Wikijunior:Big Book of Fun Science Experiments/Why does it rain?
Why Does it Rain?
Water is very important on Earth; we need it to drink and grow crops. Rain is one of the main ways people can gather the water they need to live. In order to figure out why rain happens, we need to learn a bit about water.
The Earth is a special member of the Solar System because it has liquid water on its surface. You'll never find an ocean on Mars or Saturn! Like most matter, water exists in one of three main forms: solid, liquid, or gas. Solid water is called ice, liquid water is simply called water, and water as a gas is called water vapor. Matter decides what form to be in based on its temperature—how hot it is. Very cold matter is a solid, medium temperature matter is a liquid, and hot matter is a gas. Different kinds of matter change forms at different temperatures. Water changes from a solid into a liquid at 32° Fahrenheit (0° Celsius), and into a gas at 212° Fahrenheit (100° Celsius).
Water on Earth is constantly moving. We call this journey "The Water Cycle".
Most of the water on Earth exists as a liquid. Our oceans, lakes, ponds, rivers, and streams are all made of liquid water. After being outside in the Sun, some of the liquid water starts to get hot. When this happens the liquid water turns into a gas and moves into the air; this is called "evaporation". The amount of water that is in the air is called "humidity". When a person says that it is very humid outside they are actually saying the air is very full of water!
As the water rises into the sky, it gets colder, and it eventually turns into a liquid cloud. When a gas turning into a liquid, it is called "condensation"; the water "condenses" when it turns into a cloud. The air can hold a lot of water. A lot of the water stays in clouds high up in the air. When the air begins to get too full, the clouds may turn back into liquid water and fall down to Earth. This is called rain, which is a form of "precipitation". The rain then flows back into oceans, lakes, ponds, rivers, streams, the ground, and more. Eventually, the water will evaporate again, and it will start its journey all over again.