Atmospheric Circulation and Weather Systems

Introduction

The unequal heating of the Earth's surface creates pressure differences that drive the movement of air — wind. This chapter explains how atmospheric pressure belts form, how global wind patterns emerge, and how mid-latitude and tropical weather systems produce the weather we experience day to day.

Atmospheric Pressure

Atmospheric pressure is the force exerted by the weight of air per unit area of surface. It is measured with a barometer and expressed in millibars (mb) or hectopascals (hPa). Standard sea-level pressure is 1013.2 mb.

Pressure decreases with altitude (fewer air molecules above). Pressure also varies horizontally, and these variations drive wind.

Pressure Belts of the World

Global pressure belts are created by differential heating and Earth's rotation:

1. 1.Equatorial Low (ITCZ): Intense solar heating warms air, causing it to rise. This creates a belt of low pressure near the equator (0 degrees). Rising air produces clouds and heavy rainfall. Called the Inter-Tropical Convergence Zone (ITCZ).
2. 2.Sub-tropical Highs (Horse Latitudes): Air that rose at the equator descends near 30 degrees N and S, creating high-pressure belts. Descending air suppresses rainfall — most major deserts lie here.
3. 3.Sub-polar Lows: At 60 degrees N and S, cold polar air meets warm subtropical air; the polar front causes air to rise, creating low pressure.
4. 4.Polar Highs: At poles, cold dense air descends, creating high pressure.

Winds

• Wind moves from high pressure to low pressure. Several forces modify wind direction:
• Pressure Gradient Force (PGF): The force due to pressure difference; always directed from high to low pressure.
• Coriolis Effect: Due to Earth's rotation, moving objects deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It increases with latitude (zero at equator).
• Friction: Near the surface, friction slows wind and deflects it across isobars.

Planetary Wind Systems

1. 1.Trade Winds: Blow from sub-tropical highs toward the equatorial low. NE trades in Northern Hemisphere, SE trades in Southern Hemisphere. Consistent and reliable.
2. 2.Westerlies: Blow from sub-tropical highs toward sub-polar lows, in a west-to-east direction. Dominant winds in 40 degrees–60 degrees latitudes; bring weather systems to Europe and temperate regions.
3. 3.Polar Easterlies: Blow from polar highs toward sub-polar lows, in an east-to-west direction.

Seasonal and Local Winds

• Monsoons: Large-scale seasonal reversal of wind direction. In summer, the Asian landmass heats up, creating a low-pressure area that draws in moist ocean winds (SW monsoon over India). In winter, the continent is cold and high pressure, reversing the flow (NE monsoon).
• Land and Sea Breezes: During the day, land heats faster than sea; air rises over land, drawing in a sea breeze. At night, the pattern reverses — land breeze blows from land to sea.
• Mountain and Valley Breezes: By day, mountain slopes heat up faster; warm air rises (valley breeze). At night, slopes cool faster; cold air drains downslope (mountain breeze).
• Foehn/Loo: Warm, dry winds on the leeward side of mountains (Foehn in Alps; Loo in India).

Air Masses

• An air mass is a large body of air with relatively uniform temperature and humidity, formed by staying over a source region for days or weeks. Air masses are classified as:
• cP (continental polar): Cold and dry
• mP (maritime polar): Cold and moist
• cT (continental tropical): Hot and dry
• mT (maritime tropical): Warm and moist

Fronts and Mid-Latitude Cyclones

• A front is the boundary between two contrasting air masses. Fronts produce clouds, precipitation, and rapid weather changes.
• Cold front: Cold air displaces warm air; steep slope, intense but narrow rainfall band.
• Warm front: Warm air rides over cold air; gentle slope, widespread light rainfall.
• Occluded front: Cold front catches up to warm front; complex weather.

A mid-latitude (temperate) cyclone forms along the polar front. It is a low-pressure system with anticlockwise winds in the Northern Hemisphere. Weather sequence: warm sector with cirrus clouds → warm front rains → brief clearance → cold front thunderstorms → clearance.

Tropical Cyclones

Tropical cyclones (called hurricanes in Atlantic, typhoons in Pacific) form over warm ocean water (sea surface temperature > 26 degrees C). They are intense low-pressure systems with very strong winds spiralling inward.