WATER IN THE EARTH SYSTEM

The Water Cycle.

The Water Cycle

You may not be able to see water all around you, but it's there. Water exists in the air in different forms and changes from one form to another. This continual process is known as the water cycle.

Water changes from a liquid to a gas form, called water vapor, through a process called evaporation. As liquid is heated by the sun's warmth, it changes into a gas form and rises in the atmosphere. In the air, water vapor cools and returns to a liquid form. This process is called condensation.

These water droplets cling together and form clouds. When the droplets become heavy enough, they fall to the ground as precipitation.

This storm cloud carries a lot of precipitation.

Rain, snow, sleet, hail

Precipitation can take on different forms, but scientists think it begins with frozen crystals in the clouds. As the crystals begin to fall and pass through warm air, they melt and become raindrops.

A rainy street.

Crystals that fall through very cold air reach the ground as snow. Sometimes the crystals begin to melt, and then refreeze. This is called sleet. Water freezes when temperatures reach 32° Fahrenheit, or 0° Celsius.

Sometimes, when strong gusts of air are present in the clouds, crystals are bounced up and down. They become coated with layer upon layer of ice until they are so heavy that they escape the gusts of air and fall to the ground as hail.

This usually happens in warmer weather, during thunderstorms. Hailstones range from less than a centimeter to several inches in size. Imagine hail the size of softballs falling from the sky!

Hailstones.

Various forms of water appear in other ways as well. When you wake up in the morning and discover drops of water covering plants, grass, and outdoor objects, this water is not the result of precipitation. These water droplets are called dew, and they are a form of condensation.

At night, when temperatures drop, objects begin to cool. In the morning, as the temperature of the air is rising, many surfaces remain cool. These cool surfaces, like plants and grass, cause the water vapor in the air that surrounds them to condense and turn into liquid droplets.

Dew drops on a vine.

Dew has gathered on these leaves.

In the same way, water condenses on the surface of windows in the morning. This is because their surfaces remain cool as the early morning temperature rises. If the temperature is below the freezing point, the droplets of liquid freeze on the cold surface and become frost.

Create Condensation

For this activity, you will need a jar with a lid and some ice cubes. Fill the jar with ice cubes and put the lid on tight. Observe the jar. After a few minutes, do you notice water droplets forming on the outside of the jar? If so, you have created condensation. The cold jar is cooling the air around it. When the air around the jar cools, water vapor in the air condenses and changes to a liquid state.

Condensation on a glass.

Early morning fog on a lake.

Fog

Have you ever walked outside on a foggy day and wondered if you were stepping into a cloud? Fog is similar to clouds because it is made of water vapor that has cooled, or condensed, to form tiny water droplets.

However, unlike clouds, fog forms from the ground up. Fog is formed when the air, which contains water vapor, is cooled by the ground or a body of water.

Sometimes you might see fog as a mist hanging over a lake. This happens because the sun is warming the air, while the lake is still cool. When the water vapor in the air is cooled by the lake's temperature, it condenses, and tiny water droplets cling together, creating fog.

Foggy mountain valleys.

Clouds

Clouds are formed in the air, because when water vapor rises from the Earth, it cools and condenses into tiny water droplets. Clouds can affect our weather. They can cool the temperature by shading us from the sun's light and heat. Precipitation occurs when clouds become saturated and the water droplets are heavy.

Common Types of Clouds

Some common types of clouds are stratus, cirrus, and cumulus.

Stratus clouds are low-hanging clouds that spread across the sky and cover it like a blanket. These clouds often signal gray days and possibly light rain.

Cumulus clouds are the puffy, white, cotton-candy clouds that you usually see on warm, sunny days.

Cirrus clouds are thin and wispy. They appear high in the sky and often occur during cold weather, but usually signal that warmer weather is on the way.