Category : UPSC






Atmosphere is a gaseous envelope surrounding the earth extending thousands of kilometers above the earth's surface. Life on earth exists at the bottom of the atmosphere where it meets with the lithosphere and the hydrosphere. The atmosphere directly or indirectly influences the vegetation pattern, soil type and topography of earth. About 99% of the total atmosphere mass concentrated found within 32 kms from earth's surface. The atmosphere is held close to earth because of the earth's gravity and is energized by the sun.





The atmosphere is composed of different types of gases, dust particles and water vapour. Such compositions in the atmosphere is hot static but it changes with time. The composition of the atmosphere are as follows:

  • Atmosphere is a mixture of various gases containing huge amount of solid and liquid particles collectively known as aerosols.
  • Pure dry air in the atmosphere consists of nitrogen, oxygen, argon, carbon dioxide, hydrogen, helium and ozone. Besides these gases, atmosphere also contains water vapour, dust particles, smoke, salts, etc. are also present in the air.
  • Nitrogen and oxygen are the two main gases, comprised of 99% of the total volume of the atmosphere. Nitrogen does not form a chemical union with other substances present in the atmosphere. It is an agent of dilution and regulates combustion,
  • Oxygen combines well with all other elements present in the atmosphere. It is also easily combustible.
  • Carbon dioxide exists in a very small percentage in the atmosphere and it absorbs most of radiant energy emitted by earth and reradiates it back to the earth.
  • Carbon dioxide increases the temperature of lower atmosphere and the phenomena is called as the greenhouse gas effect. The percentage of carbon dioxide in atmosphere is increasing due to burning of fossil fuels (coal, petroleum and natural gas) and by the process of deforestation.
  • Water vapour, gaseous form of water present in atmosphere has made life possible on earth.
  • Large amount of water vapour usually found in hot-wet regions while least amount is found in the dry regions. The amount of water vapour on increasing from high latitudes to low latitudes and the vice-versa.
  • Water vapour reaches in the atmosphere through evaporation (takes place in the oceans, seas, rivers, ponds and lakes) and transpiration (takes place from the -plants, trees and living beings).
  • Dust particles are found in the lower layers of the atmosphere in the form of sand, smoke and oceanic salt. Dust particles help in the condensation of water vapour - in the form of droplets by which process the clouds are formed and precipitation is made possible on the earth.



Percent by Volume


78.08 %


20.94 %



Carbon dioxide














The atmosphere consists of concentric layers of gases, with varying density and temperature. Generally the atmosphere extends upto 1600 kms from the surface of the earth. On the basis of chemical composition, the atmosphere is divided into:


Homosphere and Heterosphere.



1. Homosphere


This extends up to the height of 90 kms and is characterized by uniformity in chemical composition. It consists of three thermal layers - troposphere, stratosphere and mesosphere.


[a] Troposphere

Lowest layer of the atmosphere.

  • Height of troposphere is 18 kms on the equator and 8 kms on the poles.
  • Temperature decreases with height in this layer roughly at the rate of 1°C per 165 metres, which is known as normal lapse rate.
  • Upper limit of the troposphere is called
  • All weather phenomena are restricted to troposphere. In this layer the characteristics of both the layers - troposhere and ionosphere are found.


[b] Stratosphere

  • It lies above the troposphere is spread upto the heighi of 50 kms from the earth's surface which has an extent of 40 kms.
  • In this layer the temperature remains almost same upto the height of 20 kms above the earth's surface.
  • Due to presence of ozone (15-35 kms) in lower stratosphere a bacteria can survive in this layer.
  • Ozone is blue irritating gas with pungent odour.
  • Upper limit of stratosphere is called as stratopause.
  • This layer is considered ideal for flying of aircrafts mostly due to low temperature, which results in optimum fuel burn and low density of air reduces parasitic drag on airframe. This layer even provide smooth movement as this layer has very less weather turbulence.


[c] Mesosphere

  • It lies above the stratosphere extends upto the height of 80 kms from the earth's surface and its extent is 30 kms.
  • The upper limit is called
  • Temperature decreases with height and drops upto -\[110{}^\circ C\].
  • The phenomena of foiling stars and meteors happen in this layer.


2. Heterosphere


Proportions of gases are not constant in this layer.

[a] Thermosphere

It lies 80 kms above the mesosphere. In this layer the temperature increases rapidly with increase in height. The gases become very light due to extremely low density. Further it divided into two layers:

(i)         Ionosphere spread between 80 km to 400 km above the surface of the earth. There are number of ionic layers in the sphere, e.g. D layer, E layer, F layer and G layer.

-           D layer disappears with sunset as it is associated with solar radiation.

-           E layer is known as Kennelly-Heaviside which is confined to the height between 99 km - 130 kms.

-           It reflects the medium and high frequency radio waves.

-           \[{{F}_{2}}\] layer is called as appleton layer, lying between 150km-380km.

-           \[{{F}_{1}}\]and \[{{F}_{2}}\] area the two sub layers of \[{{E}_{2}}\] layer  combinedly known as Appleton Layer.

-           G layer is beyond 400 kms.

(ii)        Exosphere lies extend to beyond 400 kms above the surface of the earth. Density of air is very less here because of no gravitational force and the atmosphere resembles as nebula because it is highly rarefied.




  • The primary source of energy on earth is the sun. The energy received from the sun is radiated in every direction into space through short waves which is also called as solar radiation. The incoming solar radiation through short waves is called as insolation.
  • There are three major sources of heat energy through which earth is warmed up, i.e. Solar radiation, Gravity and Endogenetic.
  • Out of the above three sources of heat energy, solar radiation is the most significant source of terresterial heat energy.
  • The radiant energy received from the sun is transmitted in the form of short waves. Which travels with a speed of 1,86,000 miles per second.


Distribution of Insolation


  • The tropical zones on earth receives the maximum insolation while it gradually decreases towards poles. Insolation is more in summer season while less in winter seasons
  • There are also temporal variation of insolation received at different latitudes at different period of a year.
  • There is very little variation of insolation during winter and summer season from equator upto tropics.
  • At middle latitude (23.5° to 66° latitudes), both the hemisphere receive maximum sunlight during summer solstice (in northern hemisphere) and winter solstice (in southern hemisphere).
  • Polar zone between 66° to 90° latitudes in both the hemisphere receives maximum and minimum insolation once a year only. No direct ray of the sun is received beyond 66° latitude throughout the year.


Factors affecting the Distribution of Insolation

The non-uniformity in the amount of insolation received on the earth's surface is mostly due to following factors:

(i) Angle of incidence of the sun's rays depends on the latitude of a place. Higher the latitude less is the angle it makes with the surface of the earth causing slanting rays. The area covered by slanting rays is larger than vertical rays. The energy is distributed over a larger area. The net energy received per unit area decreases. Also slanted rays pass through a greater depth of the atmosphere resulting in more absorption, scattering and diffusion.

(ii) Length of the day : During summer, days are longer than nights. The situation is reversed with winter. Longer the day, greater will be the insolation received.

(iii) Distance between the earth and the sun: The distance between the earth and the sun is not uniform throughout the year. It is nearest to the sun (147 million km) on 3rd January and farthest (152 million km) on 4th July. It is mainly to the orbit of the earth which is elliptical in shape. When the earth is nearest to the sun it is called as perihelion and when farthest is called as aphelion.

(iv) Sunspots: Sunspots created on the outer surface due to periodic disturbance and explosions. The number of sunspots varies from year to year. Its cycle is completed in every 11 years. The energy radiated from the sun increases. When the number of sunspots increases and therefore the amount of insolation received by the earth surface is also increases.


Factors responsible for heating the atmosphere



The process of heat transfer in the form of emitted electromagnetic waves is known as thermal radiation. The process of radiation of heat energy back from ground is known as ground radiation.

When part of ground radiation is radiated back to earth after getting absorbed by earth's atmosphere is known as counter- radiation.

  • Solar energy received in the form of solar radiation heats the earth's surface and the atmosphere, resulting in change of pressure gradient. It initiating hydrological cycle, recycle nutrients and chemical elements in biosphere.
  • There are two basic laws which govern the nature and flow of radiation such as

(i) Wien's displacement law: According to this law wave length of the radiation is inversely proportional to the absolute temperature of the emitting body.

(ii) Stefan-Boltzman law: This law states that flow or influx, of radiation is proportional to the fourth power of the absolute temperature of radiating body.

  • Continuous emission of photons from the sun causes bonds of radiations with certain wavelength.
  • The radiation is mostly emitted in the form of electromagnetic waves and is known as electromagnetic radiation.
  • The electromagnetic radiation radiated from me outer surface of the sun, consisting of four spectrums which are as follows:

(i) The first spectrum includes, gama rays, hard x-rays, soft x-rays and ultra violet rays. Measured in angstrom (10~8 cm) and have short wave lengths.

(ii) The second spectrum consists of visible rays. It is measured in micron and ranges between 0.4 to 0.07 micron.

(iii)The third spectrum of electromagnetic waves covers infrared spectrum. It ranges between 0.7 to 300 microns.

(iv) The fourth spectrum consists of long waves which include micro-waves, radar waves and radio waves. These waves are measured in cm and m.



The transfer of heat energy due to the movement of a mass or substance from one place to another mostly vertical is known as convection.



The transfer of heat through molecules of an object is known as conduction. Which can be accomplished via two ways:

  1. From one part of the body to another part of the same body having unequal temperature.
  2. From one body to the other body.



Advection is the lateral or horizontal transport of heat from one place to another. It generally takes place in the ocean in the form of ocean currents.


Heat Budget


  • The process of insolation and terrestrial radiation simultaneously takes place on the surface of earth like other object earth also radiates the heat energy so, the annual mean temperature on the surface of the earth is always constant. The balance between insolation and terrestrial radiation on earth is termed as the heat budget of the earth.
  • It is estimated that out of 100 units of incoming solar energy. 35 units received at the outer limit of the atmosphere is reflected and scattered back into space by clouds (27 units), dust particles (6 units) and by surface of the earth (2 units) in its original short wave form.
  • Remaining 65 units enters atmosphere out of which 51 units are received by the earth's surface and 14 units are absorbed by atmospheric gases along with water vapour. The 51 units which are received by the earth surface – 34 units received through direct radiation and rest 17 units are received as diffuse day light.
  • 34 units are returned to atmosphere in the form of outgoing terrestrial radiation (long wave) out of which 6 unit absorbed by the atmosphere, 9 units are spent in convection and turbulence and 19 units are spent through evaporation.
  • Heat budget of atmosphere includes absorption of incoming solar radiation (14 units) and heat received from terrestrial radiation (34 units). This is equal to 48 units together.


Regional Distribution of Temperature


The three major heat zones based on the distance from the equal on the earth are temperate zone, the Torrid Zone and the Frigid Zone.

Torrid Zone (Tropical Zone)

This is the hottest zone of the earth. The region that lies from the Tropic of Cancer \[({{23.5}^{0}}N)\], across equator \[({{0}^{0}})\]to the Tropical of Capricorn \[({{23.5}^{0}}S)\] is considered as the torrid zone (Tropical Zone)  Zone). The sun's rays directly fall at least once a year in this region.



Frigid Zone

This is the coldest zone on the earth. This region lies to the north of Arctic circle \[({{23.5}^{0}}N)\] and to the south of the Antarctic circle \[({{23.5}^{0}}S)\] and is permanently frozen. There is no sunlight   for most of the months in a year in this zone.


Temperate Zone

  • This is the habitable heat zone on the earth. There are two temperate zones lie in between \[{{23}^{1/{{2}^{0}}}}\]and \[{{66}^{1/{{2}^{0}}}}\]in both the hemisphere. These regions have moderate, tolerable temperature.


Importance of the Heat Zones

This division of the Earth into different heat zones helps in understanding the climate changes and to study weather conditions acners the world.




A column of air exerts weight in terms of pressure on the surface of the earth. The weight of the column of air at a given place and time is called atmospheric pressure. Air pressure is maximum at sea level. Pressure exerted by air at a particular point is determined by temperature and density.

There is inverse relation between temperature and pressure.

Air pressure is measured as a force per unit area. The unit of measuring air pressure is milibar or mb measured by an instrument called barometer.

The distribution of atmospheric pressure is shown on a map by isobars. An isobar is an imaginary line drawn through points of equal atmospheric pressure at the sea level. The spacing of isobars expresses the rate and direction of pressure change and is called pressure gradients.


Major Pressure Belts


  • The horizontal distribution of air pressure across the latitudes is charactrised by high or low pressure belts.
  • At the equator warm air rises over the equatorial region and create a low-pressure belt at the surface, called the equatorial low pressure. It is also known as Reaching the troposphere the warm air bends towards the pole and descends at \[30-35{}^\circ \] latitude or the tropics in both the hemisphere create a belt of sub-tropical high pressure.
  • Subtropical high pressure zone is also known as horse Sub-tropical highs are the source of air moving along the surface towards the poles.
  • At \[60-65{}^\circ \]latitudes surface air streams coming from polar high pressure and sub-tropical high pressure converge and move up. This upward movement of air creates a low pressure system at the surface which is the sub-polar low
  • At the poles, due to the low temperature air compresses and creates a zone of high pressure. This thermally created high pressure system is called polar high pressure. This dense cold air flows equator-wards away from the area of high pressure.

The planetary System of Pressure and Winds at the Equinoxes


Shifting of Pressure Belts


During late June, sun is overhead at the tropic of cancer. The doldrum (low pressure belt) moves northward from the equator along with other belts shifting in the northern hemisphere. In late December, when the sun is overhead at the tropic of Capricorn, the belts move southwards in a similar.





Air in motion called wind, can move in any direction. Wind is caused by spatial differences in atmospheric pressure and flows from areas of high pressure to those of low pressure. These differences are caused by uneven absorption of solar radiation at the earth's surface. Wind speed is greater during daytime when the greatest spatial extremes in atmospheric temperature and pressure exist. Wind direction is measured as the direction from where a wind comes from.


  • Direction is measured by a wind vane. Wind speed can be measured using the Beaufort wind scale.


Coriolis Force


Rotation of Earth creates Coriolis force. The consequence of Coriolis force is that the moving air get deflected from their original path. Instead of wind blowing directly from high to low pressure, rotation of the earth causes wind to deflect. In the northern hemisphere, wind is deflected towards their right, while in the southern hemisphere it is deflected towards their left. This is also known as the Parrel's law. Magnitude of the Coriolis force varies with the velocity and the latitude of the object. Coriolis force is absent along equator but its strength progressively increases towards the poles.


Geostrophic Wind

Air under the influence of both the pressure gradient and

Coriolis force moves parallel to isobars in conditions where friction is low (1000 meters above the surface of the earth) and isobars are straight. Such winds are called geostrophic winds.


Gradient Wind


Wind above the earth's surface do not travel in straight lines. The winds blow along the curved isobars of a high (anticyclone) or low (cyclone) pressure center. A wind that blows around curved isobars above the level of friction is called a gradient wind.


Types of Winds


  • Winds can occur on a large scale and can change its direction depends on the season and region on the earth.
  • On the bases of their area of influence they are divided into two types:

(1) Planetary winds or permanent winds

     (i) Trade wind

     (ii) Westerlies

     (iii) Polar wind

(2) Periodic winds

(i) Seasonal wind

(ii) Local wind

(iii) Land and sea breeze

(3) Local winds


Local Winds







Chinook (snow eater)





North India



S. Africa






Indian subcontinent

Santa Ana


Coastal Southern California



New South Wales Burster




Harmattan (Doctor)


Guinea Coast



S.E. France












S. America


Warm, dry

New Zealand


Warm, dry

Eastern part of sahara



  1. Planetary Winds or Permanent Winds

Planetary winds or permanent winds blow from high pressure region to low pressure region in a fixed direction throughout the year. They are a major part of general global circulation of air. They occur due to temperature and pressure variance existing throughout the world.

They are of following types:

(i) Trade winds or easterly winds  

(ii) Westerlies

(iii) Polar winds


(i)         Trade Winds or Easterly Winds

Blowing from the subtropical highs or horse latitudes (between \[30{}^\circ N\] and \[30{}^\circ S\]) towards the equatorial low pressure are the trade winds or easterly winds.

  • In the northern hemisphere, the trade wind lowing from the northeast also are known as the Northeast trade wind in the southern hemisphere, the wind blowing from the southeast are known as the southeast trade wind.


(ii)        Westerlies Winds

The winds that move polwards in the middle latitudes between \[35{}^\circ \] and \[65{}^\circ \] latitude, blowing from high pressure area in the latitudes towards the poles are called as Westerlies wind.

  • The winds are predominantly from the southwest in the northern hemisphere and from the northwest in the southern hemisphere and bring extra-tropical cyclones with them.
  • In southern hemisphere they are stronger and more constant in direction than those of the northern hemisphere because of the vast expanse of water.
  • They are best developed between \[40{}^\circ \] and \[65{}^\circ \] so; latitudes. These latitudes are hence often cab
  • Roaring forties, furious fifties and shrieking sixties


(iii)       Polar Wind

Winds blowing in the arctic and the Antarctic latitudes polar winds. They blow from polar high pressure towards sub-polar low pressure belt. In the northern hemisphere they blow from north-east, and are called the north-east, and are called polar winds. In the southern hemisphere, they blow from south-east and are called south-east polar winds. As these winds blow from ice-capped landmass, which are   cold.


  1. Periodic Winds

Periodic winds changes its direction with the change seasons.


(i)         Seasonal Winds

  • Monsoon Winds: Monsoons are regional wind systems that predictably change direction with the passing of the seasons. Monsoons occur over distances of thousands of kilometers, and their two dominant patterns of wind flow act over an annual time scale.
  • South-West Monsoon: During summers, monsoon winds blow from cooler ocean surfaces toward the warmer continents during the months April to September. The continents become warmer than the oceans due to:

-           Specific heat differences between land and water.

-           Greater evaporation over water surfaces.

-           Subsurface mixing in ocean basins, which redistributes heat energy to a deeper layer.

During summer the monsoon winds blow from sea towards in land area while during winter from in land area towards the seas.

  • North-West Monsoon: During the months of October and November the wind patterns reverses, as ocean surfaces are now warmer. With little solar energy available, continents begin cooling as long wave radiation is emitted to space. Ocean surface retains its heat longer as water has high specific heat and subsurface mixing. Winter monsoons bring clear weather and winds that flow from land to sea. It bring rain to A.P and T.N as they pick moisture from Ocean
  • Monsoon Winds of SE Asia: Asiatic monsoon caused by a complex climatic interaction between distribution of land, water, topography, and tropical and mid-latitudinal circulation.
  • During summer, a low-pressure forms over northern part of India and Southeast Asia because of high soar insolation
  • Warm moist air is drawn into the thermal lows from air masses over the Indian Ocean.
  • Summer heat creates a strong latitudinal pressure gradient and development of an easterly jet stream (15 km) at the latitude of \[{{25}^{0}}N\]
  • The jet stream enhances rainfall in Southeast Asia, Arabian Sea, and South Africa. When autumn returns to Asia, thermal extremes between land and ocean decrease. Westerlies of the mid-latitudes move in. Easterly jet stream is replaced with strong westerly winds in the upper atmosphere. Subsidence from an upper atmosphere cold low above the Himalayas produces outflow that creates a surface high-pressure system that dominates the weather in India and Southeast Asia. Besides, Asian continent, monsoon wind systems exist in Australia, Africa, South America, and North America.


(ii)      Local winds: Such winds blow in comparatively small area and have special characteristics. They are generated by purely local factors (local temperature differences) and their zone of influence is quite limited. They play an important role in the weather of a particular locality.

(iii)       Land and Sea Breezes: Generally, the land and seat breezes are prevalent in narrow strips along the coast or a lake. Sometimes local conditions may set air in continuous motion. Even in calm days in summer, heated air rising from land surface may cause strong breezes to move in from over the cooler sea. During night, land cools more rapidly than the sea, cool air may move seawards as land breeze, usually a gentle flow. In some countries, if the hot desert borders with the sea, strong winds may develop. Sea breeze will be felt in miles inland.

Jet Streams: These are strong, rapid and narrow air currents circumpolar westerlies blowing in upper atmosphere or into troposphere.

  • Blow from west to east spreading over a few hundred kilometers at the height of 7.5-14 km
  • Found between poles and \[{{20}^{0}}\] latitude in both the hemisphere.

The minimum velocity is 108 km/hr and the average summer velocity is 130 km/hr and average winter velocity is 65 km/hr. The maximum velocity is 480 km/hr.





Air Mass


An air mass is defined as a large part of atmosphere, having similar physical properties (especially temperature and humidity) spreading over hundreds of kilometers.

  • In order to acquire the similar physical properties air mass must be stationary for a longer period of time on the source regions.

The notable centre for the development is anticyclone area which is characterised by high pressure and low pressure regions.


Classification of Air mass

Generally the air mass is classified into 4 types:

(1) Polar Air mass (P) - It originates in polar region between \[{{60}^{0}}\]N and S. Polar air mass is cold.

(2) Tropical Air mass (T) - It originates between\[{{60}^{0}}\]N and S upto equator. Tropical air mass is warm.

(3) Continental Air mass (C) - It originates over continents (land mass) and is dry in nature.

(4) Marine Air mass (M) - It originates over the oceans and in moist in nature.


These four air masses are combined with one other to form four principal air masses such as:

(1) Continental polar mass air (CP) - Cold, air and stable.

(2) Maritime polar mass air (MP) - Cold, air and unstable.

(3) Continental Tropical air mass air (CT) - hot, dry, stable (at height) and at surface Instable.

(4) Maritime Tropical air mass air (MT) - warm, moist and unstable.




It is the transition zone between two air masses (warm and cold) with different properties (temperature, humidity, density, pressure and wind direction). An extensive transitional zone between two converging air masses is called as frontal zone on frontal surface. The process associated with the creation of new fronts or the regeneration decaying fronts is called   frontogenesis. The process of destruction or dying of fronts is called frontolysis.


Types of Fronts

Fronts are of four principal types on the basis of their different characteristic features such as warm front, cold front, occluded front and Stationary front.

(1)         Warm front

In other wards when the warm air mass moves upwards over the cold air mass is termed as front. It has gentle sloping frontal surface along with warm and light air. As they are warm and light they rise up slowly over cold and dense air. This warm air is cooled adiabatically. It saturated and gets condensed resulting in precipitation over a larger area for several hours. The precipitation varies from moderate to gentle. It is associated with Ci, Cs, As, Ns, St and fog.


(2)        Cold front

It can be defined as the boundary between warm and cool air masses wherein cold air displaces warm air. It has a steep edge and move aggressively towards the warm air territory. As they remain denser at the ground, forcing the warm air to rise. The cold air mass is associated with bad weather-thick clouds, which further heavy downpour with thunderstorms and lightning. At times it results in snowfall and hailstorms. Cloud, associated with this type of front is Ci, Cs and Cb.


(3)        Occluded front

When the cold front completely overtakes warm front, displacing the warm air by cold air from the ground system.


(4)        Stationary front

It is the stage when the two contrasting air masses become parallel to each other and there is no ascenstet of air.




Cyclone in centre of low pressure surrounded by elliptical arrangement isobars. The pressure increases outward and the air move inward as the centre has low pressure. In the northern hemisphere it has anticlockwise movement and clockwise in the southern hemisphere.

It can be circular, elliptical or 'V shape.

It is of two types

  • Extratropical / Temperature cyclone
  • Tropical cyclone


(1) Extra tropical/Temperate cyclone

It is a simple depression or atmospheric disturbance having low pressure in the centre and pressure increases outward. It is found in the middle latitude \[({{35}^{0}}-{{65}^{0}})\]in both opposing air masses (warm, moist and light tropical air mass, front is created where these two air masses converge and lead to the development of temperate cyclone. The size of cyclones varies from 150 km to 3000 km. On an average vertical extent is 10-12 km. Its average summers velocity is 32 km per hours for winter it is 48 km per hour.



(1) Life Cycle of Cyclone


The period of cyclone from its inception (cyclogenesis) to its termination (eyclolysis) is called as life cycle of cyclone. There are six stages in a life cyclone of a cyclone:


  1. First stage involves the convergence of two air masses of contrasting physical properties and direction.
  2. Second stage is a stage when the warm and cold air masses penetrate into the territories of one other, and a wave-life front is formed. It is also called incipient stage.
  3. Third stage is the mature stage when the cyclone is fully developed and the isobars are almost circular in shape.
  4. Fourth stage in this stage where warm air mass is narrowed by the cold air mass.
  5. Fifth stage is related with the starting of occlusion of cyclone. Here, the cold front fully overtake the warm front.
  6. Sixth stage warm sector completely disappears and occluded front is eliminated and ultimately cyclone dies out.


(2) Tropical Cyclone


The development of the cyclone over the tropical region lying between tropic of cancer to Capricorn are called as Tropical Cyclones. Generally the tropical cyclones are formed along the zone of confluence of south-east and north-east trade winds. The confluence zone of these two winds is also known as the Inter Tropical Convergence Zone (ITCZ).

  • Its average diameter varies between 80 km to 300 km. At times it is restricted to 50 km or even less in diameter.
  • Velocity varies between 32 km/hr to 180 km/hr or more when it is co-verted into a hurricane.
  • Tropical cyclone becomes more vigorous and move with high velocity over ocean but over land it becomes feeble as it reaches interior portion of the continents.
  • There are less number of isobars and are more or less in circular shape. This results in rapid rush of wind towards the centre. Every year it occurs at a particular period of a, year, mainly during summer season.
  • It is not generated near equator as the coriolis force is negligible at equator.


Types of Tropical Cyclone

Tropical cyclone are divided into 4 major types:

(1) Tropical disturbance              

(2) Tropical depression

(3) Tropical storms                        

(4) Hurricane or typhoon


  1. Tropical disturbance

It is associated with easterly trade winds. The easterly wave develops between 5° to 20° N latitude in the western part of the oceans. It is associated with large amount of cumulus or cumulonimbus cloud. These clouds bring heavy to moderate rainfall.


  1. Tropical Depression

This region is the centre of low pressure and characterized by closed isobars, which are small in size. The wind velocity at the centre is 40-50 km/hr. It is usually influenced by summer weather of India and Australia. Sometimes, it becomes strong and give heavy rainfall, further resulting in floods.


  1. Tropical Storm

Tropical storm is related with low pressure centre, closed isobars which rushes towards the centre with the velocity ranging between 40 to 120 km/hr. Generally, it developes over Caribbean sea, near Philippines and Bay of Bengal causing devastation of lives and properties in the coastal regions.


  1. Hurricane or Typhoon

It is a form of massive tropical cyclone surrounded by several closed isobars. It moves with an average speed of 120 km/ hr. Hurricane has more symmetrical and circular isobars.

The pressure increases sharply from centre towards the outermargin, resulting in pressure gradient. Heavy downpour occurs, which is often uniformly distributed over a larger area as compared to other types of tropical cyclones.


Tropical Cyclones are known by different names:

Hurricanes - Carbbean and Pacific coast of Mexico.

Typhoons - Sea of China and Japan

Cyclones - India / Australia

Willy-Willies - North Australia

Tornado - South and Eastern USA.

Bagguio - Philippines


Comparison between Temperature and Tropical cyclones.


Tropical Cyclone

Extra-tropical cyclone

The tropical cyclone have a thermal origin, exclusively over the tropical seas.

Formed in middle or high latitudes, due to the development of front

 (\[{{35}^{0}}-{{65}^{0}}N\]and S)

The size of the tropical cyclone is 1/3 of temperate cyclone

It is much extensive size.

Strongest winds of tropical cyclones take place at surface.

Strongest winds of mid-latitude cyclones are higher up in atmosphere.

It is associated with single eye

There is more than one place where wind and rain is active.

Wind velocity is very high

It is low in comparison to tropical cyclone.

Its relation with upper level air is not clear

It has a distinct relation with upper level of air.




The air mass in which the pressure is high at the centre but decreases outwards is anti-cyclone. Winds move in a clockwise out-spiral in the northern hemisphre but in an anticlockwise out-spiral in the southern hemisphere.

  • They have a circular shape usually but at times assume V shape.
  • They are larger in size than temperate cyclone. 75 times the size of temperate cyclone.
  • It has the average velocity of 30-50 km/hour.
  • The high pressure at the centre causes the wind to move outward.
  • The wind descend from above at the centre and thus weather becomes clear and rainless.
  • It has no fronts.




It is the amount of water vapour present in the air at a particular period of time and place. Humidity of a place can be expressed in three ways:


Specific Humidity

Another way to express humidity as the mass of water vapour per unit weight of air or the proportion of the mass of water vapour to the total mass of air is called the specific humidity. Specific humidity is not affected by changes in pressure or temperature.

Relative Humidity

This is a ratio expressed between actual quantity of water vapour present in the air at a given temperature absolute humidity and the maximum quantity of water vapour that the atmosphere can hold at that temperature. Relative humidity determines the amount and rate of evaporation.


  • Hygrometer is an humidity. It comprises of wet and dry bulb thermometer.

  • Temperature and evaporation are directly proportional to humidity.
  • The process of transformation of liquid into gaseous form is called as
  • Oceanic and coastal regions record higher humidity capacity of air than the remote continental regions.
  • Humidity capacity decreases from equator to polewards as the temperature also decreases.
  • The air having moisture content equal to its humidity capacity is called as saturated air.
  • It balances the energy flow of the earth in the form of clouds.
  • The process of evaporation and condensation are major means by which transfer of energy takes place from earth to atmosphere, which is the basis of precipitation.





Clouds are mass of very fine water droplets, ice particles or mixture suspended in the atmosphere. They are formed by the adiabatic cooling of air when it is below its dew point. Cooling process is created by upward movement of light and warm moist air which reduces pressure, expands and reaches its dew point. There is further cooling below dew point causing condensation. Adiabatic process involves change in temperature of an ascending or descending air. The air which ascends (due to expansion of volume) results in cooling of air, whereas the air which descends results in warming a air at the rate of \[{{10}^{0}}\]per 1000 meters. This rate of shade change is called as adiabatic lapse rate. \[{{10}^{0}}\] per 1000 meters change is only before dew point (condensation level) and after dew point is achieved the rate of change in \[{{5}^{0}}\]C per 1000 m.



Types of Cloud


Usually clouds are classified in terms of their vertical arrangement in the atmosphere of high, middle and low clouds.

  1. High clouds: 6000 to 12000 meters above sea level.
  • Cirrus: Wispy, fibrous-looking cloud which indicate fair weather.
  • Cirrocumulus: A thin cloud, often globular and rippled.
  • Cirrostratus: Thin white sheet type which gives the sun and moon their haloes.


  1. Medium clouds: 2000 to 6000 meter above sea level.
  • Altocumulus: Globular, bumpy looking clouds with a flat base covering entire sky.
  • Altostratus: greyish, watery looking and a wool pack cloud.


  1. Low clouds: below 2000 meters above sea level.
  • Stratocumulus: Low rolling and grey colour, bumpy clouds.
  • Nimbostratus: Fog-like low cloud causes dull weather with drizzle. It is also associated with lightning  thunder and hailstorm.
  • Stratus: These clouds are low, grey and layered, almost fog-like in appearance, bring dull weather and often accompanied by drizzle air or snowfall.


  1. Clouds of great Vertical extent: 1500 to 9000 m.
  • Cumulus: A dome-shaped and flat bases clouds, which form a whitish grey globular mass. It sometimes becomes thunder cloud.
  • Cumulonimbus: Cumulus cloud   which   reaches up to 9000 meters, often indicates convectional rain, lightning and thunder storm.




It is defined as water in the form of liquid and solid falls on earth that falls under the gravity is called as precipitation. This could be in the form of rain, snow or hail etc. Its form depend on the temperature at which water vapour condenses.


Forms of Precipitation

The precipitation has several forms like droplets of water, ice flakes and solid balls or hails etc.



It is a form of solid precipitation consisting of large pellets or spheres of ice balls with the diameter varing between 5m to 50 mm. The falling of hail on the ground surface is called   hailstorm. It is destructive as it destroys agricultural crops and claim human and animal lives.



It is the fall of large snowflakes from clouds on the ground surface. The dew point should be below freezing point for receiving snowfall. It is a result of sublimation.



It is a mixture of snow and rain. It is a small pellets formed by  freezing of raindrops or melting snowflakes.



It is the most common form of precipitation. It is a process wherein mass of moist air ascends, saturates and condenses.

A diabatic cooling takes place when the air rises and the relative humidity becomes 100 per cent. The condensation of water vapour takes place where large hygroscopic nuclei (salt and dust) is formed. Such droplets are called as cloud dropletsshade. Rainfall occurs when cloud droplets change to raindrops which involves two processes:

  1. The warm and moist air ascends to such a height the process of condensation beings below freezing point both the water droplets and ice droplets are formed. The condensation takes place as the water droplets evaporates around ice droplets due to difference in vapour pressures. These ice droplets become large and fall when finally they are not able to be held back in the condensed ice droplets form.
  2. The suspended cloud droplets in the cloud are of varying sizes. They collide among themselves at different rate as their size varies. They combine to form a large droplet this process several cloud droplets are coalesced to to raindrops. When these cloud droplets are large enough that they are unable to hold by ascending air they being to fall.


Types of Rainfall

Rainfall can be classified into three types:

  1. Convectional Rainfall
  2. Orographic Rainfall
  3. Cyclonic or Frontal Rainfall


  1. Convectional Rainfall
  • The excessive heating of the ground surface in tropi region through the process of insolation leads to rise air to higher strata of atmosphere as they become warm light. The process of convectional rainfall mainly depends on two factors :

(i) The supply of moisture through evaporation should be abundant so that its relative humidity becomes high.

(ii) There should be intense heating through insolation   process.

The process of convectional rainfall involves intense heating ground surface though solar radiation. As the warm air rises, the vacant shape is filled by surrounding air which too up when come in contact with already warm air. Ascending warm and moist air cools according to dry adiabatic lapse rate (\[{{10}^{0}}\]C per 1000 metres) increasing the relative humidity. The moist air becomes saturated soon (relative humid becomes 100 percent) and futher rising of saturated air causes condensation and cloud formation cumulo-nimbus clouds. The air further rises and cools with moist adiabatic lapse rate (\[{{5}^{0}}\]C per 1000 metre). When the air reaches the temperature of its surrounding cumulo-nimbus cloud is formed and there instantaneous heavy rainfall.

  • It occurs daily in afternoon in the equatorial regions to short duration but point down heavily.
  • Occurs through thick dark and extensive cumulo- nimbus clouds.
  • It is accompanied by number of thunder and lightning


  1. Orographic Rainfall

When the warm and moist air rises and is obstructed by any hill or mountain, it starts ascending along the slope of the hill or mountain and get saturated after reaching a height. As a result condenses around hygroscopic nuclei. The presence of hill or mountain triggers the process of ascending of moist air, becoming cool and unstable. The slope of the mountain facing the wind is called as windward side and the other side of that mountain is known as Leeward side. Windward side receives maximum rain and leeward side receives no or little rain. The leeward side is also known as rain shadow region. This type of rainfall is called as       orographic rainfall.

  • The mountain should be acting as a barrier across the wind direction.
  • Mountain or hill should be running parallel to the coast.
  • The height of the mountain too determines the amount of orographic rainfall.
  • One of the best example of orographic rainfall in
  • India is Cherrapunji located on the southern margin of the Khasi hills in Meghalaya.



  1. Convergence or Cyclonic Rainfall

          When the air currents converge and rise, the convergence rainfall occurs. In tropical regions, the lifting of air is more or less vertical due to comparable temperatures of opposing air currents and usually accompanied by convention. When two large mass of air currents having unequal densities and temperature came into contact, the rising warm moist air mass reaches above the colder one. The rising warm moist air condenses to form clouds and then cause heavy rainfall. Such rainfall associated with thunder and lightning is also called as the frontal rainfall.

  • Frontal rainfall is associated with both type of warm and cold fronts.
  • Such rainfall is usually steady and may continue for a whole day or even longer.
  • The cooling of large air masses of humid air is an essential for convergence rainfall.

Distribution of precipitation


There is no uniformity of precipitation all over the world. The average annual precipitation for the world is 97.5 centimeters. The land area receives lesser amount of rainfall as compared to oceans while the different places on earth's surface receive different amount of annual rainfall and also in different seasons. On the basis of global pressure, wind bells, distribution of land and water bodies and nature of relief feature on earth, the distribution of precipitation can be explained:

  1. Regions of Heavy Precipitation: Rainfall more than 200 cm per year are:

(i) Equatorial regions: Amazon and Congo Basins, Malaysia, Indonesia and New Guinea.

(ii) Tropical Monsoon regions: Parts of India, South- east Asia and South China.

(iii) Mid-latitude West Margin regions: Coastal regions of British Columbia, North-west Europe, South Chile and South Island of New Zealand.

  1. Moderate rainfall (100 to 200 cm per year)

(i) Eastern margins of continents in the trade-wind belt e.g. eastern margin of China, U.S.A., Brazil, South Africa and Australia.


  1. Regions of very low rainfall (less than 50 cm.)

(i) Tropical deserts - Western margins of continents in the trade wind belt, Californian desert (USA), Atacama (South America), Kalahari (Southern Africa), Sahara, Arabian Desert and West Australian desert.

(ii) Mid-latitude desert - Interiors of large continents like Asia and North America.

(iii) Polar Regions -Arctic and Antarctic.




  1. Equatorial Climate or Tropical Rain

Forest Climate



  • It is found between \[5{}^\circ \]N to \[10{}^\circ \]N and South of equator.
  • The equatorial climate is found in the following

South America: Largest area is in the Amazon lowlands.

It also occurs along the coast of Guinea

Africa: Part of the Congo basin and Guinea coast Africa.

Southern Asia: Malaysia, Indonesia, New Guinea and parts of Philippines.



  • It is warm round the year as the sun's rays are always fall vertically. Annual average temperature is uniform at \[{{27}^{0}}\]C. The daily range of temperature is in between \[{{10}^{0}}\]C and \[{{25}^{0}}\]C. The annual range temperature is less than \[{{5}^{0}}\]C.
  • This region has no dry season. Average annual rainfall is 200 cm to 250 cm. The driest month in this region receives 6 cm of rainfall only.
  • Thermally included low pressure belt due to the uniform high temperature throughout the year.
  • Convection current is formed and results in rainfall in this zone.
  • The convergence of trade winds coming from Sub-tropical high pressure belt forms inter tropical convergence (ITC). ITC is associated with atmospheric distribution (cyclone).



High temperature and rain throughout the year produce most luxuriant vegetation in the region which are tropical rainforest or Selvas in South America.


  1. Savanna or Sudan type of climate


This transitional type of climate is bounded between equatorial rainforest and semi-arid and subtropical humid climate. This lies between \[{{5}^{0}}\] to \[{{20}^{0}}\] north and south of equator.

  • Columbia and Venezuela.
  • Africa : Sudan, parts of Senegal, Mali, Guinea,
  • Niger, Chad, Ghana, Togo, Kenya,
  • Zimbabwe, Tanzania, Angola and Uganda.
  • Australia: Queensland.



  • This zone has distinct wet and dry season.
  • The mean temperature varies between \[{{24}^{0}}\]C and \[{{27}^{0}}\]C.
  • Annual precipitation of 100 cm to 150 cm is received. From December to February there is no rainfall at all.
  • 80% to 90% of the rain occurs in rainy season only.



  • Seasonal rains allow grass to grow. These are natural grasslands. Rainfall is not sufficient to support tall trees but grass grows well. Elephant grass grows up to 4.5 metres. As we move towards the equator, trees with broad leaf and umbrella shaped grow along the banks of the streams.
  • Fauna: Elephants, giraffe, zebra, rhinoceros along with varieties of carnivores animals (lion, leopards, tiger, cheeta, hyena etc.) are found.


  1. Hot Desert Climate


Arid deserts lie close to the tropic of cancer and tropic of

Capricorn in the western margins of continents, between\[{{15}^{0}}-{{30}^{0}}\] in both the hemispheres.

  • Sahara, Arabia, Thar, Mohave and Sonoran (U.S.A.), Kalahari and Namib (Africa), Simpson, Gibson, Great Sandy (Australia). These lie in western part of the continents.



  • Climate is dominated by subsidence of air masses and sub-tropical anti cyclones.
  • Average summer temperature is between \[{{30}^{0}}\]C to \[{{35}^{0}}\]C. Mid-day temperature is recorded to be \[{{40}^{0}}\]C.
  • Daily range of temperature is great varrying between \[{{22}^{0}}\]C to \[{{28}^{0}}\]C
    • Annual range of temperature varies between \[{{17}^{0}}\]C to \[{{22}^{0}}\]C.
  • These areas are nearly rainless or receive lowest annual rainfall (Less than 12 cm).



  • Normal vegetation is cactus, thorny plants, shrubs and herbs.


  1. Steppe (Temperate continental) Climate Underline


These are also called mid-latitude grasslands. They are far away from any influence of the sea as they are in the heart of the continents. Examples:

  • Prairies (North America), Pampas (South America), Velds (South Africa), Canterbury (Newzeland), Downs (Australia) and Steppes (Russia).


  • Their climate is continental with extremes of temperate Summers are very warm and winters are very cold northern hemisphere where as in southern hemisphere temperature variation is mild.
  • These are dry lands as they are located in the deep interiors of large land masses away from the oceans. Temperas in summer season varies from \[18{}^\circ C\] to \[24{}^\circ C\] and in winter season from -\[4{}^\circ C\] to \[2{}^\circ C\]. Such range of temperature is large.
  • Rain fall occurs in spring and early summer and varies between 25 cm. and 75 cm. Rain in this region convectional type and light.



Short grass grows everywhere. Trees are found only   mountain slopes.


  1. Mediterranean Climate


The zone lies between \[{{30}^{0}}-{{40}^{0}}\]N and Southern latitudes on the western edge of the continents.

  • Mediterranean type of climate is found near
  • Mediterranean sea in the northern hemisphere (Portugal to Turkey, Morocco, Northern Algeria, Tunisia Libya) along with southern California coast.
  • In the southern hemisphere central Chile, Cape town area of South America, South and Southwest coast of Australia.
  • Average winter temperature is \[{{5}^{0}}\]C to \[{{10}^{0}}\]C whereas that of summer it is \[{{20}^{0}}\]C to\[{{27}^{0}}\]C.
  • Mean annual range of temperature is recorded to between \[{{15}^{0}}\]C to \[{{17}^{0}}\]C.
  • The zone experiences season shift of pressure belt
  • Winter rainfall is received through cyclonic storms.
  • More than \[{{75}^{0}}\]of the rainfall is in winter season.
  • The mean annual rainfall varies between 37 cm to 65 cm
  • Summer winds are generally dry and hot.



  • Woodland, dwarf forest and scrubs are found in region.
  • The leaves are thick and shiny resisting moisture loss.
  • The trees like pine, oak, cedar, madrone, walnut chestnuts grows here.
  • Citrus fruits are grown in this zone.


  1. Sub-Tropical Humid Climate/China type


  • It is found in the eastern boundary of the all continents between \[{{25}^{0}}\]C to \[{{40}^{0}}\] N and Southern latitude.
  • It is found in south-east China, PO Basin, Danube Basin South-east USA; south-east Brazil, Paraguay, Uraguay and north-eastern Argentina and Africa along with the east  coast of Africa.



  • Mean annual summer temperature lies between \[{{24}^{0}}\]C \[{{26}^{0}}\] and during winter it is 6.\[{{6}^{0}}\]C to \[{{10}^{0}}\]C.
  • The temperature do not varies spatially.
  • Rainfall varies between 75 cm to 150 cm
  • Rain decreases as we move inland from coast.
  • Tropical cyclone is found and results in rainfall. Even winter cyclones are found here as it is associated with                                               



  • Dense evergreen forests are found along with deciduous sparse forest and grassland. They have broad leaves.


  1. West European type Climate


  • The zone lies between 40° and 65° latitude in both the hemispheres along the west coast of the continents.
  • North-Western Europe, British Columbia of Canada, Washington and Oregon states of the USA, south-west coast of Chile, south-east coast of Australia and Tasmania and New Zealand.



  • Average temperature during summer ranges between \[{{15}^{0}}\]C to \[{{21}^{0}}\] C.
  • Polar front causes the development of temperate cyclone in this region under the influence of westerlies.
  • Annual range of temperature varies between 50 cm to 75 cm.



  • Broad-leaf deciduous forests (oak, birch, walnut, maple, elm, chestnut, etc).
  • Needle-leaf (coniferous) forests like pine, fir etc are found.
  • Mixed forest is also found here.


  1. Monsoon Climate


Monsoon region includes the eastern margins of continents which lies between \[{{5}^{0}}\]to \[{{30}^{0}}\] N and Southern latitude of equator.

  • Eastern Brazil (S. America), Central American countries, Natal coast (S. Africa), Indian subcontinent, South East Asia, Myanmar, Thailand, Vietnam, Philippines, etc. Parts of East Africa including Malagasy and North Australia.



  • Temperature variation results in season formation.
  • During the months of summer (March to June) the average temperature is ranging between \[{{27}^{0}}\]C to \[{{32}^{0}}\]C.
  • During the months of winter the average temperature recorded is \[{{10}^{0}}\] to \[{{27}^{0}}\]C.
  • The temperature in this zone is controlled by nearness or remoteness of the sea, latitudinal and altitudinal also influence it.
  • The region receives cyclonic rainfall mostly along with orographic rain.
  • On an average annual rainfall received in around 150 cm. Rainfall shows temporal variation.
  • 80% of the rainfall is received within 3 months (July, August and September).
  • Some parts of Indian sub-continent receives winter rainfall (Tamil Nadu and Andhra Pradesh) from north- east monsoons.
  • Monsoon rainfall is basically cyclonic in character.
  • Generally, dry season is found here



  • Trees are mostly deciduous. The forests are open and less luxuriant. Most of the forests yield valuable timber like teak and other kinds of timber are sal, acacia and eucalyptus.


  1. Taiga /Boreal/ Sub-Arctic Climate


This climate type is named after the coniferous forest cover found in the region.                                       

  • This region lies between \[{{55}^{0}}\]and in northern hemisphere. It forms a continuous belt across southern Canada, northern Europe and Russia.
    • South Alaska, southern Canada, parts of Norway,

Sweden, Finland, Northern Russia, Northern Siberia, and Sakhalin Island.



  • Winters are very cold and severe lasting for 6 to 7 months .This region has Verkhoyansk the "cold pole" colder than the arctic region. Summers are short lasting for 3 or 4 months and days are long; at \[{{60}^{0}}\]N the sun shines for 18 hours.
  • Rainfall varies from 25 to 100 cm. There is more rainfall near the coast.
  • Most of the rain comes from cyclonic weather. It falls throughout the year but maximum in summer as frequent showers.
  • In winter it is in the form of snow which remains on the ground for 5 to 7 months.


  • Vegetation in this climate type is softwood coniferous forests (Spruce, fir, pine).
  • Fauna like reindeer, deer, elk, moose and wild cat are found here.


  1. Tundra Climate


  • The northern most parts of Asia, Europe and North America (include Alaska and Canadian Islands).



  • Average annual temperature is\[-12{}^\circ C\].
  • Long, bitterly cold and severe winter are experienced.
  • Summers are short but cool.
  • Precipitation is below 40 cm and as snowfall.



  • Very short growing season.
  • Dwarf willows and birches grow here.
  • Fauna like reindeer, polar bear, fox, musk, ox and arctic hare are commonly found here.


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