Atmospheric pressure
Air pressure is the weight of the column of air above a place. It is measured with a barometer in millibars (mb) or hectopascals (hPa). Average sea-level pressure is about 1013.2 mb.
- Vertical change: pressure falls quickly with height, about 1 mb for every 10 m near the ground.
- Horizontal change: small differences between places drive the winds. Lines joining places of equal pressure are isobars. Close isobars = steep pressure gradient = strong wind.
Warm air expands and rises → low pressure. Cool air sinks → high pressure.
Forces affecting the speed and direction of wind
- Pressure gradient force: pushes air from high to low pressure. Bigger pressure difference = faster wind.
- Coriolis force: caused by the Earth's spin. It turns wind to the right in the Northern Hemisphere and to the left in the Southern. It is zero at the equator and largest at the poles. That is why cyclones do not form right on the equator.
- Friction: the ground slows wind in the lowest 1–3 km. Over the sea friction is small.
High up, where friction is absent, the pressure force and Coriolis balance and the wind blows parallel to isobars: the geostrophic wind. Around a low, winds spiral in (anticlockwise in the north): a cyclonic circulation. Around a high, winds spiral out (clockwise in the north): an anticyclonic circulation.
World pressure belts and planetary winds
- Equatorial low (0°–10°): strong heating, air rises; calm zone called the doldrums (ITCZ).
- Subtropical highs (~30° N and S): sinking air; calm, dry; the horse latitudes.
- Subpolar lows (~60° N and S).
- Polar highs (near the poles): very cold, heavy air.
General circulation (three cells)
Air rising at the equator moves poleward high up, sinks at 30° and returns to the equator near the ground: the Hadley cell. Between 30° and 60° is the Ferrel cell; near the poles is the polar cell.
Planetary (permanent) winds
- Trade winds: from subtropical highs to the equator; north-east trades in the north, south-east trades in the south.
- Westerlies: from subtropical highs to subpolar lows; very strong in the south (Roaring Forties).
- Polar easterlies: from polar highs to subpolar lows.
The ocean and atmosphere also work together: in some years the central Pacific warms unusually (El Niño), changing winds and often weakening India's monsoon.
Seasonal and local winds
Seasonal winds change direction with the season because the belts and the ITCZ move north and south with the Sun. The monsoon of South Asia is the best example: in summer, winds blow from sea to land (wet south-west monsoon); in winter, from land to sea (dry north-east monsoon).
Local winds
- Land and sea breeze: by day land heats faster → low pressure over land → cool sea breeze blows in. At night land cools faster → land breeze blows out to sea.
- Mountain and valley winds: by day warm air climbs the slopes (valley breeze); at night cold air slides down into the valley (mountain wind or katabatic wind).
- Others: the hot, dry loo of north India; warm dry winds that blow down mountains and melt snow (like the Chinook and Foehn).
Air masses and fronts
An air mass is a very large body of air that has nearly the same temperature and moisture across it. It gets these from its source region (a big warm ocean, a cold continent). Types: maritime tropical (warm, wet), continental tropical (warm, dry), maritime polar (cool, wet), continental polar (cold, dry), continental arctic.
A front is the boundary where two different air masses meet (frontogenesis). Types:
- Cold front: cold air pushes under warm air and lifts it steeply → tall clouds, sudden heavy rain, thunder.
- Warm front: warm air slides gently up over cold air → layered clouds, long light rain.
- Stationary front: neither side moves.
- Occluded front: a fast cold front catches up with a warm front and lifts the warm air off the ground.
Fronts are found in middle latitudes and bring sudden changes in weather.
Cyclones: extratropical and tropical
A cyclone is a low-pressure system with winds spiralling inward (anticlockwise in the north, clockwise in the south).
Extratropical (temperate) cyclones
Form along the polar front in middle latitudes where warm and cold air masses meet. Large, with warm and cold fronts; move west to east; bring changeable weather to places like Europe and the northern USA. In India, the western disturbances that bring winter rain to the north-west come from such systems.
Tropical cyclones
Violent storms born over warm tropical seas. Conditions: large sea surface with temperature above about 27 °C, Coriolis force (so not on the equator), small change in wind with height, and an existing weak low. Warm moist air rises, condenses and releases latent heat, which powers the storm. The calm centre is the eye, surrounded by the eye wall of the strongest winds and rain. On landfall they bring strong winds, very heavy rain and a storm surge (sea water pushed onto land); they weaken over land because the supply of moisture is cut off. Names: cyclones (Indian Ocean), hurricanes (Atlantic), typhoons (western Pacific), willy-willies (north-west Australia).
Thunderstorms and tornadoes
A thunderstorm forms when warm, moist air rises fast on hot days (strong convection). A tall cumulonimbus cloud grows, bringing lightning, thunder, heavy rain and sometimes hail. It is short-lived and local. In India, pre-monsoon storms like kalbaisakhi in West Bengal are examples.
A tornado is a narrow, violently spinning column of air that hangs from a thunderstorm cloud and touches the ground. It is very small but has the strongest winds on Earth and can destroy everything in its path. Over the sea it is called a waterspout. Tornadoes are common in the middle latitudes, especially the central USA.
Try it at home
Make a paper spinner: cut a spiral from paper and hang it by a thread above a lamp or a warm cup of tea (keep away from flames). Rising warm air turns it — that is convection, the start of every low-pressure system. Near a beach, notice how the breeze comes from the sea in the afternoon and from land late at night.
Key formulas and definitions
- Sea-level pressure ≈ 1013.2 mb; falls ~1 mb per 10 m of height
- Wind blows from High → Low pressure; close isobars = strong wind
- Coriolis: right in Northern Hemisphere, left in Southern; zero at equator
- Belts: Low (0°) – High (30°) – Low (60°) – High (90°)
- Winds: Trades (30°→0°), Westerlies (30°→60°), Polar easterlies (90°→60°)
- Tropical cyclone needs sea surface > ~27 °C + Coriolis + little wind shear
- Cyclone: low centre, winds in, anticlockwise (N); Anticyclone: high centre, winds out, clockwise (N)
Worked examples
1. Pressure at sea level is 1013 mb. Roughly what is it at 100 m height?
It falls about 1 mb per 10 m → 10 mb fall. About 1003 mb.
2. Why do sea breezes blow in the afternoon at Chennai beach?
By day the land heats faster than the sea. Air over land rises, making low pressure. Cooler, higher-pressure air over the sea flows in to fill it.
3. Wind leaves the subtropical high in the Northern Hemisphere and heads south towards the equator. Which way does it turn and what is it called?
Coriolis turns it to the right, so it comes from the north-east: the north-east trade wind.
4. Why do tropical cyclones not form between about 5° N and 5° S?
The Coriolis force is almost zero near the equator, so the air cannot be set spinning around the low.
5. A cyclone hits the Odisha coast and then moves 200 km inland. What happens to it and why?
It weakens fast. Over land it loses its supply of warm, moist air (its fuel), and friction with the land slows the winds.
Common mistakes
- Thinking wind blows from low to high pressure. It is always high → low.
- Saying Coriolis turns winds left in India. India is in the Northern Hemisphere, so winds turn right.
- Mixing up the two cyclone types: tropical cyclones need warm seas; extratropical ones form on fronts.
- Thinking a tornado and a cyclone are the same size. A tornado is a few hundred metres wide; a cyclone can be hundreds of km.