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Humidity and Precipitation

  • Water vapour in air varies from zero to five per cent by volume of the atmosphere (averaging around 2% in the atmosphere).
  • The actual amount of the water vapour present in the atmosphere is known as the absolute humidity. It is the weight of water vapour per unit volume of air and is expressed in terms of grams per cubic metre. The absolute humidity differs from place to place on the surface of the earth. The ability of the air to hold water vapour depends entirely on its temperature (Warm air can hold more moisture than cold air).
  • The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature is known as the relative humidity. With the change of air temperature, the capacity to retain moisture increases or decreases and the relative humidity is also affected.
  • Relative humidity is greater over the oceans and least over the continents (absolute humidity is greater over oceans because of greater availability of water for evaporation).
  • The relative humidity determines the amount and rate of evaporation and hence it is an important climatic factor.

Humidity and Precipitation

  • Air containing moisture to its full capacity at a given temperature is said to be ‘saturated’. At this temperature, the air cannot hold any additional amount of moisture. Thus, relative humidity of the saturated air is 100%.
  • If the air has half the amount of moisture that it can carry, then it is unsaturated

Relative Humidity can be changed by:

  • By adding moisture through evaporation (by increasing absolute humidity): if moisture is added by evaporation, the relative humidity will increase and vice versa.
  • By changing temperature of air (by changing the saturation point): a decrease in temperature (hence, decrease in moisture-holding capacity/decrease in saturation point) will cause an increase in relative humidity and vice versa.

Dew Point

  • The air containing moisture to its full capacity at a given temperature is said to be saturated.
  • It means that the air at the given temperature is incapable of holding any additional amount of moisture at that stage.
  • The temperature at which saturation occurs in a given sample of air is known as dew point.

Adiabatic Lapse Rate

  • The adiabatic lapse rate is the rate at which the temperature of an air parcel changes in response to the compression or expansion associated with elevation change, under the assumption that the process is adiabatic, i.e., no heat exchange occurs between the given air parcel and its surroundings.
  • The Dry Adiabatic Lapse Rate (DALR) is the rate of fall in temperature with altitude for a parcel of dry or unsaturated air (air with less moisture, to keep it simple) rising under adiabatic conditions.
  • When an air parcel that is saturated (stomach full) with water vapour rises, some of the vapour will condense and release latent heat [Additional Heat from inside]. This process causes the parcel to cool more slowly than it would if it were not saturated.
  • The moist adiabatic lapse rate varies considerably because the amount of water vapour in the air is highly variable.

Atmospheric Stability and Instability

  • Different forms of precipitation (dew, fog, rainfall, frost, snowfall, hailstorm etc.) depend on stability and instability of the atmosphere. The air without vertical movement is called stable air while unstable air undergoes vertical movement (both upward and downward). An air mass ascends and becomes unstable when it becomes warmer than the surrounding air mass while descending air mass becomes stable.
  • The stability and instability depend on the relationships between ‘normal lapse rate’ and ‘adiabatic change of temperature’. Adiabatic rate is always constant whereas normal lapse rate of air temperature changes.
  • When the normal lapse rate is higher than dry adiabatic rate, the air being warmer rises and becomes unstable. On the other hand, when the normal lapse rate of temperature is lower than dry adiabatic rate, the air being cold descends and becomes stable.



Condition of Stability

At ground surface if the temperature of a parcel of air is 40°C, the dry adiabatic lapse rate and normal (environmental) lapse rate are 10°C per 1000m and 6.5° C per 1000 m respectively, then at the height of one kilometre (or 1000 m) from the ground surface the temperature of the ascending air would be 30°C (40° -10°= 30°C) while the temperature of surrounding air at that height would be 33.5° C (40°-6.5° =


Sometimes, the normal lapse rate in a certain layer of the atmosphere is about 4.6° C per 1000 metres. In such conditions if the normal lapse rate is less than wet adiabatic lapse rate even at condensation point, further vertical motion of air is stopped and thus such air is said to be absolutely stable and such atmospheric condition is called absolute stability..

Condition of Instability

When normal lapse rate is greater than dry adiabatic lapse rate of ascending parcel of air the rising air continues to rise upward and expand and thus becomes unstable and is in unstable equilibrium. In other words, atmospheric instability is caused when the rate of cooling of rising air (dry adiabatic lapse rate) is lower than the normal lapse rate.

For example, if the temperature of a certain parcel of air at ground surface is 40°C, the dry adiabatic and normal lapse rates are 10°C and 11°C per 1000m respectively, then the temperature of ascending air at the height of 1000m (one kilometre) would be 30°C (40°-10° = 30°C) while the temperature of the atmosphere at that height would be 29°C (40°-11°C = 29°C).

Thus, the rising air being warmer (30°C) than the surrounding air (29°C) continues to rise and expand to cause atmospheric instability. If the wet adiabatic lapse rate is also less than normal lapse rate, the rising air further continues to rise upward. Such state of continued upward movement of air is called absolute instability.

Condensation and its forms

  • The transformation of water vapour into water is called condensation.
  • Condensation is caused by the loss of heat (latent heat of condensation, opposite of latent heat of vaporization).

Forms of Condensation

  • Dew Frost
  • Fog
  • Mist
  • Clouds





Convectional Rainfall

The, air on being heated, becomes light and rises up in convection currents. As it rises, it expands and loses heat and consequently, condensation takes place and cumulus clouds are formed. This process releases latent heat of condensation which further heats the air and forces the air to go further up.

Convectional precipitation is heavy but of short duration, highly localised and is associated with minimum amount of cloudiness. It occurs mainly during summer and is common over equatorial doldrums in the Congo basin, the Amazon basin and the islands of south-east Asia.




Orographic Rainfall

When the saturated air mass comes across a mountain, it is forced to ascend and as it rises, it expands (because of fall in pressure); the temperature falls, and the moisture is condensed.

The chief characteristic of this sort of rain is that the windward slopes receive greater rainfall. After giving rain on the windward side, when these winds reach the other slope, they descend, and their temperature rises. Then their capacity to take in moisture increases and hence, these leeward slopes remain rainless and dry. The area situated on the leeward side, which gets less rainfall is known as the rain-shadow area (Some arid and semi-arid regions are a direct consequence of rain-shadow effect. Example: Patagonian desert in Argentina, Eastern slopes of Western Ghats). It is also known as the relief rain.



Frontal Rainfall

When two air masses with different temperatures        meet, turbulent conditions are produced.        Along the front convection occurs and causes precipitation (we studied this in Fronts). For instance, in north-west Europe, cold continental air and warm oceanic air converge to produce heavy rainfall in adjacent areas.



Cyclonic Rainfall

Cyclonic Rainfall is convectional rainfall on a large scale.


World Distribution of Rainfall


World Distribution of Rainfall

  • Different places on the earth’s surface receive different amounts of rainfall in a year and that too in different seasons. In general, as we proceed from the equator towards the poles, rainfall goes on decreasing steadily.
  • The coastal areas of the world receive greater amounts of rainfall than the interior of the continents. The rainfall is more over the oceans than on the landmasses of the world because of being great sources of water.
  • Between the latitudes 35° and 40° N and S of the equator, the rain is heavier on the eastern coasts and goes on decreasing towards the west. But, between 45° and 65° N and S of equator, due to the westerlies, the rainfall is first received on the western margins of the continents and it goes on decreasing towards the east.
  • Wherever mountains run parallel to the coast, the rain is greater on the coastal plain, on the windward side and it decreases towards the leeward side.


Air Masses

What is an Air mass?

  • An airmass is a large body of air whose physical properties, especially temperature, moisture content and lapse rate are more or less uniform horizontally for hundreds of kilometres.

Nature and degree of uniformity of air mass properties are determined by:

  1. Properties of source area and direction of movement
  2. Changes introduced in the air mass during journey
  3. Age of air mass

The vertical distribution of temperature in an air mass, and moisture content of the air are two basic properties of an air mass which control the weather conditions of the area affected by that airmass.


Types of Air masses

  • Continental Polar Air mass (cP)
  • Maritime Polar Air mass (mP)
  • Continental Tropical Air mass (cT)
  • Maritime Tropical Air mass (mT)


Front is the sloping boundary which separates two opposing air masses having contrasting characteristics in terms of air temperature, humidity, density, pressure and wind direction.

An extensive transitional zone between two converging air masses is called frontal zone.

The process of creation of new fronts or regeneration of decaying fronts is called frontogenesis.   The  process    of      destruction      or     dying of      fronts        is       called frontolysis


Warm Front

Warm front is that gently sloping frontal surface along which warm and light air becomes active and aggressive and rises slowly over the cold and dense air.

This gradually rising warm air is cooled adiabatically, gets saturated and after condensation precipitation occurs over a relatively large area for several hours in the form of moderate to gentle precipitation.


Cold Front

Cold front is that sloping frontal surface along which cold air becomes active and aggressive and invades the warm air territory and being denser remains at the ground but forcibly uplifts the warm and light air.

It has a greater slope than the warm front. A cold front is associated with bad weather characterised by thick clouds (cumulonimbus), heavy downpour with thunderstorms, lightning etc.


Distribution of Fronts


  • Polar frontal zone = cP + mT
  • Arctic Frontal Zone = cP + mP
  • Intertropical frontal zone
  • Mediterranean zone


  • Centres of low pressure surrounded by closed isobars having increasing pressure outward and closed air circulation from outside towards the central low pressure in such a way that air blows inward anticlockwise in the Northern Hemisphere and Clockwise in the Southern


  • They range in shape from circular, elliptical to V shape.

Temperate Cyclones

  • Extratropical cyclones/wave cyclones/ disturbances/ depressions
  • Formed in the regions extending between 35-65 degrees North and South.
  • They are formed due to the convergence of two contrasting air masses e.g.

warm, moist and light tropical air masses and cold and dense polar air masses.

  • After their formation, they move from west to east along with the westerlies.
  • They vary in shape and size. They can be circular, semi-circular, elliptical, elongated or V-shaped.
  • Average diameter is around 1900 km with short diameters being 1000 km.


Formation of Temperate Cyclone


Occluded Front

It is formed when cold front completely overtakes the warm front and warm air is completely displaced from the ground surface.

Weather Associated with Temperate Cyclones

  • Arrival of Cyclone
  • Warm Frontal Precipitation
  • Warm Sector
  • Cold Frontal Precipitation
  • Cold Sector

Wind direction changes constantly in different parts



Tropical Cyclones

  • Cyclones developing in regions lying between the tropics of Capricorn and Cancer are called tropical cyclones.
  • Known as typhoons, hurricanes, tropical disturbances.

Origin of Tropical Cyclones

  • Continuous supply of abundant warm and moist air. Tropical cyclones originate over warm oceans having surface temperature of 27 degree celsius.
  • Higher value of coriolis force is required for the origin of these cyclones.
  • There should be anticyclonic conditions in the upper troposphere.
  • Not very strong vertical wind shear (absence of cyclones in South Atlantic)
  • Low pressure leading to wind convergence


In the South Atlantic and South-Eastern Pacific regions in tropical latitudes, cyclone does not originate. What is the reason?

  • Sea surface temperatures are low
  • Inter-Tropical Convergence Zone seldom occurs
  • Coriolis force is too weak
  • Absence of land in those regions
Distribution of Tropical Cyclones





Naming of Cyclones

For the Indian Ocean region, deliberations for naming cyclones began in 2000 and a formula was agreed upon in 2004. Eight countries in the region – Bangladesh, India,

Maldives, Myanmar, Oman, Pakistan, Sri Lanka and Thailand – all contributed a set of names which are assigned sequentially whenever a cyclonic storm develops.

Saffir-Simpson Hurricane Scale

  • The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 rating based on a hurricane’s sustained wind speed. This scale estimates potential property damage.
  • Hurricanes reaching Category 3 and higher are considered major hurricanes because of their potential for significant loss of life and damage.
  • Category 1 and 2 storms are still dangerous, however, and require preventative measures.
  • In the western North Pacific, the term “super typhoon” is used for tropical cyclones with sustained winds exceeding 150 mph.


Saffir-Simpson Scale


Questions for the Day

  • Discuss distribution of precipitation around the world (250 marks)
  • Differentiate between Tropical and Temperate Cyclones. (150 words)
  • Discuss the concept of air mass and explain its role in macro-climatic changes. (200 words)