Atmospheric Circulation and Weather Systems Class 11 Geography Chapter 9 CBSE Notes 2025-26

Understanding atmospheric circulation and weather systems is key to grasping Earth's climate and weather phenomena. Variations in temperature across the planet lead to changes in air pressure. Warm air expands, rises, and reduces pressure in local regions, while cool air sinks, increasing atmospheric pressure. This pressure difference causes air to move from high-pressure areas to low-pressure zones, creating wind. These winds help redistribute heat and moisture, ultimately keeping Earth's temperature relatively balanced.

Atmospheric pressure is the force exerted by the weight of air above a particular point and is measured in millibars (mb). At sea level, the average atmospheric pressure is about 1,013.2 mb. As you move higher above sea level, the air becomes less dense and pressure falls rapidly—approximately 1 mb drop for every 10 meters in elevation in the lower atmosphere. For example, at 1 km above sea level, pressure drops to 898.76 mb, and at 10 km, it is only 265 mb, as shown below:

Vertical differences in pressure are much greater than horizontal differences but are balanced by gravity, preventing strong upward winds.

Small pressure changes across the surface can drive strong winds and influence climate patterns. On weather maps, isobars (lines connecting equal pressure points) are used for this purpose. At sea level, certain pressure belts encircle the globe—equatorial low near the equator, subtropical highs at about 30° N and S, subpolar lows at around 60°, and polar highs. These belts shift seasonally with the Sun’s apparent movement.

Three main forces affect wind: pressure gradient force, frictional force, and Coriolis force.

• Pressure gradient force moves air from high to low pressure; strength increases as isobars get closer.
• Frictional force is strongest near the Earth's surface and can slow or divert winds, especially over land.
• Coriolis force results from Earth’s rotation, deflecting wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, strongest at poles and zero at the equator.

The interaction of these forces means winds rarely move directly from high to low pressure. Near the equator, due to the absence of Coriolis force, cyclones do not form.

Above the friction layer (about 2–3 km), the wind is mostly controlled by a balance between the pressure gradient and Coriolis forces, causing it to move parallel to isobars as a "geostrophic wind." At the surface, friction modifies this, so winds angle from high to low pressure but are deflected.

Air rises and converges at low pressure, forming clouds and precipitation, while air descends and diverges at high-pressure areas.

Global wind circulation results from latitudinal heating, pressure belts, Earth’s rotation, and the arrangement of continents and oceans. There are three main wind belts in each hemisphere, including trade winds, westerlies, and polar easterlies, all adjusted with seasonal shifts due to the Sun’s position.

• Heating causes air to rise at the equator (low pressure) and sink at subtropical regions (high pressure).
• Seasonal migration of these belts influences monsoon and seasonal winds.
• Interaction between wind and ocean currents shapes regional climates.

The general circulation drives slow, large-scale ocean currents, exchanging heat and moisture with the atmosphere. In the Pacific, El Niño refers to unusually warm water off Peru, linked with pressure changes known as the Southern Oscillation. Together, they form ENSO, impacting global rainfall and drought patterns.

Seasonal shifts in wind and pressure create monsoons, especially dominant in Southeast Asia. Local winds include:

• Land and sea breezes: During daytime, the land heats faster, creating a sea breeze (wind from sea to land); at night, land cools faster, causing land breeze.
• Mountain and valley winds: Daytime heating causes upslope valley breeze; nighttime cooling causes denser mountain wind to flow downward. Cold katabatic winds and warm winds on leeward slopes can also form.

Large parcels of air with nearly uniform properties are called air masses. Based on source region, major types are:

• Maritime tropical (mT): warm and humid
• Continental tropical (cT): hot and dry
• Maritime polar (mP): cool and humid
• Continental polar (cP): cold and dry
• Continental arctic (cA): extremely cold and dry

The boundary where two different air masses meet is called a front. There are four main types: cold front, warm front, stationary front, and occluded front. Fronts are common in middle latitudes and bring sudden weather changes, often leading to clouds and rain.

Cyclonic systems (lows) and anticyclonic systems (highs) control large-scale weather. Winds circulate in an anticlockwise direction around lows in the Northern Hemisphere and clockwise around highs (and vice versa in the Southern Hemisphere). Surface wind patterns often align with those in the upper atmosphere.

Extra tropical cyclones form in mid- and high-latitudes, usually along polar fronts. The system consists of cold and warm fronts. Warm air is wedged between cold fronts and rises, creating clouds and rain; as the cold front closes in, it pushes the warm air upward further.

Tropical cyclones are intense storms forming over warm ocean areas with sea surface temperatures above 27°C, sufficient moisture, and a pre-existing low-pressure area. The Coriolis force helps in their development. The eye, a calm, low-pressure centre, is surrounded by intense spiraling winds and heavy rain in the “eye wall.” On making landfall, cyclones lose energy and dissipate.

• Cyclones in different locations have local names: hurricanes (Atlantic), typhoons (Western Pacific), and willy-willies (Australia).
• Tropical cyclones move slowly and can cause storm surges and coastal flooding.

Thunderstorms develop due to rising warm, moist air, quickly forming tall cumulonimbus clouds, thunder, lightning, and sometimes hail. Downdrafts occur later, bringing cool air and rain. Tornadoes, highly destructive but short-lived, may form from severe thunderstorms—marked by a spinning column of air, like a trunk, with very low pressure at the centre.

Thunderstorms and tornadoes are atmospheric responses to energy imbalances. After these events, the atmosphere stabilizes until energy redistributes again.

Class 11 Geography Chapter 9 Notes – Atmospheric Circulation and Weather Systems: Structured Revision Guide

Our Class 11 Geography Chapter 9 Notes cover every key point on atmospheric circulation and weather systems, following the NCERT textbook structure. Students can quickly review important tables, diagrams, and summary points—helpful for last-minute exam preparation and understanding tough concepts.

These revision notes present all chapters in logical order using easy language and clear headings. Difficult processes like cyclone formation, pressure belts, and local winds are explained simply, making it easier for students to score well and strengthen their fundamentals in Geography.