When warm wet air rises, it cools and water vapour condenses to form clouds. A cloud is made of small drops of water or ice crystals, depending on its height and how cold its surrounding air is. Most rain originates in nimbus or in towering cumulonimbus clouds.
To form rain, water vapour needs what's called a condensation nucleus, which can be tiny particles of dust, or pollen, swept up high into the atmosphere. When the condensing droplets that form the cloud get large and heavy enough to overcome the upward pressure of convection, they begin to fall.
Although all clouds contain water, some produce precipitation and others drift away placidly without giving rain. First all the droplets in a cloud are less than 20 micrometer in diameter. In a cloud there are lot hygroscopic particles and normally drops form by absorbing moisture by these particles.
Rain is restricted to drops of water that fall from a cloud. They have a typically diameter of at least 0.5 mm. A raindrop large enough to reach the ground without evaporating contains roughly a million times the water of a cloud droplet (typical diameter is 0.012 mm). No matter what the intensity of rain is the size of the drop rarely exceeds about 5 mm. Larger drops do not survive as the process of surface tension which holds the drop together is exceeded by the frictional drag of air and therefore larger drops break apart into smaller ones.
Raindrops as they descend, initiate a chain reaction, a downward trend of the water droplets, with the larger drops always breaking — a common feature observed when one forcefully disgorge the contents of a glass of water.
Most rainfall begins as snow crystals or other solid forms. Entering the warmer air below the cloud, these ice particles often melt and reach the ground as raindrops.
A raindrop starts falling and then picks up speed due to gravity. When one drop starts falling a wake follows in the cloud. (Wake is a clearance that is normally found behind
Answered by
ramsheela
, an ibibo Advisor,
at
12:09 AM on August 19, 2007
