NCERT Class 11 Geography Chapter 10: Atmospheric Circulations and Weather Systems YouTube Lecture Handouts

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NCERT Class 11 Geography Part 1 Chapter 10: Atmospheric Circulations and Weather Systems
  • Atmospheric Pressure determines rise and sink of air
  • Wind – redistributes heat and moisture & maintain constant temperature of planet

Atmospheric Pressure

Weight of column of air contained in unit area from mean sea level to top of atmosphere – measured in mb or Pascals [1 hectopascal ]

Atmospheric Pressure

At sea level average atmospheric pressure is 1013.2 mb

Air at surface is denser and has higher pressure

Pressure is measured by mercury barometer or aneroid barometer

Pressure decreases with height and air gets verified and one feels breathless

Wind moves from HP to LP

Decrease amounts to about 1 mb for each 10 m increase in elevation

Horizontal distribution by isobars at constant levels (join places of equal pressure)

Equal Pressure

Distribution

At equator – sea level pressure is low called equatorial lows

At – HP as subtropical highs

Polewards – LP as subpolar lows

At poles – HP as polar highs

Pressure belts are not permanent and oscillate with sun Examrace YouTube Channel s movement

In North Hemisphere in winter they move southwards and in summer northwards

Forces Affecting Velocity and Direction of Wind

  • Air in motion is called as wind
  • Wind blows from HP to LP
  • Wind at surface experience friction
  • Rotation of earth affects wind movement – force exerted by rotation is Coriolis force
  • Pressure Gradient Force – caused by differences in atmospheric pressure (strong where isobars are close and weak where they are apart)
  • Frictional Force - affects speed and is maximum at surface upto 1 - 3 km; minimal over sea
  • Coriolis force – discovered in 1844 is apparent force – winds deflect to right in NH and left in SH – deflection is high when velocity is high. It is also proportional to angle of latitude – maximum at pole and absent at equator
  • At the equator, Coriolis force is zero and wind blows perpendicular to the isobars. LP gets filled instead of getting intensified & so tropical cyclones are not formed near equator.

Pressure and Wind

  • Velocity and direction of wind are net result of wind generating force
  • Wind circulation around a low is called cyclonic circulation & around a high it is called anticyclonic circulation
Low Convergence and High Divergence

When isobars are straight and when there is no friction, the pressure gradient force is balanced by the Coriolis force and the resultant wind blows parallel to the isobar. This wind is known as the geostrophic wind.

General Circulation (pattern of movement of planetary winds) depends on:

  • Latitudinal variation of atmospheric heating
  • Emergence of pressure belts
  • Migration of belts following apparent path of sun
  • Distribution of continents and oceans
  • Rotation of earth

Pattern of movement of planetary wind is called general circulation of atmosphere

Polar Vortex
Pressure System
  • At ITCZ – air rise is due to high insolation & LP is created
  • Winds from tropics converge at LP and rise along convective cell – it reaches top of troposphere upto 14 km and moves to poles – so air accumulates at - it sinks to ground and forms subtropical highs – at surface air flows as easterlies
  • Easterlies converge in at ITCZ – circulations are known as cells
  • In tropics it is called Hadley Cell
  • Mid latitude – Ferrel Cells – westerlies (warm air that blows from subtropical highs)
  • At poles – cold dense air subsides near the poles and blows as polar easterlies (polar cell)
  • The transfer of heat energy from lower latitudes to higher latitudes maintains the general circulation.
  • Seasonal Winds – Monsoons (SE Asia)
  • Local Winds (difference in heating and cooling of earth surface)
  • Oceans in turn provide input of energy and water vapour into the air.
  • Warm water of central Pacific Ocean slowly drifts towards South American coast and replaces cool Peruvian current
  • El Nino – Appearance of warm water off the coast of Peru – pressure changes in Central Pacific & Australia (this known as Southern Oscillation)
  • SO + El Nino = ENSO (strong, large-scale variations in weather occur over the world. The arid west coast of South America receives heavy rainfall, drought occurs in Australia and sometimes in India and floods in China)

Land Breeze & Sea Breeze

Land Breeze & Sea Breeze

Mountain & Valley Breeze

  • Day – Valley Breeze slope are heated and air moves upslope to fill the gap air from valley blows up
  • Night – slopes are cool and air descends in valley as mountain wind (cool air draining in valley is katabatic wind)

Air Mass

Air remains over homogenous region for long time – it acquires characteristics of area – has unique temperature and humidity

Source region – homogenous surface over which air mass forms

  1. Warm tropical and subtropical oceans
  2. Subtropical hot deserts
  3. Relatively cold high latitude oceans
  4. Very cold snow covered continents in high latitudes
  5. Permanently ice covered continents in the Arctic and Antarctica.

Types of Air Masses

  1. Maritime tropical (mT) - warm
  2. Continental tropical (cT) - warm
  3. Maritime polar (mP) - cold
  4. Continental polar (cP) - cold
  5. Continental arctic (cA) - cold

Fronts

Boundary when two different air masses meet – warm, cold, stationary and occluded

Frontogenesis – formation of fronts

  • Stationary Front – front remains stationary
  • Cold front – cold air moves over warm air mass
  • Warm front – warm air moves over cold air mass
  • Occluded front – Air mass is fully lifted above the ground

Extra-Tropical (Mid Latitude) Cyclones

System in mid and high latitude beyond the tropics – abrupt changes in weather conditions

Direction of Motion of Centre

Cyclonic circulation leads to extra tropical cyclone with warm and cold front

The warm air glides over the cold air and a sequence of clouds appear over the sky ahead of the warm front and cause precipitation

Cold front pushes the warm front up (cumulus cloud develop along cold front) – cold front moves faster and overtakes warm front

Warm air is completely lifted up and the front is occluded and the cyclone dissipates

Image of Extra Tropical Cyclone and Tropical Cyclone
Extra-tropical CycloneTropical Cyclone
Clear frontal systemNot present
Large areaSmaller area
Both land and seaOnly seas
Wind speed is lessHigh speed wind and destructive
Move from west to eastMove from east to west

Tropical Cyclone

  • Violent storms over oceans in tropical areas – heavy rain and storms
  • Cyclones in Indian Ocean, Hurricanes in the Atlantic, Typhoons in the Western Pacific and South China Sea, and Willy-willies in the Western Australia

Conditions

  • Large sea surface with temperature higher than 27° C
  • Presence of the Coriolis force
  • Small variations in the vertical wind speed
  • A pre-existing weak low- pressure area or low-level-cyclonic circulation
  • Upper divergence above the sea level system

Towering cumulonimbus clouds surrounding the storm. With storm form the sea, moisture is strengthened – on reaching land moisture is cut off and storm dissipates

Place where a tropical cyclone crosses the coast is called the landfall of the cyclone.

Cyclones which cross 20° N latitude recurve and they are more destructive

Strong spirally circulating wind around center (eye) – diameter can vary from 150 to 250 km

Eye wall has maximum velocity wind with 250 kmph – torrential rainfall

Direction of the Storm
  • Thunderstorms are caused by intense convection on moist hot days.
  • Thunderstorm is a well-grown cumulonimbus cloud producing thunder and lightening.
  • When the clouds extend to heights where sub-zero temperature prevails, hails are formed and they come down as hailstorm.
  • If there is insufficient moisture, a thunderstorm can generate duststorms.
  • Thunderstorm has intense updraft of rising warm air – causes clouds to grow big and cause rainfall
  • Spiralling wind descends like trunk of elephant with great force and low pressure at center – tornado (in mid latitudes & over sea are called water sprouts)

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