Lecture 19 & 20 : Geography

Topics Covered in this Lecture

  1. Local Winds
  2. Concept of Jet Streams
  3. Types of Jet Streams and Weather Associated
  4. Concept of Air Mass
  5. Types and Significance of Air Mass

Jet Streams

As per WHO, A narrow meandering upper air circulation moving from west to east having high velocity and several velocity maxima and minima influencing the ground climatic condition with local cyclonic and anti-cyclonic condition is Jet Streams.

In simple words,

A narrow upper air circulation, generally moving from west to east with very high velocity is known as Jet Streams.

These are an example of geostrophic winds. Geostrophic winds are those winds that moves parallel to the isobars.

Geo strophic winds

  • normally the winds flow to isobars but the final direction of wind is the results of different forces acting on the winds.

  • Winds in the upper atmosphere above (1km) are free from frictional force and hence they start flowing with high velocities. As a result the deflection if the winds increases.

  • So in the upper atmosphere, the pressure gradient force and the Coriolis force try to balance each other and hence the resultant direction of the winds is it flows parallels to the isobars. Such winds are known as Geostrophic winds.

Example- jet streams.

Characteristics of jet streams

  1. Jet stream flows in the upper atmosphere between 6 to 12 kilometers.

  2. The flow of the jet streams is highly mean daring in nature. This meandering nature of the jet streams was discovered by ross by and hence they are known as Ross by waves.

  3. They are thousands of kilometers in length. In other words, they are circular polar in nature.

  4. They are hundreds of kilometers in width and a few kilometers in depth.

  5. There are general directions is from West to the east except for sub tropical easterly jet.

  6. It is also known as upper air Westerlies.

  7. They flow with very high Velocity varying from 150 to 600km/hour.

  8. The jet streams influence the weather conditions in the areas they visit(significance of jet streams)

  9. Jet streams are more strong and permanent during the winter season because the temperature differences between any two places are greater during winters.

Mechanism of formation of jet stream

Jet streams will originate in the upper atmosphere in the region where there’s maximum temperature difference causing pressure difference such zones are the areas of either the divergence of two different winds (sub tropical high pressure belt) or convergence of two different winds (sub polar low) on the ground. As there’s no difference in the air masses over the equator jet streams do not originate in the regions ITCZ.

The importance of jet streams

  1. The polar front jet stream
  2. The subtropical westerly jet stream
  3. The tropical easterly jet stream
  4. Polar night jet stream
  5. Polar front jet streams It is the most powerful jet stream blows throughout the year from west to east.
  • Sub tropical westerly jet stream-

It is the stronger during winter season. It is responsible for bringing temperate cyclones in India which causes western disturbances.

  • Tropical easterly jet stream-

It flows east to West over the monsoon region’s of Asia. The intensity of monsoon rainfall is largely controlled by tropical easterly jet.

Note – causes of monsoon in India.

            Causes of deflection of winds.

Intensity duration of easterly jet stream is responsible for variation in India monsoon.

  • Polar night jet stream-

Originate in polar regions only during winters. It originate because of greater temperature difference between the cold troposphere and warm stratosphere. It is  responsible for taking the pollutants from the troposphere to the stratosphere and hence farming ozone.

Significance of jet stream-

  1. Impact on weather and climate

Jet streams influence the weather conditions of areas it visits.

  1.  Polar jet streams take the temperature cyclones from West to East and hence they are responsible for giving precipitation in the interiors of Russia  (Siberia), and Canada, USA.
    1. 1 Thus, the climate of higher Lattitudes is greatly influenced by temperate cyclones and jet streams.
    1. 2 Western disturbance- the subtropical westerly jet streams which comes to India brings the moist winds from the black sea, Caspian sea regions, due to the height of the Himalayas, it is bifurcated into two parts-

The northern part is responsible for snowfall while the southern part gives much-needed rainfalls in northern plains known as western disturbances. ( for Rabi crops) the perennial nature of Indian rivers owes its origin the Western disturbances and hence canal irrigation dominates India.

  •  The easterly jet stream which comes to India during the southwest monsoon season is responsible for bringing low-pressure centers from the Bay of Bengal towards India. The easterly jet stream also influences the direction of tropical cyclones making the eastern coast of India more vulnerable to tropical cyclones.

Importance in terms of environment

Jet streams Remove the air pollution that bulids over the northern plains with the arrival of winters. But in recent years due to stubble burning in Pakistan and North western India, thr pollution has increased.

After the winters the jet streams are also responsible for bringing heat waves and dust storms. It is also responsible for thunderstorms in Central India which is a major pulses and oil seeds growing region.

  • Transportation-

Modern aviation industry makes the use of direction of a jet stream by placing a fight/ place in the jet stream or outside the jet stream depending upon the direction when it is going.

Topics Covered in this Lecture

  1. Airmass:- Conditions and Types
  2. Modifications of Air Mass
  3. Fronts :- Types and Weather Conditional Associated
  4. Temperate Cyclones :- Formation and Associated weather Conditions.


An air mass can be defined as a large body of air which is having uniform properties (in terms of temperature pressure and humidity) along its length.

In an air mass horizontal gradients of temperature pressure and humidity should be uniform and also the verticle gradients should be more or less similar.

Conditions for the formation of air masses-

  1. These should be uniform or homogenous surface. Such areas are located only a few regions of the world which are having either uniform continent or uniform ocean surface regions.

  2. For an air mass to develop the air must become stabilize over the source region for some days. Therefore, air masses do not develop in the regions of the equatorial low-pressure belt.

The major source regions are:

  1. Vast continental areas example:- Canada and Siberia region.
  2. Huge oceanic surfaces.
  3. Vast desert regions of the tropics.

Classification of air mass-

On the basis of origin the air mass can be classified into

M- Maritime Airmass

C- continental Airmass

P- Tropical Airmass

T- tropical Airmass

On the basis of temperature, it is classified into

W- warm Airmass – the Airmass whose temperature is greater than the area is visit.

K- cold Airmass- an Airmass whose temperature is lesser than the area is visits.

On the basis of weather conditions, it is classified into-

S- stable Airmass – dry conditions

U- unstable Airmass -Precipitations

Source: Britannica

Significance of air mass –

  • They will modify the weather conditions of the areas they visit.

  • Example- Indian subcontinent from June to September is influenced by monsoon which is an example of MTWU Airmass.

  • The convergence of warm and cold Airmass results in the formation of fronts. Results into the formation of fronts.

  • Protogenesis is the reason for the formation of temperature cyclone and temperate cyclones give much-needed precipitation in the higher latitude and influence weather conditions as well as climate of the higher latitudes.

  • For example- temperate cyclones give year long Percipitation (summer – rain, winter, snow). In the interiors of Russia and canada giving rise to taiga biome that is evergreen coniferous forests.

  • Airmass also results in extreme weather events such as tornadoes .(understand in fronts).


This is a sloping boundary between two contrasting and converging air masses. The process of formation of fronts is known as Frontogenesis and the process of destruction of fronts is frontolysis.

Conditions for the formation of Fronts

  • The presence of two contrasting air masses that is the air masses should differ in terms of temperature and pressure.

  • The air masses should converge.

Note- Fronts do not develop along the ITCZ because both the air masses are similar.


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

Warm front

Fronts are formed in the zone of convergence of warm westerlies wind and the cold polar warm front is a generally slopping boundary. It occurs when a warm air mass is active, enters the territory of the cold air mass, and rises along a gentle slope.

Weather condition

It is associated with gentle to medium precipitation which occurs for longer duration and over a larger area.

Cold front

It occurs in the regions where the cold air being active enters the territory of warm air. It violently uplifts the warm air and throws it along the steep slopes known as cold front.

Weather conditions

  • It is associated with the formation of a cumulonimbus cloud which gives very heavy rainfall along with hail, lightning, and thunder.
  • When the temperature difference between two different air masses is very high, tornadoes are formed along the cold front. For example, in North America due to absence of East-West mountains easterlies and warm westerlies meet and form tornadoes.

Occluded front

It occurs when the cold air completely displaces the warm air from the ground or when the cold front overtakes the warm front. Occluded front denotes frontolysis and also the death if temperate cyclones.

Stationary Front

It occurs when the two air masses do not converge with each other.

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