Precipitation (Rainfall) : Forms, Types and Distribution

What is Precipitation :

Rainfall is the product of condensation of atmospheric water vapour that falls under gravity and reaches the surface of the earth.

Hydrological Cycle

Continuous movement of water among the three spheres is known as Hydrological Cycle.

Hydrological cycle involves evaporation, condensation, precipitation, advection, interception, evapotranspiration, infiltration, percolation and runoff to the ocean.

Evaporation is the process by which water in liquid state changes into vapour state using heat energy from Sun. Evaporation is maximum when the temperature is high, on the large expanse of water and when dry winds blow over water surface.

Condensation is the process by which water vapour cools to form water droplet by loosing temperature. The condensation occurs when dew point is reached in the atmosphere.

Precipitation is the process by which all forms of water particles fall from the atmosphere and reach the ground.

Various Forms of Precipitation :

Rainfall:

• When water droplets of more than 0.5 mm diameter falls from the atmosphere to the ground it is called as ‘Rainfall’.
• If the diameter is less than 0.5mm, it is called as ‘Drizzle’.

Hail:

When precipitation occurs at sub zero temperature, the water droplets crystallise and fall as ice pellets with the size of 5 to 50 mm or some times more. This is called as ‘Hail’.

Sleet :

Precipitation occurs as falling of raindrop along with ice pellets less than 5 mm diameter or snow, called as ‘Sleet’.

Snow:

Precipitation occurs at below freezing point and falls as thin ice flakes or powdery ice, called as ‘Snow’.

Dew:

Condensation of water droplets on the objects at the surface of the earth such as leaves and grasses are called as ‘Dew’.

Mist: Foggy weather.

Types of Precipitation:

Precipitation can be classified based on the causes for the rising up of air,

1. Convectional rainfall

2. Orographic or Relief rainfall

3. Cyclonic or Frontal rainfall

1. Convectional Rainfall:

• As a result of heating of the surface air, the warm moist air expands and is forced to rise to a great height.
• As the air rises, it cools, reaches dew point and condenses to form clouds. This process influences the upper tropospheric circulation. By further cooling, precipitation takes place as rainfall. This rainfall occurs throughout the year near the equator in the afternoon. It is called as 4 ‘O’ clock rainfall region.
• In middle latitudes, convectional rainfall occurs in early summer in the continental interiors.

2. Orographic or Relief Rainfall:

• It occurs when large mass of air is forced to rise across land barriers, such as high mountain ranges, plateaus, escarpments, or over high hills.
• On the windward side of the region the warm moist air raises, temperature of the air falls below its dew point, forming clouds which give subsequent rainfall.
• As the wind moves to the leeward side it has emptied itself of moisture and thus descends the slope as warm dry winds.  The leeward side of the mountain therefore is called as the rain shadow region.

3. Cyclonic or Frontal Rainfall:

This type of precipitation is associated with a cyclonic activity (Tropical and Temperate) and also occurs along the frontal zone.

Cyclonic rainfall is associated with Cumulo Nimbus (CuNi) clouds. The rainfall is very heavy and accompanied with lightning and thunder and high speed winds which has the potential to cause damage.

‘Frontal rainfall’ is associated with fronts which form due to collision of different air masses. Warm front is formed due to advent of warm air masses which leads to moderate rainfall. In the same way cold front is formed due to advent of cold air mass which leads to heavy rainfall with lightning and thunder.

Also Read : Desert Landforms and Erosion Mechanism

Why causes Lightning and Thunder ?

Lightning and Thunder are caused by differences in the electrical charge of different parts of the cloud. The top of the cloud becomes positively charged and the bottom is mostly negatively charged. When the difference is great lightning occurs. Differences in the charge between cloud and the earth surface also cause lightning. 

Thunder is caused by rapid expansion of the air that is heated as the lightning passes through it.

  

Amount of Precipitation (World Distribution):

• Rainfall from equator to polar decreases steadly.
• Precipitation at equator is more than 200 cm.
• Rainfall at Tropical region is 100-200 cm.
• Precipitation at Sub-tropical and Temperate is 50-100 cm.
• In arid and semi-arid region, precipitation is less than 50 cm.
• High rainfall at coastal area in the world.
• Between 35 degree to 40 degree North and South of Equator – High rainfall at East coast.
• Between 45 degree to 65 degree North and South of Equator – High rainfall at West Coast due to the westerlies.

 

Benefits:

• Agriculture, E.g. More than 60% of Indian agriculture depends on the rainfall.
• It reduces air pollution.
• It is best source of drinking water.
• Maintain atmospheric temperature.
• Provide greener environment.
• Tourist attraction places. E.g. Mawsynram (Meghalaya) and Western Ghat of India.

 

Challenges:

• Flood – Loss of life and property.
• Drought – It is one of major problem of the world.
• Cloud burst – devastating effects. E.g. In Uttarakhand .
• Acid Rain.

Acid Rain:

Acid rain is a rain that is unusually acidic, it has elevated levels of hydrogen ions. It is caused by a chemical reaction of compounds like sulphur dioxide and nitrogen oxides that are released into the air from anthropogenic activities and from volcanic eruptions. These substances can rise very high into the atmosphere, where they mix as hygroscopic nuclei and react with water vapour, oxygen, and other gases to form more acidic water which falls to the ground as acid rain. It is harmful for plants, animals, human and environment. Normally the rainfall is slightly acidic because of the presence of dissolved carbonic acid. The pH of normal rain has been given a value of 5.6. 

Acid rain has the pH value of less than 5.6. A great way to reduce acid rain is by using renewable energy resources, such as solar and wind power and reducing the use of fossil fuels.

Also Read : Ocean Tides and Tidal Bulge

References

NCERT

E-book