G4 – The Water Cycle 700 – 800L_02

Do you ever think about the water you drink, maybe where it came from or how old is it? Maybe you just opened a new bottle of water or you just filled your glass from the faucet. In both cases, you are drinking water that is older than you can possibly imagine. It’s true! Each drop of water in your glass or bottle could be billions of years old. How is that even possible? Simple! Water is recycled through a process called the water cycle.

The majority of Earth’s surface is covered by water. It is found from the equator to the poles. It can exist in different forms, from ice to a gas. You have learned that water can change its form, or state, based on its temperature. In colder regions, water can exist as ice in glaciers on land or as frozen sea water. Both the north and south poles of Earth are covered by ice. In warm regions, water can be found as a liquid in lakes, streams, and oceans. Even the air contains water in the form of a gas. Water in a gaseous state is called water vapor. You may be wondering, “What does this have to do with water being really old?”

The Sun powers the recycling process of the water cycle. Heat energy from the Sun causes liquid water to change into water vapor. This process is called evaporation. Most evaporation takes place in the oceans because they cover so much of Earth’s surface. During evaporation, water vapor rises into the atmosphere. Wind currents carry the water vapor to other locations. 

Liquid water in plants also ends up in the atmosphere as water vapor. Roots absorb water from the soil and deliver it to the rest of the plant. Some of the water is released as water vapor through leaves and rises into the atmosphere. This process is called transpiration. Once in the air as a gas, water needs a way to turn back into a liquid so it can be recycled. Can you think of a way this could happen?

Condensation is the opposite of evaporation. During the process of condensation, water vapor is turned back into liquid water.

As air rises higher into the atmosphere, it cools. Cool air can hold less water vapor than warm air, so some of the water vapor condenses into water droplets. Those water droplets may form the clouds you see in the sky. In the clouds, water droplets join with other droplets becoming bigger and bigger.