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If you are searching for the NCERT Class 11 Geography Fundamentals of Physical Geography Chapter 9: Atmospheric Circulation and Weather Systems Notes, then you’re in the right place. This blog provides simple and clear notes to help you understand the key concepts of this chapter. Whether you’re preparing for exams or need a quick revision, these notes will help you grasp the essential ideas without going through the entire textbook. So, let’s start!
Table of Contents
- 1 Introduction
- 2 Atmospheric Pressure
- 3 Vertical Variation of Pressure
- 4 Horizontal Distribution of Pressure
- 5 Forces Affecting the Velocity and Direction of Wind
- 6 General Circulation of the Atmosphere
- 7 General Atmospheric Circulation and Its Effects on Oceans
- 8 Air Masses
- 9 Fronts
- 10 Extratropical Cyclones
- 11 Tropical Cyclones
- 12 Thunderstorms and Tornadoes
- 13 Important Definitions in NCERT Notes Class 11 Geography Fundamentals of Physical Geography Chapter 9: Atmospheric Circulation and Weather Systems Notes
- 14 FAQs
Explore Notes of Class 11 Fundamentals of Geography
Introduction
Atmospheric circulation is driven by the uneven heating of the Earth’s surface, causing variations in atmospheric pressure. This leads to air movement from high to low pressure, creating winds. Winds redistribute heat and moisture globally, maintaining a stable planetary temperature. Vertical air movements form clouds and precipitation. This chapter explains pressure differences, forces controlling wind, air masses, and weather phenomena like cyclones, thunderstorms, and tornadoes.
Atmospheric Pressure
Atmospheric pressure is the force exerted by the weight of air per unit area, measured in millibars (mb) using mercury or aneroid barometers. At sea level, the average pressure is 1,013.2 mb. Pressure decreases with altitude due to less dense air and varies spatially due to temperature differences. Warm air expands, creating low pressure, while cold air contracts, forming high pressure. These variations drive wind from high to low-pressure areas.
Vertical Variation of Pressure
In the lower atmosphere, pressure decreases rapidly at about 1 mb per 10 meters of elevation. The rate varies, but pressure at 5,500 meters is roughly half of sea-level pressure. The vertical pressure gradient force is strong but balanced by gravity, preventing strong upward winds.
Horizontal Distribution of Pressure
Horizontal pressure distribution is shown by isobars, lines connecting places of equal pressure reduced to sea level. Low-pressure systems (cyclones) have converging, rising air, while high-pressure systems (anticyclones) have subsiding, diverging air. Global pressure belts include:
- Equatorial Low: Low pressure near the equator due to high temperatures.
- Subtropical Highs: High pressure at 30°N and 30°S from descending air.
- Subpolar Lows: Low pressure at 60°N and 60°S where air masses meet.
- Polar Highs: High pressure at the poles due to cold, dense air.
These belts shift with the sun’s apparent movement, moving south in January and north in July.
Also Read:
- NCERT Class 6 Geography: Chapter 5 Major Domains of the Earth
- NCERT Class 7 Geography Chapter 3 ‘Our Changing Earth’: Notes and Solutions (Free PDF)
Forces Affecting the Velocity and Direction of Wind
Winds result from pressure differences and are influenced by three forces:
- Pressure Gradient Force: Drives wind from high to low pressure; stronger gradients (closer isobars) produce faster winds.
- Frictional Force: Slows winds near the surface, extending up to 1–3 km; minimal over seas.
- Coriolis Force: Deflects winds right in the northern hemisphere and left in the southern hemisphere, proportional to latitude (zero at the equator, maximum at the poles).
Geostrophic winds occur in the upper atmosphere (2–3 km), balancing pressure gradient and Coriolis forces, blowing parallel to isobars. Near the surface, friction causes winds to blow across isobars. Cyclonic circulation occurs around lows, and anticyclonic circulation around highs, with directions varying by hemisphere.
General Circulation of the Atmosphere
The general circulation redistributes heat from low to high latitudes, driven by latitudinal heating variations, pressure belts, the sun’s migration, the continent-ocean distribution, and the Earth’s rotation. Key components include:
- Hadley Cell: Air rises at the Intertropical Convergence Zone (ITCZ) due to high insolation, creating an equatorial low, and sinks at 30°N and 30°S, forming subtropical highs. This drives trade winds (easterlies).
- Ferrel Cell: In mid-latitudes, westerlies flow between subtropical highs and subpolar lows.
- Polar Cell: Cold air sinks at the poles, forming polar highs, and blows as polar easterlies toward subpolar lows.
These cells maintain global heat balance through air circulation.
General Atmospheric Circulation and Its Effects on Oceans
Atmospheric winds drive ocean currents, redistributing heat. Trade winds push equatorial currents westward, westerlies drive currents like the Gulf Stream eastward, and polar winds influence polar currents. Oceans provide energy and water vapour to the atmosphere, interacting slowly over large areas.
Seasonal Winds
Wind patterns shift seasonally due to changes in heating and pressure belts. The monsoon in Southeast Asia is a key example, with winds reversing direction due to the ITCZ’s shift. Other seasonal winds include:
- Land and Sea Breezes: Daytime heating creates low pressure over land, drawing sea breezes; at night, land cools faster, causing land breezes.
- Mountain and Valley Winds: Daytime heating causes valley breezes up slopes; nighttime cooling causes mountain (katabatic) winds down slopes. Warm, dry winds on leeward slopes (e.g., due to adiabatic heating) can melt snow.
Local Winds
Local winds result from regional temperature and pressure differences. Examples include warm, dry winds on leeward mountain slopes that can rapidly melt snow due to adiabatic heating.
Air Masses
Air masses are large bodies of air with uniform temperature and humidity, formed over homogeneous source regions like oceans or plains. Types include:
- Maritime Tropical (mT): Warm, moist air from tropical oceans.
- Continental Tropical (cT): Warm, dry from subtropical deserts.
- Maritime Polar (mP): Cold, moist from high-latitude oceans.
- Continental Polar (cP): Cold, dry from high-latitude continents.
- Continental Arctic (cA): Very cold, dry from the Arctic/Antarctic regions.
Fronts
Fronts are boundaries between air masses, causing weather changes. Types include:
- Cold Front: Cold air displaces warm air, causing precipitation.
- Warm Front: Warm air rises over cold air, leading to clouds and rain.
- Stationary Front: Air masses stall, causing prolonged weather changes.
- Occluded Front: Cold front overtakes warm front, lifting warm air, causing complex weather.
Fronts occur in mid-latitudes with steep temperature and pressure gradients.
Extratropical Cyclones
Extratropical cyclones form in mid- and high latitudes along the polar front, with warm and cold fronts. Warm air rises over cold air, forming clouds and precipitation, while cold air pushes warm air up, creating cumulus clouds. These cyclones move from west to east, cover large areas, and can form over land or sea. They dissipate when the warm air is fully lifted (occluded).
Also Read:
- NCERT Class 6 Geography: Chapter 3 Motions of the Earth
- NCERT Class 6 Geography: Chapter 1 The Earth in the Solar System
Tropical Cyclones
Tropical cyclones are violent storms over warm tropical oceans (>27°C), forming between 5° and 20° latitude. Conditions include Coriolis force, low vertical wind shear, and a pre-existing low-pressure area. They feature a calm eye, a stormy eye wall with maximum winds (up to 250 km/h), and rain bands. They cause heavy rain, storm surges, and destruction, dissipating over land. Names include cyclones (Indian Ocean), hurricanes (Atlantic), typhoons (Pacific), and willy-willies (Australia).
Thunderstorms and Tornadoes
Thunderstorms form on hot, moist days with intense convection, producing cumulonimbus clouds, thunder, lightning, and heavy rain or hail. Dust storms occur when the moisture is low. Tornadoes are intense, spiraling winds descending from thunderstorms, causing destruction in mid-latitudes. Over water, they are called waterspouts. These storms convert potential and heat energy into kinetic energy, stabilising the atmosphere.
Important Definitions in NCERT Notes Class 11 Geography Fundamentals of Physical Geography Chapter 9: Atmospheric Circulation and Weather Systems Notes
Here we have explained the key concepts and terms of this chapter to make it easy for you to understand.
- Atmospheric Pressure: The force exerted by the weight of air per unit area, measured in millibars (average 1,013.2 mb at sea level).
- Isobars: Lines connecting places of equal atmospheric pressure, reduced to sea level.
- Pressure Gradient Force: The force driving wind from high to low pressure, proportional to isobar spacing.
- Coriolis Force: An apparent force from Earth’s rotation, deflecting winds right in the northern hemisphere and left in the southern.
- Geostrophic Wind: Winds in the upper atmosphere balancing pressure gradient and Coriolis forces, flowing parallel to isobars.
- Hadley Cell: A tropical circulation where air rises at the ITCZ and sinks at 30°N and 30°S, driving trade winds.
- Air Mass: A large body of air with uniform temperature and humidity, formed over homogeneous regions.
- Front: The boundary between two air masses, causing weather changes.
- Extra Tropical Cyclone: A mid-latitude cyclone with fronts, moving west to east, affecting large areas.
- Tropical Cyclone: A violent storm over warm tropical oceans, with a calm eye and destructive winds.
- Thunderstorm: A storm with cumulonimbus clouds, producing thunder, lightning, and heavy precipitation.
- Tornado: A spiralling, destructive wind descending from a thunderstorm, common in mid-latitudes.
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FAQs
Winds are caused by differences in atmospheric pressure, moving from high to low pressure areas, influenced by pressure gradient, Coriolis, and frictional forces.
Pressure belts (equatorial low, subtropical highs, subpolar lows, polar highs) drive global winds like trade winds, westerlies, and polar easterlies, redistributing heat via Hadley, Ferrel, and polar cells.
Tropical cyclones require a sea surface temperature above 27°C, Coriolis force, low vertical wind shear, a pre-existing low-pressure area, and upper-level divergence.
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