Atmospheric Circulation and Weather Systems Class 11 Geography Notes

This chapter explains atmospheric pressure, pressure belts, wind systems, air masses, fronts, cyclones and general circulation of atmosphere. It highlights how pressure differences and Earth’s rotation create global wind systems and weather phenomena across the world.

• Atmospheric pressure
• Pressure distribution
• Pressure gradient force
• Coriolis force
• Geostrophic wind
• General circulation of atmosphere
• Hadley Cell
• Ferrel Cell
• Polar Cell
• ENSO and El Nino
• Local winds
• Air masses
• Fronts
• Tropical cyclones
• Extra tropical cyclones
• Thunderstorms and tornadoes

Meaning of Atmospheric Pressure

Atmospheric pressure is the weight of air column exerted over unit area from mean sea level to the top of atmosphere.

Unit of Atmospheric Pressure

• Measured in millibar (mb)
• Average sea level pressure is 1013.2 mb

Instruments Used to Measure Pressure

1. Mercury barometer
2. Aneroid barometer

Vertical Variation of Pressure

• Pressure decreases with height
• Average decrease is 1 mb for every 10 m increase in elevation

Standard Pressure and Temperature

Isobars

Isobars are lines joining places having equal pressure.

Low Pressure System

• Lowest pressure at centre
• Surrounded by isobars

High Pressure System

• Highest pressure at centre
• Surrounded by isobars

World Distribution of Pressure

Equatorial Low Pressure Belt

• Located near equator
• Low pressure zone

Subtropical High Pressure Belt

• Located around 30° N and S

Subpolar Low Pressure Belt

• Located around 60° N and S

Polar High Pressure Belt

• Located near poles

Pressure Belts Shift

Pressure belts shift north and south with the apparent movement of the Sun.

Pressure Gradient Force

Pressure gradient is the rate of pressure change with distance.

• Strong gradient – Isobars close together
• Weak gradient – Isobars far apart

Frictional Force

• Slows wind speed
• Maximum near Earth’s surface
• Influence up to 1–3 km height

Coriolis Force

Coriolis force is caused by Earth’s rotation and named after Gaspard Coriolis.

Deflection of Winds

• Northern Hemisphere – Deflects to right
• Southern Hemisphere – Deflects to left

Characteristics of Coriolis Force

• Maximum at poles
• Zero at equator

Geostrophic Wind

Wind blowing parallel to isobars when pressure gradient force balances Coriolis force.

Cyclonic Circulation

• Cyclone is a low pressure system
• Anticlockwise in Northern Hemisphere
• Clockwise in Southern Hemisphere

Anticyclonic Circulation

• Anticyclone is a high pressure system
• Clockwise in Northern Hemisphere
• Anticlockwise in Southern Hemisphere

Convergence and Divergence

• Low pressure – Surface convergence and rising air
• High pressure – Surface divergence and descending air

Factors Affecting General Circulation

1. Latitudinal heating
2. Pressure belts
3. Migration of pressure belts
4. Distribution of land and water
5. Earth’s rotation

Inter Tropical Convergence Zone (ITCZ)

• Low pressure zone near equator
• Trade winds converge here
• Warm air rises upward

Hadley Cell

• Tropical circulation cell
• Air rises at equator
• Air sinks near 30° latitude

Ferrel Cell

• Mid-latitude circulation cell
• Surface winds are westerlies

Polar Cell

• Polar circulation system
• Surface winds are polar easterlies

El Nino

El Nino is a warm ocean current near the Peru coast.

Southern Oscillation

Pressure changes over Pacific Ocean are known as Southern Oscillation.

ENSO

• Combined effect of El Nino and Southern Oscillation
• Causes floods and droughts
• Influences global weather patterns

Seasonal Winds

• Change due to shifting pressure belts
• Main example – Monsoon winds

Sea Breeze

• Occurs during daytime
• Wind blows from sea to land

Land Breeze

• Occurs during nighttime
• Wind blows from land to sea

Valley Breeze

• Warm air rises upslope during daytime

Mountain Breeze

• Cool dense air descends into valley during nighttime

Katabatic Wind

Cold dense air flowing downhill.

Meaning of Air Mass

Air mass is a large body of air with uniform temperature and humidity.

Source Regions of Air Masses

1. Tropical oceans
2. Hot deserts
3. Cold oceans
4. Snow-covered continents
5. Arctic and Antarctic regions

Types of Air Masses

Meaning of Front

Front is the boundary between two air masses.

Frontogenesis

Formation process of fronts.

Types of Fronts

1. Cold front
2. Warm front
3. Stationary front
4. Occluded front

Cold Front

Cold air moves towards warm air.

Warm Front

Warm air moves towards cold air.

Meaning of Extra Tropical Cyclones

• Cyclones beyond tropics
• Also called middle latitude cyclones

Formation

• Form along polar front
• Contain warm and cold fronts

Features

• Larger area coverage
• Move west to east
• Occur over land and sea

Meaning of Tropical Cyclones

Tropical cyclones are violent storms formed over tropical oceans.

Names in Different Regions

Conditions for Formation

1. Sea temperature above 27°C
2. Coriolis force
3. Weak low pressure area
4. Small vertical wind variation
5. Upper divergence

Eye of Cyclone

Calm central region of cyclone.

Eye Wall

• Strong spiralling winds
• Heavy rainfall
• Wind speed up to 250 km/hour

Storm Surge

Rise of sea water during cyclone causing coastal flooding.

Thunderstorms

• Violent cumulonimbus cloud activity
• Associated with thunder and lightning
• Heavy rainfall occurs

Tornado

• Spiralling destructive wind
• Very low pressure centre
• Causes massive destruction

Waterspout

Tornado occurring over sea is called waterspout.

Very Short Answer Questions

1. What is atmospheric pressure?
2. What are isobars?
3. What is Coriolis force?
4. What is ITCZ?
5. What is ENSO?

Short Answer Questions

1. Explain pressure gradient force.
2. Discuss Coriolis force.
3. Differentiate cyclone and anticyclone.
4. Explain land and sea breezes.
5. What are air masses?

Long Answer Questions

1. Explain the factors affecting wind direction and speed.
2. Discuss the general circulation of atmosphere.
3. Explain tropical cyclone formation and structure.
4. Differentiate tropical and extra tropical cyclones.
5. Explain fronts and their types.

1. What is atmospheric pressure?

Atmospheric pressure is the weight of air exerted over unit area.

2. What are isobars?

Isobars are lines joining places having equal pressure.

3. What is Coriolis force?

It is the force caused by Earth’s rotation that deflects winds.

4. What is ITCZ?

ITCZ is the low pressure zone near the equator where trade winds converge.

5. What is the eye of a cyclone?

The calm central region of a tropical cyclone is called the eye.

• Atmospheric Pressure:
  • Weight of air column
• Sea Level Pressure:
  • 1013.2 mb
• Isobars:
  • Equal pressure lines
• Pressure Belts:
  • Equatorial low
  • Subtropical high
  • Subpolar low
  • Polar high
• Wind Forces:
  • Pressure gradient force
  • Frictional force
  • Coriolis force
• Coriolis Effect:
  • Right in Northern Hemisphere
  • Left in Southern Hemisphere
• Geostrophic Wind:
  • Parallel to isobars
• Circulation Cells:
  • Hadley
  • Ferrel
  • Polar
• ENSO:
  • El Nino + Southern Oscillation
• Local Winds:
  • Sea breeze
  • Land breeze
  • Valley breeze
  • Mountain breeze
• Air Masses:
  • mT
  • cT
  • mP
  • cP
  • cA
• Tropical Cyclones:
  • Warm ocean storms
• Eye:
  • Calm centre
• Eye Wall:
  • Maximum wind and rainfall
• Tornado:
  • Violent spiralling wind