Water has three forms. It may be a liquid, a solid called ice, or a gas called water vapor or steam. Rain is the liquid form of water that falls from the sky in drops.

How Rain Forms

Rain is a part of Earth's endless water cycle. At the beginning of the cycle, water on Earth's surface heats up. The heat causes the water to evaporate, or to turn into water vapor. This water vapor rises into the air. As the water vapor cools, it turns back into water drops through a process called condensation. Many of these drops form clouds by collecting around dust and other particles. The water drops in clouds grow larger by joining with other drops. Eventually they become too heavy to stay in the cloud. They fall to Earth as rain. Then the water cycle begins again.

Sometimes the water drops in clouds freeze. As these ice crystals fall toward the ground, they melt. This is another way that rain forms.

Where Rain Falls

Rain falls almost everywhere on Earth. One of the world's rainiest places is Mount Waialeale in Hawaii. It rains about 350 days a year there. About 460 inches (1,170 centimeters) of rain fall there every year. One of the driest places on Earth is the Atacama Desert in Chile. It receives less than ¹/₂₅ inch (0.1 centimeter) of rain a year. It has not rained in some parts of this desert in hundreds of years.

Acid Rain

Rain washes dust and dirt from the air. But rain itself is not always pure. Sometimes polluting chemicals from cars, factories, and power plants become trapped in clouds. The rain from these clouds contains those harmful chemicals. This polluted rain, known as acid rain, can damage plants, animals, people, and property.