The uneven distribution of temperature over the earth’s surface creates differences in atmospheric pressure. When air is heated, it expands; when it is cooled, it gets compressed. This variation in pressure sets air in motion from high pressure areas to low pressure areas. Air in horizontal motion is called wind.Atmospheric pressure also decides whether air will rise or sink. Rising moist air cools, forms clouds and causes precipitation. Winds redistribute heat and moisture across the planet and help maintain the overall temperature balance of the earth.
Atmospheric pressure is the weight of a column of air contained in a unit area from mean sea level to the top of the atmosphere. It is measured in millibars (mb).At sea level, the average atmospheric pressure is 1013.2 mb. Due to gravity, air near the surface is denser and therefore has higher pressure. Air pressure is measured by a mercury barometer or an aneroid barometer.Pressure decreases with height. Its variation from place to place is the primary cause of air motion.
In the lower atmosphere, pressure decreases rapidly with height. On average, pressure decreases by about 1 mb for every 10 metres increase in elevation, though the rate is not always uniform.
| Level | Pressure in mb | Temperature |
|---|---|---|
| Sea Level | 1013.25 | 15.2°C |
| 1 km | 898.76 | 8.7°C |
| 5 km | 540.48 | –17.3°C |
| 10 km | 265.00 | –49.7°C |
The vertical pressure gradient force is much larger than the horizontal pressure gradient force, but it is generally balanced by the opposite gravitational force. Therefore, strong upward winds are not normally experienced.
Small differences in pressure are very important for determining wind direction and velocity. Horizontal pressure distribution is shown on weather maps through isobars.Isobars are lines joining places having equal pressure. To remove the effect of altitude, pressure values are reduced to sea level before comparison.A low-pressure system is enclosed by one or more isobars with the lowest pressure at the centre. A high-pressure system is enclosed by one or more isobars with the highest pressure at the centre.
Near the equator, sea level pressure is low. This is called the Equatorial Low. Around 30° N and 30° S, high-pressure belts occur, known as Subtropical Highs. Around 60° N and 60° S, low-pressure belts occur, called Subpolar Lows. Near the poles, pressure is high and is known as the Polar High.
| Pressure Belt | Location |
|---|---|
| Equatorial Low | Near Equator |
| Subtropical Highs | Around 30° N and 30° S |
| Subpolar Lows | Around 60° N and 60° S |
| Polar Highs | Near Poles |
These pressure belts are not permanent. They shift with the apparent movement of the sun. In the Northern Hemisphere, they move southwards in winter and northwards in summer.
Wind is controlled mainly by three forces near the earth’s surface: pressure gradient force, frictional force, and Coriolis force. In addition, gravitational force acts downward.
The difference in atmospheric pressure produces the pressure gradient force. It is the rate of change of pressure with distance. Where isobars are close together, the pressure gradient is strong and wind speed is higher. Where isobars are far apart, the pressure gradient is weak.
Friction affects the speed of wind. It is greatest at the surface and generally influences wind up to 1–3 km above the surface. Friction is minimum over the sea surface.
The rotation of the earth causes the Coriolis force. It deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The deflection increases with wind velocity.The Coriolis force is directly proportional to latitude. It is maximum at the poles and absent at the equator. Because Coriolis force is zero at the equator, tropical cyclones do not form near the equator.
The velocity and direction of wind are the result of wind-generating forces. In the upper atmosphere, about 2–3 km above the surface, winds are free from surface friction and are controlled mainly by pressure gradient force and Coriolis force.When isobars are straight and friction is absent, the pressure gradient force is balanced by the Coriolis force. The wind then blows parallel to the isobars. Such wind is called Geostrophic Wind.
Wind circulation around a low-pressure centre is called cyclonic circulation. Wind circulation around a high-pressure centre is called anticyclonic circulation.
| Pressure System | Centre Condition | Northern Hemisphere | Southern Hemisphere |
|---|---|---|---|
| Cyclone | Low Pressure | Anticlockwise | Clockwise |
| Anticyclone | High Pressure | Clockwise | Anticlockwise |
Over a low-pressure area, air generally converges and rises. Over a high-pressure area, air subsides from above and diverges at the surface. Rising air is important for the formation of clouds and precipitation.
The pattern of planetary winds depends on:
This broad pattern of planetary wind movement is called the general circulation of the atmosphere. It also sets ocean waters in motion and influences the earth’s climate.
At the Inter Tropical Convergence Zone (ITCZ), intense heating causes convection and creates low pressure. Winds from the tropics converge here and rise up to the top of the troposphere, nearly 14 km. The air then moves towards the poles and accumulates near 30° N and 30° S, where part of it sinks and forms subtropical highs.This tropical circulation is called the Hadley Cell. In the middle latitudes, circulation between subtropical highs and polar air forms the Ferrel Cell. Near the poles, cold air sinks and moves towards middle latitudes as Polar Easterlies, forming the Polar Cell.
| Cell | Location / Feature |
|---|---|
| Hadley Cell | Tropical circulation |
| Ferrel Cell | Middle latitude circulation |
| Polar Cell | Polar circulation |
These three cells help transfer heat energy from lower latitudes to higher latitudes.
General atmospheric circulation also affects the oceans. Large-scale winds initiate slow-moving ocean currents. Oceans supply energy and water vapour to the atmosphere.The warming and cooling of the Pacific Ocean is especially important. When warm water of the central Pacific slowly drifts towards the South American coast and replaces the cool Peruvian Current, it is called El Niño.El Niño is closely linked with pressure changes over the central Pacific and Australia. This pressure change is called the Southern Oscillation. Together, El Niño and Southern Oscillation are called ENSO.During strong ENSO years:
ENSO is closely monitored for long-range weather forecasting.
Wind circulation changes with seasons due to the shifting of regions of maximum heating, pressure belts and wind belts. The most pronounced effect is seen in the monsoon system, especially over Southeast Asia.
Local winds are created by differences in heating and cooling of land and water surfaces. These winds may develop daily or annually.
During the day, land heats faster than the sea. Air over land rises and creates low pressure, while the sea remains relatively cool and has higher pressure. Wind blows from sea to land as Sea Breeze.At night, land cools faster than the sea. The pressure gradient reverses and wind blows from land to sea as Land Breeze.
| Time | Wind | Direction |
|---|---|---|
| Day | Sea Breeze | Sea to Land |
| Night | Land Breeze | Land to Sea |
During the day, mountain slopes heat up and air moves upslope. Air from the valley moves upward to fill the gap. This is called Valley Breeze.At night, slopes cool and dense air descends into the valley. This is called Mountain Wind. Cold air draining from high plateaus and ice fields into valleys is called Katabatic Wind.A warm dry wind may occur on the leeward side of mountain ranges. Moisture condenses and precipitates while crossing the mountains. As the dry air descends on the leeward side, it warms by the adiabatic process and can melt snow quickly.
When air remains over a homogeneous area for a long time, it acquires the characteristics of that area. A large body of air having little horizontal variation in temperature and moisture is called an air mass.Homogeneous surfaces over which air masses form are called source regions.
| Source Region | Air Mass Type |
|---|---|
| Warm tropical and subtropical oceans | Maritime Tropical (mT) |
| Subtropical hot deserts | Continental Tropical (cT) |
| Cold high-latitude oceans | Maritime Polar (mP) |
| Snow-covered high-latitude continents | Continental Polar (cP) |
| Permanently ice-covered Arctic and Antarctica | Continental Arctic (cA) |
Tropical air masses are warm, while polar air masses are cold.
When two different air masses meet, the boundary zone between them is called a front. The process of front formation is called frontogenesis.
| Type of Front | Meaning |
|---|---|
| Cold Front | Cold air moves towards warm air |
| Warm Front | Warm air moves towards cold air |
| Stationary Front | Front remains stationary |
| Occluded Front | Warm air is fully lifted above the surface |
Fronts occur mainly in middle latitudes. They have steep gradients of temperature and pressure. They bring abrupt weather changes, lift air, form clouds and cause precipitation.
Extra tropical cyclones develop in the middle and high latitudes, beyond the tropics. They are also called middle latitude cyclones. They form along the polar front.Initially, the front is stationary. In the Northern Hemisphere, warm air blows from the south and cold air from the north. When pressure drops along the front, warm air moves northwards and cold air moves southwards, creating anticlockwise cyclonic circulation.A well-developed extra tropical cyclone has:
Warm air glides over cold air and forms a sequence of clouds ahead of the warm front, causing precipitation. The cold front moves faster than the warm front and eventually overtakes it. The warm air is lifted completely, the front becomes occluded, and the cyclone dissipates.
| Basis | Extra Tropical Cyclone | Tropical Cyclone |
|---|---|---|
| Region | Middle and high latitudes | Tropical oceans |
| Fronts | Clear frontal system present | No frontal system |
| Origin | Land and sea both | Only over seas |
| Area Covered | Larger area | Smaller area |
| Wind Velocity | Lower | Higher |
| Destruction | Less destructive than tropical cyclone | More destructive |
| Movement | West to East | East to West |
Tropical cyclones are violent storms that originate over warm tropical oceans and move towards coastal areas. They cause destruction through violent winds, very heavy rainfall and storm surges.They are known by different names in different regions:
| Region | Name |
|---|---|
| Indian Ocean | Cyclones |
| Atlantic | Hurricanes |
| Western Pacific and South China Sea | Typhoons |
| Western Australia | Willy-willies |
Tropical cyclones form and intensify under the following conditions:
The energy of the storm comes from condensation in towering cumulonimbus clouds around the centre. Continuous moisture supply from the sea strengthens the storm. After reaching land, the moisture supply is cut off and the cyclone weakens.The place where a tropical cyclone crosses the coast is called landfall. Cyclones crossing 20° N latitude generally recurve and become more destructive.
A mature tropical cyclone has strong spiral winds around a calm centre called the eye. The diameter of the circulating system may vary between 150 and 250 km.The eye is calm and has subsiding air. Around the eye lies the eyewall, where air rises strongly up to the tropopause. The maximum wind velocity occurs in the eyewall and may reach 250 km per hour. Torrential rain also occurs here.Rain bands of cumulus and cumulonimbus clouds extend outward from the eyewall. Over the Bay of Bengal, Arabian Sea and Indian Ocean, the diameter of the storm may be 600–1200 km. The system moves slowly, about 300–500 km per day.Tropical cyclones create storm surges, which inundate low-lying coastal areas. The storm weakens after moving over land.
Thunderstorms and tornadoes are severe local storms. They occur over small areas for short periods but are violent.
Thunderstorms are caused by intense convection on moist hot days. A thunderstorm is a well-developed cumulonimbus cloud producing thunder and lightning.If clouds rise to heights where sub-zero temperatures exist, hail forms and falls as a hailstorm. If moisture is insufficient, thunderstorms may generate dust storms.A thunderstorm has strong updrafts of warm air, which cause clouds to grow higher. This leads to precipitation. Later, downdrafts bring cool air and rain down to the surface.
From severe thunderstorms, a spiralling wind may descend like the trunk of an elephant with great force. It has very low pressure at the centre and causes massive destruction. This is called a tornado.Tornadoes generally occur in middle latitudes. A tornado over the sea is called a waterspout.
Atmospheric circulation is the result of unequal heating, pressure differences and earth’s rotation. It controls winds, air masses, fronts, cyclones, thunderstorms and tornadoes. These weather systems represent the atmosphere’s adjustment to uneven energy distribution, where potential and heat energy are converted into kinetic energy, helping the atmosphere return towards stability.