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Atmosphere- Air Masses, Fronts, Cyclones and Anti-cyclones

August 04,2017

Air Masses

  • Over a homogeneous area, when air remains for a sufficiently long time, it acquires the characteristics of that area.
  • Homogeneous area can be vast plains or oceans where only little horizontal variation occurs in temp. & moisture.
  • Homogeneous regions over which air masses form are called source regions


  • When two different type of air masses meet, the boundary zone between them is called a front
  • Process of its formation is known as frontogenesis

Warm Front

  • When a warmer & lighter air mass moves against a colder & denser air mass
  • the former rises over the latter one & warm front is formed

Cold Front

  • When a colder air mass forces its way under a warmer air mass & pushes the latter upward, cold front is formed

Occluded Front

  • When a cold front overtakes a warm front & lift it off from the surface of the earth
  • After occlusion, air masses loses their earlier characteristics & form new fronts
  • Meeting of warm & cold air masses in temperate zone gives birth to temperate cyclones along the fronts of warm & cold air masses
  • In each case, precipitation is likely to occur but more in case of cold front


  • A low pressure area surrounded by high pressure area from all from all the sides along with winds moving from all the sides towards central low
  • Cyclones moves in Anti clockwise in N – Hemisphere & in Clockwise direction in S – Hemisphere under the effect of westerlies due to coriolis effect
  • No Cyclones at equator as coriolis force is 0 there

Temperate Cyclones

  • Also known as wave cyclones or Extra Tropical
  • Originate mainly in zones between 35* – 65* N & S of latitudes

Polar Front Theory  Forms due to collision of 2 air masses of contrasting characteristics (in terms of temp. & humidity at about 60* latitude). Here they do not meet each other readily but forms a front known as polar front

  • Cold air mass pushes the warm air mass upwards & a void is created due to decrease in pressure.
  • Air from surrounding area rushes to fill the void & a temperate cyclone if formed
  • Average speed of extra tropical cyclone is 32 km/hr in summer & 49 km/hr in winters

Tropical Cyclones

  • Also known as Typhoons or Hurricanes
  • Originate mainly in zones between 5* – 30* N & S of latitudes
  • Are the violent storms that originate over oceans in tropical areas & move to coastal areas
  • Bring large scale destruction, caused by violent winds, heavy rainfall & storm surges
  • Favorable conditions for formation of tropical cyclones are
  • Large sea surface with temp. > 27*C
  • Presence of coriolis force
  • Small variation in vertical wind speed
  • Upper divergence above sea level

  • Pre-existing weak low pressure area or low level cyclonic circulations
  • Energy that intensifies the storm comes from the condensation process in towering cumulonimbus clouds, surrounding center of the storm.
  • Hence, with constant supply of moisture from the sea, storm is further strengthened
  • On reaching land, moisture supply is cut off & the storm dissipates
  • Place where tropical cyclone crosses the land is called landfall of the cyclone
  • Central low pressure is known as eye of the cyclone Calm with subsiding air having lowest pressure & highest temp.
  • Surrounding this area is zone of strong winds with clouds extending vertically
  • Surrounding the eye is eye wall, a place of strong spirally ascending winds to a height reaching tropopause, having max. wind velocity

Tropical Cyclone Distribution & its various names worldwide




Cyclone Indian ocean, Arabian sea & Bay of Bengal
Hurricane Atlantic sea (West indies) & USA
Typhoons China sea + Japan sea
Willy Willies Western Australia

Difference between Tropical & Extra Tropical Cyclones

Tropical Cyclones

Extra Tropical Cyclones

Moves from east to west Moves from east to west
Wind velocity is very high & more destructive Low wind velocity & less destructive
Originate only on sea & dissipates on reaching land Affect much larger area & can originate on land as well as sea


  • An anticycloneis just opposite to a cyclone
  • Basically it is a large-scale circulation of winds around a central region of high atmospheric pressure
  • Clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere
  • Anticyclones are form from air masses, cooling more than their surroundings, which causes the air to contract slightly making the air denser
  • Since dense air weighs more, the weight of the atmosphere overlying a location increases, causing increased surface air pressure
  • Anticyclones herald fair weather, clearing skies, calm air with high temperature in summers & cold in winters
  • Fog can also form overnight within a region of higher pressure


Article by AZAD


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