• Water is continuously exchanged between the atmosphere, the oceans and the continents through the processes of evaporation, transpiration, condensation and precipitation.
  • The moisture in the atmosphere is derived from water bodies through evaporation and from plants through transpiration (evapotranspiration). Evaporated water undergoes condensation and forms clouds.
  • When saturation is reached, clouds give shed water in the form of precipitation.
  • Since the total amount of moisture in the entire system remains constant, a balance is required between evapotranspiration and precipitation. The hydrological cycle maintains this balance.







  • Water vapour is around 2% in the atmosphere.
  • Water vapour absorbs both incoming and outgoing radiation and hence plays a crucial role in the earth’s heat budget.
  • The amount of water vapour present decides the quantity of latent energy stored up in the atmosphere for development of storms and cyclones.
  • The atmospheric moisture affects the human body’s rate of cooling by influencing the sensible temperature.



  • Water vapour present in the air is known as

A) Absolute Humidity

  • 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.

B) Relative Humidity

  • The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature is known as the relative humidity.

C) Dew point

  • Saturated air has at the given temperature is incapable of holding any additional amount of moisture.
  • The temperature at which saturation occurs in a given sample of air is known as dew point.
  • Dew point occurs when Relative Humidity = 100%.

D) Specific Humidity

  • It is expressed as the weight of water vapour per unit weight of air (grams of water vapour per kilogram of air).
  • The only way of changing specific humidity is by adding (evaporation) or removing (precipitation) of moisture.



  • Evaporation is a process by which water is transformed from liquid to gaseous state.
  • The oceans contribute 84% of the annual total and the continents 16%.
  • The highest annual evaporation occurs in the sub-tropics of the western North Atlantic and North Pacific because of the influence of the Gulf Stream and the Kurishino Current, and in the trade wind zone of the southern oceans.
  • The land maximum occurs in equatorial region because of high insolation and luxuriant vegetation.

Factors Affecting Rate of Evaporation

  • Amount of water available.
  • Area of evaporating surface.
  • Temperature
  • Relative humidity: low relative humidity has more space for moisture and hence evaporation increases.
  • Wind: Movement of air replaces the saturated layer with the unsaturated layer. Hence, greater the wind speed, the greater is the evaporation.
  • Whenever there is a combination of high temperature, very low relative humidity and strong winds, the rate of evaporation is exceptionally high. This leads to dehydration of soil to a depth of several inches.
  • Air Pressure: Evaporation is also affected by the atmospheric pressure exerted on the evaporating surface. Lower pressure over open surface of the liquid results in a higher rate of evaporation.
  • Composition of water: Evaporation is inversely proportional to salinity of water.
  • Rate of evaporation is always greater over fresh water than over salt water. [Because of the reduction in the vapour pressure (ability of the water molecules to bounce off the surface)
  • Under similar conditions, fresh water evaporates 5% faster than ocean water.
  • More evaporation by plants: Water from plants generally evaporates at a faster rate than from land.



  • 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 vaporisation).
  • When moist air is cooled, it may reach a level when its capacity to hold water vapour ceases (Saturation Point = 100% Relative Humidity = Dew Point reached).
  • Then, the excess water vapour condenses into liquid form. If it directly condenses into solid form, it is known as sublimation.
  • Condensation also takes place when the moist air comes in contact with some colder object and it may also take place when the temperature is close to the dew point.
  • Condensation, therefore, depends upon the amount of cooling and the relative humidity of the air.
  • After condensation, the water vapour or the moisture in the atmosphere takes one of the following forms — dew, frost, fog and clouds.
  • Condensation takes place when the dew point is lower than the freezing point as well as higher than the freezing point.

Processes of Cooling for causing Condensation :

A) Adiabatic Temperature Changes

  • When the air rises, it expands. Thus, heat available per unit volume is reduced and, therefore, the temperature is also reduced.
  • temperature change does not involve any subtraction of heat, and cooling of air takes place only by ascent and expansion, is termed ‘adiabatic change’.
  • The vertical displacement of the air is the major cause of adiabatic and katabatic (cold, dense air flowing down a slope) temperature changes.
  • Near the earth’s surface, most processes of change are non-adiabatic because horizontal movements often produce mixing of air and modify its characteristics.

B) Non-Adiabatic Temperature Changes

  • Non-adiabatic processes include cooling by radiation, conduction or mixing with colder air.
  • The non-adiabatic processes of cooling produce dew, fog or frost. As they are incapable of producing a substantial amount of precipitation.
  • In case there is direct radiation from moist air, the cooling produces fog or clouds, subject to presence of hygroscopic nuclei in the air.
  • Cooling by contact with a cold surface produces dew, frost or fog depending on other atmospheric conditions.



  • The forms of condensation can be classified on the basis of temperature at which the dew point is reached.
  • Condensation can take place when the dew point is
  • lower than the freezing point,
  • higher than the freezing point.
  • White frost, snow, hailstones and some clouds (cirrus clouds)are produced when the temperature is lower than the freezing point.
  • Dew, fog and clouds result even when the temperature is higher than the freezing point.
  • Forms of condensation may also be classified on the basis of their location, i.e. at or near the earth’s surface and in free air.
  • Dew, white frost, fog and mist come in the first category, whereas clouds are in the second category.

A) Dew

  • When the moisture is deposited in the form of water droplets on cooler surfaces of solid objects (rather than nuclei in air above the surface) such as stones, grass blades and plant leaves, it is known as dew.
  • The ideal conditions for its formation are clear sky, calm air, high relative humidity, and cold and long nights.

For the formation of dew, it is necessary that the dew point is above the freezing point.



B) White Frost

  • Frost forms on cold surfaces when condensation takes place below freezing point (0° C), i.e. the dew point is at or below the freezing point.
  • The excess moisture is deposited in the form of minute ice crystals instead of water droplets.
  • The ideal conditions for the formation of white frost are the same as those for the formation of dew, except that the air temperature must be at or below the freezing point.



C) Fog

  • When the temperature of an air mass containing a large quantity of water vapour falls all of a sudden (mostly due to temperature inversion), condensation takes place within itself on fine dust particles.
  • So, the fog is a cloud with its base at or very near to the ground.
  • Because of the fog and mist, the visibility becomes poor to zero.



  • In urban and industrial centres smoke provides plenty of nuclei which help the formation of fog and mist. Such a condition when fog is mixed with smoke is described as
  • Radiation fog results from radiation, cooling of the ground and adjacent air. These fogs are not very thick and are usualin winters.
  • Fogs formed by condensation of warm air when it moves horizontally over a cold surface, are known as advectional fog. These fogs are thick and persistent. Occurs over warm and cold water mixing zones in oceans.
  • Frontal or precipitation fog is produced due to convergence of warm and cold air masses where warm air mass is pushed under by the heavier cold air mass.
  • Precipitation in the warm air mass condenses to produce fog at the boundary of the two air masses. These are called frontal or precipitation fog.
  • In fog visibility is less than one kilometre.

D) Mist

  • The difference between the mist and fog is that mist contains more moisture than fog.
  • In mist, each nucleus contains a thicker layer of moisture.
  • Mists are frequent over mountains as the rising warm air up the slopes meet a cold surface.
  • Water droplets also form mist, but with less merging or coalescing. This means mist is less dense and quicker to dissipate.
  • Fogs are drier than mist, and they are prevalent where warm currents of air come in contact with cold currents.
  • In mist, visibility is more than one kilometre but less than two kilometres.



E) Smog

  • Smog = smoke + fog (smoky fog) caused by the burning of large amounts of coal, vehicular emission and industrial fumes (primary pollutants).
  • Smog contains soot particulates like smoke,sulphur dioxide, nitrogen dioxide and other components.



  • Smog is a combination of airborne particulate matter, like soot, and invisible toxic gases including ozone (O3), carbon monoxide (CO), sulphur dioxide (SO2), which are carcinogens (cancer-causing agents).
  • The atmospheric pollution levels of Los Angeles, Beijing, Delhi, Mexico City and other cities are increased by inversion that traps pollution close to the ground.
  • It is usually highly toxic to humans and can cause severe sickness, shortened life or death.
  • Temperature inversions are accentuated, and precipitation is reduced.
  • Smog-related Haze lowers visibility.



F) Haze

  • In a haze dust, smoke and other dry particles obscure the clarity of the sky.
  • There is no condensation in haze. Smog is similar to haze, but there is condensation in smog.
  • Sources for haze particles include farming (ploughing in dry weather), traffic, industry, and wildfires.

G) Clouds ( discussed below )



  • Cloud is a mass of minute water droplets or tiny crystals of ice formed by the condensation of the water vapour in free air at considerable elevations.
  • Clouds are caused mainly by the adiabatic cooling of air below its dew point.
  • As the clouds are formed at some height over the surface of the earth, they take various shapes.
  • According to their height, expanse, density and transparency or opaqueness clouds are grouped under four types: (i) cirrus; (ii) cumulus; (iii) stratus; (iv) nimbus.

A) Cirrus Clouds

  • Cirrus clouds are formed at high altitudes (8,000-12,000m). They are made of ice crystals.
  • They are thin and detached clouds having a feathery appearance. They are always white.

B) Cumulus Clouds

  • Cumulus clouds look like cotton wool. They are generally formed at a height of 4,000-7,000 m.
  • They exist in patches and can be seen scattered here and there. They have a flat base.

C) Stratus Clouds

  • As their name implies, these are layered clouds covering large portions of the sky.
  • These clouds are generally formed either due to loss of heat or the mixing of air masses with different temperatures.

D) Nimbus Clouds

  • Nimbus clouds are black or dark grey. They form at middle levels or very near to the surface of the earth.
  • These are extremely dense and opaque to the rays of the sun.
  • Sometimes, the clouds are so low that they seem to touch the ground.
  • Nimbus clouds are shapeless masses of thick vapour.

A combination of these four basic types can give rise to the following types of clouds:

  • High clouds – cirrus, cirrostratus, cirrocumulus;
  • Middle clouds – altostratus and altocumulus;
  • Low clouds – stratocumulus and nimbostratus (long duration rainfall cloud; rain bands in tropical cyclones) and
  • Clouds with extensive vertical development – cumulus and cumulonimbus (thunderstorm cloud)







  • Condensation of water vapour followed by release of moisture is known as precipitation.
  • The process of continuous condensation in air helps the condensed particles to grow in size.
  • When the resistance of the air fails to hold them against the force of gravity, they fall on to the earth’s surface as different forms of precipitation. Precipitation may happen in liquid or solid form.
  • Precipitation in the form of drops of water is called rainfall when the drop size is more than 0.5 mm.
  • It is called Virage when raindrops evaporate before reaching the earth while passing through dry air.
  • Drizzle is light rainfall with drop size being less than 0.5 mm, and when evaporation occurs before reaching the ground, it is referred to as
  • When the temperature is lower than the 0° C, precipitation takes place in the form of fine flakes of snow and is called snowfall. Moisture is released in the form of hexagonal crystals.
  • Besides rain and snow, other forms of precipitation are sleet and hail, though the latter are limited in occurrence and are sporadic in both time and space.
  • Sleet is frozen raindrops and refrozen melted snow-water. When a layer of air with the temperature above freezing point overlies a subfreezing layer near the ground, precipitation takes place in the form of sleet.
  • Raindrops, which leave the warmer air, encounter the colder air below. As a result, they solidify and reach the ground as small pellets of ice not bigger than the raindrops from which they are formed.
  • Sometimes, drops of rain after being released by the clouds become solidified into small rounded solid pieces of ice and which reach the surface of the earth are called hailstones.
  • These are formed by the rainwater passing through the colder layers. Hailstones have several concentric layers of ice one over the other.
  • Rainfall: drop size more than 0.5 mm.
  • Virage: raindrops evaporate before reaching the earth.
  • Drizzle: light rainfall; drop size less than 0.5 mm.
  • Mist: evaporation drizzle occurs before reaching the ground leading to foggy weather.
  • Snowfall: fine flakes of snow fall when the temperature is less than 0° C.
  • Sleet: frozen raindrops and refrozen melted snow; mixture of snow and rain or merely partially melted snow.
  • Hail: precipitation in the form of hard rounded pellets (5 to 50 mm) is known as hail.



  • On the basis of origin, rainfall may be classified into three main types – the convectional, orographic or relief and the cyclonic or frontal.



A) Convectional Rainfall

  • The air on being heated, becomes light and rises in convection currents.
  • As it rises, it expands and loses heat, and consequently, condensation takes place, and cumulous clouds are formed (when convection is rapid and intense cumulonimbus clouds are formed).
  • This process releases latent heat of condensationwhich 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.



B) Orographic Rainfall

  • This type of precipitation occurs when warm, humid air strikes an orographic barrier (a mountain range).
  • Because of the initial momentum, the air is forced to rise. As the moisture-laden air gains height, it expands (because of fall in ambient pressure) and the temperature falls (adiabatic).
  • Condensation sets in, and soon saturation (dew point) is reached. The surplus moisture falls as orographic rainfall along the windward slopes.
  • After giving rain on the windward side, the winds are relatively dry and cold. They reach the leeward slope and descend (katabatic wind), and their temperature rises due to increase in ambient pressure.
  • Their capacity to take in moisture increases (relative humidity decreases) and hence the 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, etc.).
  • The rainfall in rain shadow area is known as the relief rain. Example: Mahabaleshwar, situated on the windward side of Western Ghats, receives more than 600 cm of rainfall, whereas Pune, lying in the rain shadow area, receives only about 70 cm.



C) Frontal Rainfall

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



D) Cyclonic Rain

  • Cyclonic Rainfall is convectional rainfall on a large scale
  • The precipitation in a tropical cyclone is of convectional type while that in a temperate cyclone is because of frontal activity.

E) Monsoonal Rainfall

  • This type of precipitation is characterized by seasonal reversal of winds which carry oceanic moisture (especially the south-west monsoon) with them and cause extensive rainfall in south and southeast Asia.



  • Tornado is a small-diameter column of violently rotating air developed within a convective cloud and in contact with the ground.
  • Tornados occur most often in association with thunderstorms during the spring and summer in the mid-latitudes of both the Northern and Southern Hemispheres.
  • Tornadoes generally occur in middle latitudes because of convergence of warm and cold air masses.



A) Formation

  • When warm, humid air meets a cold airmass, horizontally spinning winds are created.
  • As the warm air rises, it begins rotating vertically forming a mesocyclone in the centre of the Cumulonimbus cloud. This is a supercell.
  • The rotating warm air condenses into rain which in turn pulls the mesocyclone closer to the ground; then the tornado begins to form.
  • Heavy rains in front of the tornado cause downdrafts.
  • These whirling atmospheric vortices can generate the strongest winds known on Earth: wind speeds in the range of 500 km (300 miles) per hour.
  • They are often referred to as twisters.



B) Waterspout

  • Waterspout is an intense columnar vortex (usually appearing as a funnel-shaped cloud) that occurs over a body of water.



  • They are connected to a towering cumulonimbus cloud.
  • They are weaker than most of its land counterparts, e. tornadoes.
  • Most waterspouts do not suck up water; they are small and weak rotating columns of air over water.

C) Distribution of tornadoes

  • The temperate and tropical regions are the most prone to thunderstorms and tornadoes.
  • Tornadoes have been reported on all continents except Antarctica.
  • United States has the most violent tornadoes.
  • Canada reports the second largest number of tornadoes.
  • In the Indian sub-continent, Bangladesh is the most prone country to tornadoes.


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