Wind and Its Deflection
Leaves outside the window sway, yet they do not move on their own. The moving air that sways them is wind.
The movement of air is wind. Air moves horizontally from a high pressure area to a low pressure area and winds are generated.
Key idea 1 : Why does wind blow? — We feel wind but cannot see it. If pressure differs between two places air flows from the higher to the lower pressure. Air rushing out when a balloon’s mouth opens is a small form of the same.
Key idea 2 : Speed of wind — The bigger the pressure difference the faster the wind. If the difference is small winds are gentle. Wind speed is measured in knots or kilometres per hour and varies from place to place.
Key idea 3 : Effect of rotation — The earth rotates from west to east. So winds do not move straight but turn. In the northern hemisphere winds turn to the right of their original direction, in the southern hemisphere to the left. Curved arrows show this in diagrams. That is why the direction of the same wind differs between hemispheres.
- Easy : Wind blows from where to where? — High to low pressure.
- Medium : In the northern hemisphere winds turn? — Right.
- Exam-type : Speed is measured in? — Knots or km/h.
In the figure: Wind from high to low pressure.
High → low; north = right, south = left.
Wind does not go straight; rotation deflects it.
Memory clue: North right, south left.
Spot the difference
| Hemisphere | Deflection | Hint |
|---|---|---|
| Northern | To the right | N = right |
| Southern | To the left | S = left |
Practice questions
- 1What is wind?
- 2How does wind generate?
- 3On what does wind speed depend?
- 4In which units is wind speed measured?
- 5What effect does rotation have on winds?
- 6Which way do winds turn in the northern hemisphere?
- 7Which way do winds turn in the southern hemisphere?
- 8What happens to wind if the pressure difference is small?
Show answers (try them yourself first)
- 1Movement of air.
- 2Air flows from high to low pressure.
- 3On the pressure difference.
- 4Knots or kilometres per hour.
- 5Winds turn from their original direction.
- 6To the right.
- 7To the left.
- 8The wind will be gentle.
Planetary Winds
Some winds blow all year in a fixed direction like a river current. The earth’s large winds of this type are planetary winds.
Winds that blow regularly all year from high pressure belts to low pressure belts over a large part of the earth are planetary winds: Easterlies, Westerlies and Polar winds.
Key idea 1 : Easterlies — Winds blow from the 25°–35° high pressure belt towards the equatorial low pressure belt. Due to rotation they blow from north-east to south-west in the northern hemisphere and from south-east to north-west in the southern. From both sides they converge near the equator. They are Easterlies.
Key idea 2 : Westerlies and Polar winds — Winds blow from the mid-latitudinal high to the sub-polar low near 60°. In the southern hemisphere they blow from north-west to south-east and in the northern from south-west to north-east; these are Westerlies. Winds from the polar high to the sub-polar low (55° to 65°) are Polar winds, generally from east to west. A wind is named for the direction it comes from; Westerlies come from the west.
Key idea 3 : Strong southern winds and calm belts — The southern hemisphere is mostly ocean with almost no land obstacles, so winds blow faster there. Beyond 40° S they blow tremendously: the Roaring Forties. Around 50° S they are stormy: the Furious Fifties. Near 60° S they make great noise: the Screeching Sixties. Within 5° N and S of the equator winds mostly do not blow; this calm zone is the Doldrums. The 25°–35° high pressure region is also calm and is called the Horse Latitudes.
- Easy : Easterlies blow from where to where? — 25°–35° to the equator.
- Medium : Westerlies come from? — The west.
- Exam-type : Calm belt within 5° N-S? — Doldrums.
In the figure: The three planetary winds.
Named for where it comes from: Westerlies = from the west.
Westerlies do not blow towards the west; they come from the west.
Memory clue: 0–30 Easterlies; 30–60 Westerlies; 60–90 Polar.
Spot the difference
| Winds | From → to | N. hemisphere direction |
|---|---|---|
| Easterlies | 30° → 0° | NE → SW |
| Westerlies | 30° → 60° | SW → NE |
| Polar | 90° → 60° | East → west |
Practice questions
- 1What are planetary winds? Name their types.
- 2How do Easterlies blow?
- 3How do Westerlies blow?
- 4Why do winds blow faster in the southern hemisphere?
- 5What are the Doldrums?
- 6What are Horse Latitudes?
- 7From where to where do Polar winds blow?
- 8Where are the Roaring Forties found?
Show answers (try them yourself first)
- 1Regular all-year winds: Easterlies, Westerlies, Polar.
- 2North-east to south-west in the northern hemisphere.
- 3South-west to north-east in the northern hemisphere.
- 4More ocean; no land obstacles.
- 5Calm belt within 5° N–S of the equator.
- 6The calm belt at 25°–35°.
- 7From polar high to sub-polar low.
- 8Beyond 40° S.
Local Winds and Seasonal Winds
At the seashore the cool wind comes from the sea in the afternoon and reverses at night. Such winds blow only in specific areas.
Winds that last a short time, arise in specific regions and blow over a limited area are local winds. Large winds that reverse by season due to uneven heating of land and sea are monsoon winds.
Key idea 1 : Valley, mountain, sea and land breezes — By day hilltops heat quickly, air warms and rises and pressure falls there; cool air from the foothills moves up: the valley breeze. At night the mountain cools fast, pressure rises there and wind blows down to the valley: the mountain breeze. By day land heats fast and pressure falls; over water pressure stays high so wind goes from sea to land: the sea breeze. At night land cools fast, pressure rises and wind goes from land to sea: the land breeze.
Key idea 2 : Why land and water heat differently — Land is dense, stable and opaque so heat spreads fast and in greater amount and land heats quickly. Water is comparatively less dense, transparent and unstable, so it heats slowly. So the air pressure over land and water differs and this difference causes winds.
Key idea 3 : Named local winds and monsoon — The Loo is a hot dry wind blowing in the afternoons of summer in the north Indian plains; it comes from the Thar desert. The Simoom is a destructive hot dry wind of great velocity from the Sahara and Arabia. The Chinook blows warm and dry down the eastern slopes of the Rockies and melts snow, hence “snow eater”. Mistral and Bora are cold dry winds from the Alps; the Fohn is hot and dry on the Alps’ northern slopes. Monsoon winds blow from sea to land in summer and from land to sea in winter; they are sea and land breezes on a large scale. South-east Asia, East Africa and north Australia feel them strongly. In India the south-west monsoon brings moisture from June to September and the north-east monsoon, from September to December, is dry.
- Easy : Daytime sea-to-land wind? — Sea breeze.
- Medium : Hot summer wind of north India? — Loo.
- Exam-type : South-west monsoon months? — June to September.
In the figure: Direction changes by day-night and by season.
Day from sea; night from land.
Monsoon winds do not always bring rain; the north-east monsoon is dry.
Memory clue: Name = where it comes from / its nature.
Spot the difference
| Wind | Nature | Region |
|---|---|---|
| Loo | Hot, dry | North India |
| Chinook | Warm, dry | Rockies |
| Mistral | Cold, dry | Spain, France |
Practice questions
- 1What are local winds?
- 2How does a sea breeze form?
- 3When and why does the land breeze blow?
- 4Why do land and water heat differently?
- 5Differentiate valley breeze and mountain breeze.
- 6What is the Loo?
- 7Why is the Chinook called the snow eater?
- 8How do monsoon winds form?
Show answers (try them yourself first)
- 1Winds blowing briefly over a limited area.
- 2Land heats giving low pressure; wind from sea.
- 3At night; land cools giving high pressure.
- 4Land is dense; water heats slowly.
- 5Valley breeze goes up by day; mountain breeze down at night.
- 6A hot dry summer wind from the Thar.
- 7It melts the snow.
- 8Winds reversing by season due to uneven heating.
Cyclones and Anticyclones
Water draining from a bucket spins round with a dip in the middle. Air in a cyclone also spins around a centre.
When a low pressure is surrounded by high pressure winds spiral inward: a cyclone. When high pressure is surrounded by low, winds blow outward from the centre: an anticyclone.
Key idea 1 : Cyclone — When a low pressure area is surrounded by high pressure, winds start moving towards the centre. Due to rotation they spin anticlockwise in the northern hemisphere and clockwise in the southern. The sky is cloudy, winds are very fast and rain is heavy. The affected area is limited, but duration, speed, direction and origin are unpredictable. Weather maps mark the centre with ‘L’.
Key idea 2 : Storm types and names — In the western Pacific near Japan, China and the Philippines cyclones are Typhoons; they arise from June to October. Cyclones in the Caribbean Sea are Hurricanes. In these storms the minimum wind speed is 60 km per hour. Cyclones also form in the temperate zone but are weaker and not destructive. When wind speed exceeds 33 knots (60 km/h) the storm gets a name, which makes it easy to remember.
Key idea 3 : Anticyclone — When pressure in a region rises and the surrounding stays low, winds blow outward from the centre in a circle. In the northern hemisphere they go clockwise and in the southern anticlockwise. The sky is clear, winds gentle and weather pleasant. Anticyclones last a few days or a week and originate in temperate zones. Maps mark the centre with ‘H’. In high pressure belts winds always move outward, so rainfall is low.
- Easy : Cyclone centre? — Low pressure, ‘L’.
- Medium : Weather in an anticyclone? — Clear and pleasant.
- Exam-type : Cyclone near Japan-China? — Typhoon.
In the figure: Cyclone in, anticyclone out.
As per the textbook typhoons occur near Japan, China and the Philippines from June to October, Caribbean cyclones are hurricanes, and storms above 33 knots (60 km/h) are named. (as per the textbook)
L = low = cyclone; H = high = anticyclone.
An anticyclone is not stormy; it brings calm, clear weather.
Memory clue: North: cyclone = anticlockwise; anticyclone = clockwise.
Spot the difference
| Point | Cyclone | Anticyclone |
|---|---|---|
| Centre | Low (L) | High (H) |
| Winds | Spiral in, strong | Spiral out, gentle |
| Weather | Cloudy, rain | Clear, pleasant |
| N. spin | Anticlockwise | Clockwise |
Practice questions
- 1What is a cyclone?
- 2What is an anticyclone?
- 3Differentiate cyclone and anticyclone.
- 4Why do winds spin in a cyclone?
- 5What is the difference between typhoon and hurricane?
- 6Why are cyclones named?
- 7What do L and H show on weather maps?
- 8State the effects of a cyclone.
Show answers (try them yourself first)
- 1Inward-spiralling winds around a low pressure area.
- 2Outward winds around a high pressure area.
- 3One in, the other out; weather opposite.
- 4Due to the earth’s rotation.
- 5Typhoon in the west Pacific; hurricane in the Caribbean.
- 6To remember them easily.
- 7L = low centre; H = high centre.
- 8Cloudy sky, fast winds, heavy rain.