Process of Precipitation/Rainfall
Rainfall is one form of precipitation that occurs when water droplets fuse together to form large droplets. How does rainfall occur is discussed below step-by-step:
- Saturation of air: The main reason for the rainfall to occur is the increase in relative humidity (amount of moisture in the air). Water once evaporated reaches up in the atmosphere in the form of vapour which starts collecting at one place to form a cloud. At a point when it reaches its saturation point which is 100% relative humidity condensation occurs which leads to precipitation.
- Coalescence: Rainfall will only occur when the drops are heavy enough to break the air resistance and fall on the surface. Small droplets in the air start fusing (Coalescence) the air and form bigger droplets.
Type of Rainfall
The cooling of air in the atmosphere usually occurs due to 4 different mechanisms. Based on these mechanisms rainfall is classified into 3 different types which are listed below:
Based on the type of origin the rainfall has been divided into 3 major categories which are listed below:
- Convectional rainfall
- Orographic or relief rainfall
- Cyclonic or frontal rainfall
Convectional Rainfall occurs when there is a rise in the temperature of any surface more than their surrounding which leads to evaporation of the surface. Convectional rainfall generally falls as a shower for a shorter period of time. This type of rainfall is usually dominant in the area of topics or equatorial regions particularly during the summers or hotter part of the day.
Usually, there is a rain shower when there is conventional rainfall however presence of Graupel and hail can also occur due to conventional rainfall. Convectional rainfall also occurs in the region where there is weather disturbance due to differences in air density usually due to cold fronts.
This type of rainfall occurs in the mountain region and is caused when large masses of air are forced upwards from an area of lower elevation to an area of higher elevation to the side of the elevated land formation (usual mountains). The lifting up of air through the side of the mountain results in adiabatic cooling of air which thus results in condensation and precipitation. Relief or orographic rainfall usually occurs in the mountainous regions of the world which are subjected to consistent winds.
In the mountain region where there are consistent winds, a moist climate prevails on the windward side of the mountain. This moisture is removed by orographic lift and because of it, a rain shadow is observed on the leeward side of the mountain.
Cyclonic or Frontal Rainfall:
This rainfall is the result when warmer and tropical air meets the cool air in the region of low-pressure areas. Cyclonic rainfall occurs when the air of two different masses meets and less dense warm air overrides the more condensed air as a result of which the warmer air is forced to rise up. Under the right conditions, there is a saturation point in the relative humidity which results in condensation and then precipitation.
Cyclonic rainfall is abundant in the areas of weather fronts where the air of different densities are separated. Weather disturbances occur due to these disturbances leading to two situations which are cold fronts and warm fronts. Cold fronts can lead to thunderstorms while warm fronts lead to precipitation and stratus clouds.
Difference Between Conventional, Relief and Frontal Rainfall
The three types of rainfall differ on various bases like region, temperature, and other weather conditions. Major differences between the three types of rainfall are listed below:
Occur when there is a rise in the temperature of the earth surface thus causing the vapour to rise and because of which a saturation point is reached in the atmosphere causing condensation and then precipitation
Occur in the mountainous regions where consistent winds where the air is forced up to reach from low elevation to high elevation on the side of the mountain thus creating a saturation point in the atmosphere causing condensation and then precipitation.
Occur in the areas of weather disturbances where air masses of different densities and less dense or warmer overrides the colder to high denser air. Under the right conditions, condensation occurs followed by precipitation.
Occur in the region of the tropics where the temperature is usually high.
Occur during the summer at the hotter part of the day.
Occur in the mountain region where there is a moist climate due to persistent wind at the side of the mountain.
Occur in the low-pressure areas.
Occur for a shorter period of time.
Occur for a consistent and notable period of time
Duration cannot be determined