Educational Questions & Answers

Why does rain come in drops and not in a continuous stream?

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 a speeding boat.) This clearance is convenient for another drop to follow and not exactly in the same path but close to it, says Mr. C. Ranganathan of Tiruchy.

Drops that pick up speed are slowed down by the drag of the surrounding air. Indeed the smallest drop may not fall at all, being suspended or perhaps forced upward by ascending currents of air until they grow large enough to fall. As larger droplets descent, they produce an airstream around them.

The larger the cloud droplet the better the chance of its colliding with a giant droplet. So each drop falls at a different speed as their sizes are different. There are collisions between raindrops. Some collisions cause drops to coalesce, forming a large drop and some cause drops to break into smaller ones. As the number of drops grows the intensity of rain increases.

Collision does not guarantee coalescence. Experiments have indicated that the presence of atmospheric electricity may be the key to what keeps the drops together as they collide. That is when a droplet with a negative charge collides with another with a positive charge their electrical attraction may hold them together.

Rate at which drops fall is size dependent. Giant droplets fall rapidly. Thus drops keep on falling side by side and not in a continuous stream.

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Why does rain come in drops and not in a continuous stream?