The water cycle (part 2)

 Cloud Form

What happens next once a water molecule has found its way into the atmosphere? Warm air carries the water molecule higher into the atmosphere. Higher up, the air tends to become much colder. Cold air holds less water vapor than warm air. Some of the water vapor cools and condenses into liquid water. Condensed droplets of water clump together around tiny dust particles in the air, forming clouds. In even colder parts of the upper atmosphere, the water vapor sometimes forms ice crystals rather than water droplets.

Waterfalls as precipitation

As more water vapor condenses, the water droplets in a cloud grow larger and larger. Eventually, the drops become so heavy that they fall back to Earth. Water that falls to Earth as rain, snow, hail, or steel is called precipitation. Most water molecules spend only about 10 days in the atmosphere before falling back to Earth. Most precipitation falls directly into the oceans. Water in the sea may stay there for many years before evaporating, thus continuing the cycle.

When precipitation falls on land, some of the water evaporates again immediately. Some runs of the land into rivers and lakes. From there, it may eventually evaporate or flow back into the ocean. Some water trickles down into the ground and forms groundwater. Groundwater may move underground until it reaches a river, lake, or ocean. Once groundwater reaches the surface, it can continue through the cycle by evaporating again.

Before Returning to the atmosphere, some water passes through living things. Animals drink the water and eventually release it back into the environment as a waste product. Plants use the water to grow and produce food. When these living things die, their bodies are broken down slowly, and the water returns to the environment.

A global process

Precipitation is the source of all fresh water on and below the Earth's surface. The water cycle renews the usable supply of fresh water on Earth. For millions of years, the total amount of water on Earth has remained fairly constant. The worldwide amounts of evaporation and precipitation balance each other. This may not seem believable if you live in an area where there is either a lot of precipitation or very little. Parts of India can receive as much as 1000 centimeters of precipitation in a year, while the Sahara, a desert in Africa, may get only 5 centimeters. But in the world as a whole, the rates of evaporation and precipitation are balanced.