What is rain?

What is rain?
Water droplets comes out of warn air. As the warm air rises towards the sky (where the temperature is lower) it cools. Water vapor (invisible water gas in the air) always exists in our air, the amount of water vapor depends and is measured in humidity. Warm air holds quite a bit of water, you might have experienced this humid air in a green house. When this humid warm air raises and cools of smal droplets are formed and when enough of these droplets collect together, we see them as clouds. When the clouds are big enough and have enough water droplets, the droplets merges and form even bigger drops. Picture a huge room full of tiny droplets milling around. If one droplet bumps into another droplet, the bigger droplet will "eat" the smaller droplet. This new bigger droplet will bump into other smaller droplets and become even bigger--this is called coalescence. Soon the droplet is so heavy that the cloud (or the room) can no longer hold it up and it starts falling. As it falls it eats up even more droplets. We can call the growing droplet a raindrop as soon as it reaches the size of 0.5mm in diameter or bigger. If it gets any larger than 4 millimeters, however, it will usually split into two separate drops.
The gravity pulls the water back toward the ground and you'll experince rain.

What causes rain?
Both clouds and rain occures when someting makes the air rise. Many things can make air rise. Mountains, low-pressure areas, cold fronts, and even the jet stream.

How big are raindrops?
Raindrops are much smaller than you probably think! They are actually much smaller than a centimeter. Raindrops range from tiny drops with a diameter only a quater of a milimeter to big, heavy raindrops with a diameter of 4-5 milimeters. The biggest raindrops ever observed (very, very rare) are almost 1 cm in diameter. Scientist have observerd this "monster raindrops" over Brazil.

How fast do raindrops fall?
Raindrops fall between 10 and 30 kilometers per hour (3 and 8 meters per second) in still air. The reason the speed varies is because of different size of raindrops. Air friction breaks up raindrops when they exceed 25 kilometers per hour.

What's the shape of a raindrop?
Almost everyone think the raindrop is tear-shaped. Actually, real raindrops bear scant resemblance to this popular fantasy.

Smaller raindrops (radius < 1 millimeter (mm)) are spherical; larger raindrops is shaped more like that of a hamburger bun. When the raindrops get larger than a radius of about 4 mm they rapidly become distorted and then they break up into smaller drops.

This remarkable evolution in forms is the results from two different forces: the surface tension of the water and the air friction pushing up against the bottom of the drop as it falls. When the drop is small, surface tension is the stronger force and pulls the drop into a spherical shape. With increasing size, the fall velocity increases and the air friction on the bottom of the drop increases causing the raindrop to flatten and even develop a depression. Finally, when the radius exceeds about 4 mm or so, the depression grows almost explosively to form a bag with an annular ring of water and then it breaks up into smaller drops.