falls on tropical east coasts: eastern Brazil, eastern Madagascar, Queensland. The hot deserts (Sahara, Kalahari, Atacama, Australian) lie on western sides.
• The SE trades cross the equator at about 40°–60°E. They turn right and become India's south-west monsoon.
• El Niño → trade winds weaken. La Niña → trade winds strengthen and upwelling off Peru grows stronger [3]. A common trap is to swap the two.
• In the Pacific, the trades start at about 30°–40° N and S. Their average speed is about 24 km/h (15 mph) [2].

Mains Points

• Trade winds and where rain falls (GS-I):
• The easterly trades pick up moisture over warm oceans and drop it on tropical east coasts (eastern Brazil, eastern Madagascar, Queensland).
• They reach the west coasts dry. With cold ocean currents, this helps explain why the hot deserts lie on western margins.

• The pressure belts move with the sun, and this movement creates seasonal climates such as the Indian monsoon.

• Link to India's monsoon and food security (GS-I/GS-III):

• 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 [3]. El Niño years are usually linked with weaker Indian monsoons.

• This matters for kharif sowing, drought planning and contingency crop plans.

• Trade winds and the ocean–atmosphere link (GS-I):

• Strong trades pile warm water in the western Pacific and keep cold upwelling going off Peru [3][5].
• When the trades weaken (El Niño), warm water moves east and weather changes around the world [3][4].
• So the strength of the trades is a key signal for seasonal forecasting.

Related concepts

• Southeast trade winds
• Westerlies
• Roaring Forties
• Polar easterlies
• Atmospheric river

Read more

• The planetary winds: trades, westerlies and polar easterlies