The water cycle ( part 1 )

 The Water Cycle

The next time it rains, cup your hand and catch some raindrops. Think about where a single water molecule in one of those raindrops may have traveled. Most recently, it was part of the gray cloud overhead. Last year, it may have tumbled over a waterfall or floated down the Nile River. Perhaps it spent years as part of the Pacific Ocean. The same water molecule may even have fallen as rain on dinosaurs millions of years ago.

How could one water molecule reappear in so many different places and forms? In fact, all the water on Earth has been through similar changes. Water is naturally recycled through the water cycle. The water cycle is the continuous process by which water moves through the living and nonliving parts of the environment. In the water cycle, water moves from bodies of water, land, and living things on Earth's surface to the atmosphere and back to Earth's surface. The sun is the source of energy that drives the water cycle.

Water Evaporates

Water moves continuously through the water cycle. The cycle has no real beginning or end. You can follow a water molecule through one complete cycle in Exploring the water cycle on the facing page.

Think about a molecule of water floating near the surface of an ocean. The sun is shining and the air is warm. Soon, the molecule has absorbed enough heat energy to change state. It evaporates and becomes water vapor. Although the water comes from the salty ocean, it becomes fresh through the process of evaporation. The salt remains in the ocean.

Large amounts of water evaporate constantly from the surfaces of oceans and large lakes. In addition, small amounts evaporate from the soil, puddles, and even from your skin.

A significant amount of water is given off by plants. Plants draw in water from the soil through their roots. Eventually, the water is given off through the leaves as water vapor in a process called transpiration. You may be surprised to learn how much water plants release to the atmosphere through transpiration.

The thousands of leaves on a signal birch tree, for example, may give off 260 liters of water in one day - enough to fill nine kitchen sinks!

Have you ever seen your breath on a cold day? If so, you have observed another way that water vapor enters the atmosphere. Small amounts of water vapor are released by animals when they exhale. Tiny amounts of water vapor also enter the air from ice, when water passes directly from the solid state to the gaseous state.