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Summary
Geography Class 27

Revision of the previous class  in a detailed way (05:05 PM)

  • Polar Vortex (05:30 PM)
  • Polar Vortex is a large area of low pressure and cold air surrounding the Earth's pole.
  • The term Vortex refers to the counter-clockwise flow of air which keeps the cold polar air locked inside.
  • The polar winter night jetstreams keep this region covered with strong winds.
  • Occasionally when the vortex weakens due to the weakening of jetstreams the vortex expands and sends the cold air southwards along with jetstreams.
  • This brings down the temperature in southern Regions to colder levels of below zero degree Celsius.
  • Air masses (05:47 PM)
  • It is the extensive portion of the atmosphere.
  • Whose physical properties such as temperature pressure and Humidity are homogeneous horizontally and vertically for hundreds of km.
  • The temperature of the source area shall remain uniform horizontally with a very gradual change in the vertical direction.
  • The humidity conditions should also be uniform with the least variability.
  • The surface area shall remain homogenous for a few hundred km.
  • The Regions with light divergent winds, High-pressure belts, and anticyclonic circulations are more favorable for the formation of air masses due to stable conditions.
  • Types of Air masses
  • Classified based on different factors such as:
  • a) Source area:Continental(c) formed over the continents and Maritime(m) formed over oceans 
  • b) Temperature: Warm and Cold air mass
  • c) Pressure: Stable(s) and Unstable air mass(u)
  • d) Latitude:Arctic air mass(a) and Antarctic air mass(aa) Polar air mass(P) Tropical air mass(T0 Equatorial air mass(E)
  • a and aa is continental
  • P & T is continental and maritime respectively.
  • E is maritime.

Significance of Air mass (06:16 PM)

  • Air masses Transport heat from one region to another and help in the global distribution of temperature.
  • When Air mass moves away from its source area it changes the temperature and pressure conditions of the destination regions.
  • Maritime air masses bring More moisture toward the continental region increasing its humidity and causing precipitation higher precipitation along the coastal regions due to the influence of Maritime air masses.
  • The dry Continental air masses along the subtropical regions help in maintaining the aridity conditions.
  • The cold air mass formed over Siberia and Canada affects the surrounding region for long-distance causing cold conditions.
  • The maritime air mass of the Indian ocean plays an important role in precipitation over India during Monsoon.
  • The interaction of warm and cold air masses in temperate regions leads to the formation of fronts and temperate cyclones.
  • The maritime tropical and equatorial air masses in the Atlantic and Indian oceans help in the development of Tropical cyclones.
  • Temperate Cyclones (06:56 PM)
  • Front
  • A narrow zone of transition dividing two air masses of differing temperature and humidity conditions is called the front.
  • They are usually formed in Mid latitude or temperate regions.
  • The process of formation of fronts is called frontogenesis
  • The process of decay of fronts is called as
  • Types of Fronts (07:07 PM)
  • a) Stationary front
  • It is a front where the surface position of a front is not changing and the two air masses are unable to push each other.
  • When a stationary front is disturbed it results in a warm front or cold front.
  • b) Cold front
  • It is the boundary between warm and cold air mass where the cold air mass is advancing and undercutting the warm air mass.
  • It results in a steeper gradient along the front leading to rapid upliftment of warm air and the formation of cumulonimbus clouds.
  • It results in heavy rainfall over the smaller regions.
  • C) Warm front
  • It is the boundary between warm and cold air mass where the warm air mass is advancing and overriding the cold air mass.
  • The gradient of the front is gradual resulting in strato nimbus clouds.
  • It causes moderate rainfall over a larger area.
  • Occluded Front (07:39 PM)
  • It is the later stage of front formation where the air in the warm sector is no longer at the ground surface due to the meeting of a warm and cold front.
  • It is a compound zone with both warm and cold front characteristics.
  • The occluded front gradually results in frontolysis.
  • It is the later stage of front formation where the air in the warm sector is no longer at the ground surface due to the meeting of a warm and cold front.
  • It is a compound zone with both warm and cold front characteristics.
  • The occluded front gradually results in frontolysis.
  • Formation of Temperate Cyclones (07:45 PM)
  • 6 Stages
  • Stage 1:Convergence of two air masses with different characteristics. Air masses move parallel to each other and a stationary front is formed.
  • Stage 2: Warm and cold air masses penetrate into the territory of each other forming a wave-like the front.
  • Stage3: With warm and cold fronts starting to move towards each other, The warm sector is wedged between the cold sector.
  • Stage 4: The warm sector is narrowed in extent due to the advance of a cold front.
  • Stage 5: The occlusion starts with warm air rising completely above the cold air and the cold front completely overtaking the warm front.
  • Stage 6: The warm sector completely disappears, the occluded front is eliminated and the cyclones die out.
  • Temperate cyclones are also called Mid-latitude cyclones or extratropical cyclones or wave cyclones.
  • They are more prominent in the regions between 35-65 degrees North and south.
  • Role of Jet streams in Temperate Cyclones
  • Jet streams Being Quasi horizontal in nature results in upper divergence in some locations and upper divergence causes surface convergence which helps in bringing together the warm and cold air masses and the subsequent formation of fronts.

The topic for the next class: Tropical cyclones