1
Summary
Geography Class 26

Revision of the previous class (05:13 PM) 

Precipitation (05:28 PM)

  • Precipitation requires the following three condition
  • a)Upliftment mechanism: To cause moist air to rise
  • b)Saturation and cooling of air below the due point
  • c)Presence of Hygroscopic nuclei such as dust particles around which water droplets can accumulate.
  • Types of precipitation
  • a)Convectional Rainfall
  • It occurs in the region of intense heating near the ground surface.
  • Heating causes air to expand and rise resulting in the upliftment.
  • They are often accompanied by thunderstorms and lightning due to the formation of cumulonimbus clouds.
  • They are experienced throughout the year in equatorial regions and in summers in tropical regions.
  • b)Orographic Rainfall
  • When warm and moist air is forced to rise across mountain slopes it cools down causing precipitation along the windward side.
  • However, on the leeward side, the descending air will not cause precipitation creating a rain shadow region, for example, The Western Ghats of India.
  • c)Frontal Rainfall
  • It occurs along frontal zones due to the convergence of different types of air masses.
  • The warm air mass rises above the cold air mass causing precipitation.
  • Frontal precipitation is common along the temperate region.
  • Distribution of precipitation
  • Along the Equatorial region, the highest precipitation is experienced above 200 cm per annum.
  • It is due to the convectional Rainfall
  • Along subtropical regions around 30 degrees minimum rainfall experienced 25 cms per annum.
  • The Mid-latitude regions between 30 to 40 degrees of North and south experience uniform and moderate precipitation due to the frontal rainfall.
  • Due to the presence of moist maritime air mass oceans receive more precipitation than the continents.
  • Coastal regions receive higher precipitation than the interior.

Thunderstorm and Lightning (05:49 PM) 

  • Due to intense heating and strong vertical convection, Cumulonimbus clouds are formed.
  • The Raindrops in these clouds move up and down due to strong air currents creating electric charges which accumulate on opposite sides of clouds.
  • When both types of charges have attracted a flash of light is produced due to the exchange of charges called lightning.
  • Lightening causes Vaccum in the cloud due to the Rapid expansion of air.
  • It is filled by the surrounding cold air producing claps of Thunder.

Cloud Burst (06:10 PM)

  • IMD defines cloud burst as Rainfall over 10cm per hour concentrated in a small area of a few km.
  • Formation of cloud burst
  • High temperature and higher humidity favour strong and rapid convection.
  • Steep topography along the mountainous region causes a huge buildup of clouds without precipitation due to the funnelling effect.
  • The Rapid convection prevents the Raindrops to fall down.
  • Once the cloud becomes too heavy for the air to hold on it drops together in quick succession resulting in a cloud burst.
  • In India, cloud bursts can be observed along the Himalayas, Rajasthan desert, and monsoon coasts.

Tornado (06:36 PM)

  • A violently rotating column of air extends from the ground to the base of a Thunderstorm.
  • It appears like a funnel descending from a cloud.
  • The wind speed goes up to 400 km per hour.
  • It is formed with changes in wind speed and direction creating a horizontal spinning effect within a storm cell.
  • This effect is tipped vertically by rising air moving up through the thunderclouds.
  • A water spout is a whirling column of air and water mist developed over water bodies.

Jet streams (07:11 PM)

  • WMO defines jet streams as a strong narrow current concentrated along a quasi-horizontal axis in the upper troposphere or lower stratosphere characterized by strong vertical and lateral wind shear.
  • Featuring one or more velocity maxima.
  • Characteristics:
  • They are thousands of km in length and 100 km in width.
  • Normally blows from west to east.
  • winds are faster during winter.
  • The entire system follows the sun.
  • It follows the Zig Zag path in the form of Rosby waves.
  • Causes
  • Jet streams are generated due to the temperature difference between the two regions that is Tropical and Temperate, Temperate and polar, etc.
  • The difference in pressure gradient with altitude above cold and warm air mass causes strong movements of winds.
  • The larger the temperature difference, the stronger the winds.

Types of Jet streams (07:49 PM)

  • a)Polar front jet streams.
  • They are formed where the polar and Ferrel cells meet.
  • They are irregular and discontinuous.
  • They blow from west to East.
  • b)Subtropical westerly jetstream
  • Associated with temperature gradient between Headley and Ferrel cell.
  • It is more regular and strong, It blows from west to east.
  • c) Tropical Easterly Jet stream
  • Forms over India and Africa only in summer due to the intense heating of the Tibetan plateau.
  • The direction is east to west.
  • d)Polar Night jet stream
  • Formed over the polar region above the troposphere during winters.
  • They are associated with the formation of the ozone hole.
  • The direction is from west to east.
  • e)Local jet stream
  • They are formed due to local thermal and dynamic conditions.
  • Example: Somali jet streams
  • Significance of the Jet streams
  • The jet stream is responsible for cyclonic and anticyclonic conditions along the surface.
  • Upper divergence in jetstreams causes lower convergence and upper convergence causes lower divergence.
  • It changes the weather condition along the surface by suppressing or increasing precipitation.
  • It is used in Aircraft navigation.
  • The polar night jetstreams are responsible for Ozone depletion.
  • The jet streams help in the transportation of pollutants into the upper troposphere from urban regions and help in clearing fog and smog during winter.
  • It helps in the formation of temperate cyclones and intensifies them.
  • The monsoon in south Asia is controlled and affected by subtropical westerly, Tropical easterly, and Somali jet streams.
  • The sub-tropical westerly jet streams help in bringing western disturbance causing precipitations during winters in the Northwest region.
  • The interactions of jet streams with la Nina conditions cause the formation of a heat dome over North America.
  • Meandering of Jetstreams near polar regions during winters results in the outbreak of polar vortex causing freezing conditions along lower latitudes.

The topic for the next class is: Polar vortex, Continuation of climatology