Although moisture is always present in the atmosphere but it is condensed only when air is cooled and saturated with some water vapors. Usually mechanism by which air is cooled to cause precipitation is the lifting of air mass.
Based on lifting of air carrying vapors, precipitation is classified as follows:
- Cyclonic precipitation
- Convective precipitation
- Orographic precipitation
1. Cyclonic precipitation
Cyclone is a type of atmospheric disturbance by mass of air circulating clockwise in southern and anticlockwise in northern hemispheres.
What causes Cyclonic Type of Precipitation
- Cyclone is also a violently rotating wind storm.
- It is a large whirling mass all converging into a low pressure area, air will flow horizontally from surrounding area, causing air in the low pressure area to lift. The precipitation that results is called non-frontal cyclonic precipitation.
- If one air mass passes over another air mass, the precipitation is frontal cyclonic precipitation.
- The air rushes horizontally into the low pressure area changing into whirling mass because of rotary motion of the earth about its own axis. This cyclone is very large mass of air ranging from 800 to 1600km in diameter and moving with a velocity of 50 km/hr.
- The cyclonic precipitation occurs in the form of drizzle, intermediate rain or steady rain.
- Precipitation caused by cold front is intense and of short duration.
- Precipitation caused by warm front is more continuous.
2. Convective precipitation:
It is due to the upward movement of warm air. Generally this type of precipitation occurs in tropical region, where in hot days, the ground surface is heated unequally, and causing the warmer air to lift up as the colder air comes to take its place. The vertical air currents develop tremendous velocities and are dangerous for aircrafts.
- Precipitation occurs in the form of showers of high intensities and short duration،
- In summer most parts of Pakistan gets this type of precipitation.
3. Orographic precipitation:
This type of precipitation is caused by air masses striling some natural topographic barriers like mountains. As it cannot move forward it rises up causing condensation and precipitation.
- The greater amount of precipitation falls on the windward side.
- Orographic barriers tend to increase both cyclones and orographic precipitation because of the lifting of air.
- Rainfall is composed of steady rainfall.
- Southern slope of the Himalayas is a good example of this kind.
- Similarly, winds coming from ocean strike the western slopes of coastal ranges causing heavy rains.
- All three types of precipitations occur due to lifting of air.