Science: Air and Water: Water Vapour, Cloud Formation and Relative Humidity (For CBSE, ICSE, IAS, NET, NRA 2022)

Get unlimited access to the best preparation resource for CBSE/Class-10 : get questions, notes, tests, video lectures and more- for all subjects of CBSE/Class-10.

Water Vapour

  • Air contains water vapour. Its amount in the air is not the same everywhere.
  • It is the maximum in low latitudes and over oceans and is low in the atmosphere over polar regions.
  • It is also more in summers than in winters.
  • It plays an important role in heating and cooling of the atmosphere.
  • Clouds, rain, snow, fog, all result from water vapour present in the atmosphere.
  • Evaporation is a process in which water from any source change into vapour state ‘due to heat’ .
  • Water evaporates from water bodies due to heat of the sun forms clouds and then falls as rain upon condensation.

Cloud Formation

Condensation of water vapour in the atmosphere leads to the formation of clouds. Clouds are formed when moist air rises upwards. When dew point is reached, condensation of water vapour occurs resulting in the formation of very tiny droplets of water.

Cloud Formation

They cling to the dust particles in the air. These millions of very minute water droplets or tiny ice crystals almost hang in the air rather than fall. They are blown as clouds by the wind.

Dew Point

the temperature at which the water vapour begins to change into water drops.


  • When clouds rise up, they are cooled when blown into cooler regions of the atmosphere. The small droplets of water in them become still cooler and they, come closer to each other. A number of small droplets combine to form a big drop of water. These drops are so big that they can no longer float in the air and they fall down on the earth as rain. As they fall, they pickup more and more small drops of water on their way down. The falling of these big drops of water from the clouds is known as rain and the process is called precipitation.
  • The instrument used to measure rainfall is called rain gauge. Rainfall is measured in centimeters.

Relative Humidity

Relative humidity is the ratio of the mass of water vapour actually present in a certain volume of air at room temperature to the mass of water vapour required to saturate the same volume of air at that temperature. The instrument used to measure relative humidity is called hygrometer.

Atmospheric Pressure

The force of air column acting per unit area of a surface results in a pressure exerted by atmosphere. This pressure is called atmospheric pressure.

Variation of Air Pressure with Height

  • The atoms and molecules of the gases in the atmosphere like those of all other matter are subject to earth՚s gravitational pull.
  • As a consequence, the atmosphere is much denser near the surface of earth than at higher altitudes.
  • In fact, the density of air decreases very rapidly with increasing distance from earth. Therefore, atmospheric pressure also decreases with altitude.
  • Air pressure is measured by the instrument known as barometer.

Air Pollution

  • Primary pollutants which are directly emitted into the atmosphere such as carbon monoxide from exhaust of a motor vehicle
  • Secondary pollutants which are not emitted directly into atmosphere but are formed in air when primary pollutants interact.

Major Primary pollutants include:

  • Carbon monoxide (CO) is produced by incomplete combustion of fuels like petrol, natural gas, coal or wood. It is a colorless and odorless gas but very poisonous in nature.
  • Carbon dioxide (CO2) is produced by complete combustion of fuels in motor vehicles and various industries. It is a colorless, odorless and non-toxic gas. (A person dies in atmosphere of carbon dioxide due to lack of oxygen and not due to its toxic nature) . (Read details in lesson 30, section 30.8. 2)
  • Sulphur oxides (SOx) (mainly Sulphur dioxide, SO2) are produced by combustion of coal and petroleum and also produced in volcanoes. It is also produced in various industrial processes. Oxidation of sulphur dioxide (SO2) to sulphur trioxide (SO3) results in formation of sulphuric acid (H2SO4) which causes acid rain. (See Lesson-30, Section 30.8. 4)
  • Nitrogen oxides (NOx) especially nitrogen dioxide, NO2 is a reddish brown gas with pungent smell. It catalyses the oxidation of SO2 to SO3 and indirectly causes acid rain.
  • Volatile organic compounds (VOCs) include methane, benzene, toluene and xylene. While methane is a major green house gas, others are suspected to be carcinogens (cancer inducing) .
  • Particulate matter consists of tiny particles of solids or liquids suspended in air. These are also called ‘suspended particulate matter (SPM) ’ . The major sources for these include volcanoes, dust storms and burning of fuels. These can cause heart and lung diseases and breathing disorders.
  • Chloro-fluorocarbons (CFCs) are used as refrigerants in air conditioners and refrigerators and are harmful to the ozone layer which protect us from harmful ultraviolet rays. You shall read about the ozone hole in Lesson30, Section 30.8. 1)

Major Secondary Pollutants Include

  • Photochemical smog (smoke + fog) formed by the action of ultraviolet light from the sun on particulate matter or formed due to burning of coal and petrol in an atmosphere containing SO2. It prevents dissipation of pollutants and causes breathing disorders. Read in detail from Lesson-30, Section 30.8. 3
  • Ground level ozone (O3) is formed from NOx and VOCs. It is a constituent of smog. Normally ozone occurs in stratosphere and prevents UV radiations from reaching earth՚s surface. At ground level, when inhaled, it is harmful for health of humans and animals.

Developed by: