Atmosphere – Air Masses, Fronts, Cyclones, Anticyclones

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

Fronts

• When two different type of air masses meet, the boundary zone b/w 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

Cyclones

• 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 b/w 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 b/w 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

Difference between Tropical & Extra Tropical Cyclones

Anticyclones

• • • 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