Introduction
This topic details how the uneven distribution of temperature leads to atmospheric pressure variations, causing wind—the horizontal movement of air from high-pressure to low-pressure areas. Wind plays a critical role in redistributing heat and moisture globally. This movement, both horizontal and vertical, drives the planet's weather and climate.
Atmospheric Pressure and Its Variation
Atmospheric Pressure is the weight of a column of air over a unit area from sea level to the top of the atmosphere.
It's measured in millibars (mb).
The average pressure at sea level is 1,013.2 mb.
Air is densest and pressure is highest near the surface due to gravity.
Pressure is measured using a mercury barometer or aneroid barometer.
Vertical Variation of Pressure
Pressure decreases rapidly with height in the lower atmosphere, approximately 1 mb for every 10 m increase in elevation. The immense vertical pressure gradient force is generally balanced by the opposing gravitational force, preventing strong upward winds.
| Level | Pressure in mb | Temperature ∘C |
| Sea Level | 1,013.25 | 15.2 |
| 1 km | 898.76 | 8.7 |
| 5 km | 540.48 | –17.3 |
| 10 km | 265.00 | –49.7 |
Horizontal Pressure Systems
To study pressure across the surface, readings are reduced to sea level. Isobars are lines connecting places of equal pressure.
Low-Pressure System (Cyclone): Enclosed by isobars with the lowest pressure at the centre.
High-Pressure System (Anticyclone): Enclosed by isobars with the highest pressure at the centre.
World Distribution of Sea Level Pressure
The world has four major alternating pressure belts that shift seasonally with the Sun's apparent movement (southward in Northern Hemisphere winter, northward in summer).
Equatorial Low: Near the equator (warm, rising air).
Subtropical Highs: Along $30^\circ\text{N}$ and $30^\circ\text{S}$ (sinking air).
Subpolar Lows: Along $60^\circ\text{N}$ and $60^\circ\text{S}$ (converging, rising air).
Polar Highs: Near the poles (cold, dense, sinking air).
Forces Affecting Wind Velocity and Direction
Horizontal winds near the Earth's surface are the result of three principal forces:
1. Pressure Gradient Force (PGF)
This is the force generated by the difference in atmospheric pressure over distance. It dictates that wind moves from high pressure to low pressure.
Strong PGF occurs where isobars are close together, resulting in high wind speed.
Weak PGF occurs where isobars are far apart.
2. Frictional Force
This acts to slow down wind speed.
It is greatest at the surface and typically extends up to 1–3 km.
It is minimal over the sea surface.
3. Coriolis Force
This is an apparent force caused by the Earth's rotation, which affects the direction of the wind.
It deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
It is maximum at the poles and zero at the equator (hence, tropical cyclones do not form at the equator).
It acts perpendicular to the PGF.
Pressure and Wind Direction
The combined effect of these forces determines wind patterns.
Geostrophic Wind: In the upper atmosphere (above the friction layer), the PGF is balanced by the Coriolis force, causing the wind to blow parallel to the isobars.
Cyclonic Circulation (Low Pressure): Air converges (spirals in) at the surface and rises.
Anticyclonic Circulation (High Pressure): Air subsides (sinks down) and diverges (spirals out) at the surface.
| Pressure System | Pressure Condition | Pattern of Wind Direction |
| at the Centre | Northern Hemisphere | |
| Cyclone (Low) | Low | Anticlockwise |
| Anticyclone (High) | High | Clockwise |
General Circulation of the Atmosphere
The General Circulation is the pattern of planetary winds, driven by the latitudinal variation in heating, pressure belts, rotation, and land/ocean distribution. It maintains the global heat balance and drives ocean currents.
Hadley Cell (Tropics): Air rises at the Inter Tropical Convergence Zone (ITCZ) (Equatorial Low), moves poleward aloft, sinks at $30^\circ\text{N/S}$ (Subtropical Highs), and flows back to the equator as the Easterlies (Trade Winds).
Ferrel Cell (Mid-Latitudes): A secondary cell driven by the other two. It sees rising warm air from the subtropical high and sinking cold air from the poles. Surface winds are the Westerlies.
Polar Cell (Polar Latitudes): Cold, dense air subsides at the poles and flows towards middle latitudes as the Polar Easterlies.
ENSO (El Niño-Southern Oscillation)
The warming and cooling of the Pacific Ocean significantly affects general circulation.
El Niño: The appearance of warm water off the coast of Peru, replacing the cool Peruvian current.
Southern Oscillation: The corresponding change in pressure conditions between the Central Pacific and Australia.
A strong ENSO event causes global weather extremes, such as heavy rain in coastal South America and droughts in Australia and sometimes India.
Local and Seasonal Winds
Land and Sea Breezes
These are local winds caused by the differential heating of land and sea.
Sea Breeze (Day): Land heats faster (Low Pressure), so wind blows from sea to land.
Land Breeze (Night): Land cools faster (High Pressure), so wind blows from land to sea.
Mountain and Valley Winds
Valley Breeze (Day): Slopes heat up, and air moves upslope from the valley.
Mountain Wind (Night): Slopes cool, and dense air descends into the valley.
Katabatic Wind: A type of mountain wind where cool air from high plateaus/ice fields drains down a slope.
Warm Downslope Winds: Air descends the leeward side of a mountain, is compressed, and adiabatically warmed, often melting snow rapidly.
Air Masses and Fronts
Air Masses
An air mass is a large body of air with uniform temperature and moisture characteristics acquired from its source region (e.g., vast ocean or plains).
| Type of Air Mass | Characteristics | Source Region Example |
| Maritime Tropical (mT) | Warm and Moist | Tropical Oceans |
| Continental Tropical (cT) | Warm and Dry | Subtropical Hot Deserts |
| Maritime Polar (mP) | Cold and Moist | High Latitude Oceans |
| Continental Polar (cP) | Cold and Dry | Snow Covered Continents |
Fronts
A front is the boundary zone between two different air masses. The process of their formation is called frontogenesis.
Cold Front: Cold air moves towards and displaces warm air.
Warm Front: Warm air moves towards and overrides cold air.
Stationary Front: Front remains unmoving.
Occluded Front: A faster-moving cold front overtakes a warm front, lifting the warm air completely off the surface.
Fronts are common in middle latitudes and bring abrupt weather changes and precipitation.
Cyclonic Systems
Extra Tropical (Mid-Latitude) Cyclones
These form in the middle and high latitudes along the polar front. They are characterized by a clear frontal system (warm and cold fronts).
Development: They begin as a stationary front, develop a wave, and then a well-defined cyclonic circulation with a warm sector between the forward warm front and the rear cold front.
The cold front moves faster and eventually overtakes the warm front, creating an occluded front, which leads to dissipation.
They cover a larger area, can originate over land or sea, and move from west to east.
Tropical Cyclones (Hurricanes, Typhoons)
These are violent storms originating over warm tropical oceans.
Favourable Conditions: Sea surface temperature $> 27^\circ\text{C}$, presence of Coriolis force, small vertical wind speed variation, and upper-level divergence.
The energy comes from the latent heat of condensation in towering cumulonimbus clouds.
They move from east to west and dissipate upon reaching land because their moisture supply is cut off.
| Feature | Description |
| Eye | The centre; a region of calm with subsiding air and extremely low pressure. |
| Eye Wall | Surrounds the eye; strong spiraling ascent of air, maximum wind velocity (up to 250 km/hr), and torrential rain. |
Thunderstorms and Tornadoes
These are severe, short-duration local storms.
Thunderstorms: Caused by intense convection on moist, hot days, forming large cumulonimbus clouds with strong updraft and downdraft, producing thunder, lightning, and precipitation (sometimes hailstorms).
Tornadoes: A violent, spiraling column of air descending from a severe thunderstorm, characterized by a very low-pressure centre, causing extreme destruction. A tornado over the sea is called a water spout.
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