Water vapour is a highly variable component of the atmosphere. Its proportion varies from zero to four percent by volume of the atmosphere. The heat energy radiated from the sun changes water into water vapour. Humidity refers to the amount of water vapour present in a given air. It indicates the degree of dampness or wetness of the air. Humidity of the air is expressed in two ways - absolute humidity and relative humidity.
Absolute humidity is the actual amount of water vapour present in grams per cubic metres of a given air. Relative humidity is the ratio of actual water vapour content to the maximum moisture holding capacity of an air at a given temperature and it is expressed in percentage. The temperature at which a given sample of air becomes fully saturated is called dew point or saturation point.
Precipitation is defined as water in liquid or solid forms falling to the earth. It happens when continuous condensation in the body of air helps the water droplets or ice crystals to grow in size and weight that the air cannot hold them and as a result these starts falling on the ground under the force of gravity.
The precipitation in the form of tiny droplets of water and bigger water droplets are known as drizzle and rainfall respectively. When the precipitation is in the form of big ice balls, it is called snow fall.
When a mass of moist air ascends to high altitudes it cools down to lower temperatures. In doing so it attains dew point which leads to condensation and precipitation. The cooling of air occurs mainly when it rises. There are three important ways in which a mass of air can be forced to rise and each of these ways produces its own characteristic precipitation or rainfall.
1. Convectional Rainfall
Excessive heating of the earth’s surface in tropical region results in the vertical air currents. These currents, lift the warm moist air to higher strata of atmosphere. When the temperature of such a humid air starts falling below dew point continuously, clouds are formed. These clouds cause heavy rainfall which is associated with lightning and thunder. This type of rainfall is called conventional rainfall. It is very common in equatorial region.
2. Orographic or Relief Rainfall
Orographic rainfall is formed where air rises and cools because of a topographic barrier. When their temperature fall below dew point, clouds are formed. These clouds cause widespread rain on the windward slopes of the mountain range. This type of rain is called orographic rainfall.
However, when these winds cross over the mountain range and descend along the leeward slopes, they get warm and cause little rain. Region lying on the leeward side of the mountain receiving little rain is called rainshadow area. A famous example of orographic rainfall is Cherrapunji on the southern margin of the Khasi Hills in Meghalaya India.
3. Convergence or Cyclonic Rainfall
Convergence rainfall is produced where air currents converge and rise. In tropical regions where opposing air currents have comparable temperatures, the lifting is more or less vertical and is usually accompanied by convention.
Mixing of air along the front also probably contributes to condensation and therefore to the frontal rainfall. When two large air masses of different densities and temperature meet, the warmer moist air mass is lifted above the colder one. When this happens, the rising warm air mass condenses to form clouds which cause extensive down pour. This rainfall is associated with thunder and lightning.
The spatial distribution of precipitation is not uniform all over the world. Different places of the earth’s surface receive different amount of annual precipitation and that too in different seasons.
Precipitation is greatest in the equatorial region and decreases towards the poles. Precipitation is heaviest in the coastal regions and decreases towards the interior of the continents.