Following are the factors mainly responsible for the formation of precipitation:
- A lifting mechanism to produce cooling of the air.
- A mechanism to produce condensation of water vapors and formation of cloud droplets.
- A mechanism to produce growth of cloud droplets to size capable of falling to the ground against the lifting force of air.
- When air ascends from near the surface to upper
- Levels in the atmosphere it cools.
- Cooling lowers the capacity of a given volume of air to hold a certain amount of water vapor.
- As a result super saturation occurs and the excess moisture over saturation condenses through the cooling process.
Mechanism of Cooling
This is the only mechanism capable of producing the degree and rate of cooling needed to account for heavy rainfall.
Condensation of Water Vapor
- Condensation of water into cloud droplets takes place on hygroscopic nuclei which are small particles having an affinity for water.
- The source of these condensation nuclei are the particles of sea salt or products of combustion of certain sulfurous and nitrous acid and carbon dioxide.
- There are always sufficient nuclei present in the atmosphere.
- Growth of droplets is required if the liquid water present in the cloud is to reach the ground. The two processes regarded as most effective for droplet growth are:
- Coalescence of droplets through collision due to difference in speed of motion between larger and smaller droplets.
- Co-existence of ice crystals and water droplets.
- Co-existence effect generally happens in the temperature range from 100 to 20 °F.
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