What are Rainfall and precipitation? What are the types of rainfall and how does it occur? Read to know more.
It is a process of falling atmospheric moisture on the surface in any form due to gravity. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor so that the water condenses and precipitates.
There are five forms of precipitation:
- Rainfall: It is the fall of atmospheric moisture in the form of water due to gravity.
- Snow: Precipitation of white opaque crystals when cloud forms below zero degree Celcius.
- Hail: It falls in the form of small ice pellets and is a very destructive form of precipitation produced by thunderstorms or cumulonimbus clouds.
- Sleet: It is a mix of rain and snow or it is frozen rain that forms when rain passes through very cold air mass before reaching the land.
- Drizzle: Very small and uniform sized raindrops (less than 0.5 mm size)
It is the most common form of precipitation, especially in low latitudes. Monsoon or equatorial rains are good examples to understand.
Conditions for rain formation:
- Warm, moist, and unstable air
- Sufficient number of hygroscopic nuclei
The warm and moist air after being lifted upward becomes saturated and clouds are formed but the process of condensation begins only when the relative humidity of ascending air exceeds a hundred percent.
Rainfall does not occur unless these cloud droplets become so large due to coalescence that the air becomes unable to hold them.
Types of rainfall
Generally, three different types of rainfall are recognized:
- Convectional or convective rainfall occurs due to thermal convection currents caused by insolation heating of the ground surface.
- Orographic rainfall occurs due to the accent of air from highland.
- Cyclonic or frontal rainfall occurs due to upward movement of air caused by convergence of contrasting wind
Convectional rain occurs mainly in equatorial regions where daily heating of the ground surface causes convection currents. The sky becomes overcast by afternoon daily causing pitch darkness and heavy rains to follow. Thus the convectional rainfall in the equatorial region is a daily regular feature.
In this type of rainfall, there is a thermal convective rise of hot and humid air. For the hot and humid air to rise two conditions are necessary:
- Presence of moisture through evaporation to the air so that relative humidity becomes high.
- Intense heating of ground surface through incoming short wave solar radiation insolation heating.
The ground surface is intensely heated due to the enormous amount of heat received. The air in contact with a warm surface also gets heated expands and ultimately rises upward. The ascending warm and moist air cools becomes saturated, causes condensation and cloud formation, and rainfall starts.
India receives convectional rainfall during the summer season before the onset of Southwest monsoon these are known by local names in India like mango showers, Kal Baisakhi, bhardoli cheera, etc.
The rain which is caused by the physiographic barrier for falling in the path of moisture-laden air is known as orographic rainfall. The moisture in the air is forced to rise which gets cooled adiabatically leading to condensation, cloud formation, and rain. This occurs on the windward side of the hill or by any other physiographic barrier like plateau or mountain.
Necessary conditions for the occurrence of orographic rainfall are:
- They should be mountain barriers across the wind direction so that the moist air is forced on obstruction to move upwards
- There should be enough moisture content in the air and the presence of an onshore wind that is hot and humid
Features of orographic rainfall:
- More rainfall on the windward side and less on the leeward side
- Maximum rainfall near the mountain slopes decreases away from the foothills
- Windward slopes of mountains are at the time of rainfall are characterized by cumulus clouds and leeward side slopes have stratus clouds.
Amount of rainfall depends on:
- Amount of moisture present in onshore winds
- Distance of mountain barrier from the coast
- Altitude and slope of the mountain
Orographic rainfall is the most common in the world and India. Rainfall in India through Southwest monsoon and northeast monsoon is orographic.
Also read: Unseasonal Rains in India
Cyclonic or frontal rain:
This occurs due to the convergence of extensive air masses. When two contrasting air masses like cold polar air mass and warm westerly air mass coming from opposite directions converge along a line a front is formed. The warm wind is lifted upward along this front where the cold air being heavier settles down.
Such cyclonic fronts are created in temperate regions where cold polar winds and warm westerlies converge. The warm air lying over cold air is cooled and gets saturated and condensation begins around hygroscopic nuclei.
The lifting of warm air along the cyclonic front is not vertical like convective currents rather it is along an inclined plane.
The front is a zone of intensification of low pressure, cooling condensation, cloud formation, and rainfall- the entire process is known as frontogenesis.
Frontal rainfall is most common in mid-latitudes as it is a zone of convergence of warm westerlies and cold polar easterlies. Frontogenesis is also the basis of the formation of temperate cyclones. Temperate cyclones also produce frontal rainfall in India during the winter season in North-Western parts of India which are known as western disturbances.