When the air at the given temperature is not capable of holding moisture any more, and the temperature at which saturation occurs in air is known as dew point. Increase in temperature increases water absorption and retention capacity of the given parcel of air. The transformation of water vapour into water is called condensation. After condensation the water vapour takes one of the following forms, i.e. dew, frost, fog or clouds.

Types of Rainfall

Depending upon the origin, rainfall can be divided into three main categories: convectional, orographic and cyclonic or frontal.

Types of rainfall

• Rainfall may be classified into the convectional, orographic or relief and the cyclonic or frontal rainfall
  • convectional rain: when, with thunder and lightning, heavy rainfall takes place but does not last long
  • common during the summer or in the hotter part of the day
  • common in the equatorial regions and interior parts of the continents, mainly in the Northern Hemisphere
• Orographic rain: when the saturated air mass comes across a mountain and is forced to ascend, the temperature falls and the moisture is condensed
  • windward slopes receive greater rainfall
  • It is also known as the relief rain

• Cyclonic rain:
  • cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about large scale destruction caused by violent winds, very heavy rainfall and storm surges
  • This is one of the most devastating natural calamities

Thunderstorms and Tornadoes

• Thunderstorms are caused by intense convection on moist hot days
• A thunderstorm is a well-grown cumulonimbus cloud producing thunder and lightning
• From severe thunderstorms sometimes spiralling wind descends like a trunk of an elephant with great force, with very low pressure at the centre, causing massive destruction on its way, and such a phenomenon is called a tornado
• The violent storms are the manifestation of the atmosphere’s adjustments to varying energy distribution
• The potential and heat energies are converted into kinetic energy in these storms and the restless atmosphere again returns to its stable state.

World Distribution of Rainfall

• throughout the year, different amounts of rainfall is recieved on different parts of the earth’s surface during different seasons 
• rainfall decreases as one moves from the equator to the poles
• coastal areas of the continents receive more rainfall than the interiors of the continents
• rainfall falls more heavily on the oceans than on the world’s land masses due to abundant water sources
• rainfall is heavier on the eastern coasts and continues to decrease towards the west between latitudes 35 degrees and 40 degrees N and S of the equator
• with mountains running parallel to the coast, rain falls more on the coastal plain, or on the windward side, and less on the leeward side
• rainfall is first received on the western margins of the continents as the westerlies are present between 45 degrees and 65 degrees N and S of the equator, and then decreases towards the east
• based on the total amount of annual precipitation, major precipitation regimes of the world are identified as:
  • heavy rainfall (over 200 cm per annum): the equatorial belt, the windward slopes of the mountains along the western coasts in the cool temperate zone
  • moderate rainfall (100 – 200 cm per annum): interior continental areas, coastal areas of the continents receive moderate amounts of rainfall
  • low rainfall (50 – 100 cm per annum): central parts of the tropical land and the eastern and interior parts of the temperate lands
  • very low rainfall (less than 50 cm per annum): areas lying in the rain shadow zone of the interior of the continents and high latitudes.
    

    Conclusion

    The air contains water vapours and it varies from zero to four per cent by volume of the atmosphere and plays an important role in weather phenomena.The amount of water vapours in the atmosphere is present or absent due to evaporation and condensation respectively. In some regions, rainfall is distributed even throughout the year, like in the case of the equatorial belt and in the western parts of cool temperate regions.