Earth Sciences: Water Cycle

Earth Sciences: Water Cycle

This lesson discusses the processes water takes as it moves around the Earth in the water cycle. You'll get an in-depth look at condensation, precipitation, and evaporation.

Where is Water?

The water cycle, also known as the hydrologic cycle, is the process by which water moves from place to place above, on, and below the Earth's surface. This is the process by which water moves around the Earth to different places. The total amount of water on the Earth is relatively unchanging, and it has remained about the same since our planet's formation. As the planet cooled, water vapor present at its formation condensed to fill the oceans and other places, like inland lakes and rivers.

Where is Water?

The Earth's surface is 75% water and 25% land. Of the water, 97% is salt water, a surprisingly high percentage, leaving only 3% as fresh water. Most of that - two-thirds of the fresh water on Earth - is frozen in snow and ice in glaciers, ice caps, and the like. That leaves only about 1% of all the Earth's water as liquid and fresh, making it a very scarce natural resource. If that wasn't enough, most of the liquid fresh water is stored underground in aquifers. Groundwater is just what the word sounds like: water stored under the surface of the Earth. But, like rocks or other types of matter, water on the Earth is not static. It is constantly moving around the hydrosphere, the layer of the Earth where water is present. It moves around by changing in three different processes that make up the water cycle.

Multiple Choice

How much 'new' water is made through the processes of the water cycle?

• None: all water is recycled

• About 25%

• A small amount, about 3-4%

• About half, 50%

Evaporation

Evaporation is the process by which water is converted from its liquid state to the gaseous state, also known as water vapor. In other words, water leaves the Earth's surface and enters the atmosphere as a gas.

In fact, the United States Geological Survey (the USGS) says that up to 90% of the water vapor in the air comes from surface water, including oceans, lakes and rivers, with the rest coming from plants. When this happens, anything that is in the water, dissolved or undissolved, remains behind. This includes salt, rocks, minerals, and other materials that often end up in the surface water.

Evaporation is a purifying process. One of the ways you can purify sea water is to heat it so it evaporates and then collect the steam. The steam is pure water, and getting it back to a liquid requires our next phase of the water cycle.

Multiple Choice

Steam is an example of which part of the water cycle?

• Condensation

• Precipitation

• Ground absorption

• Evaporation

Condensation

Condensation is the process by which water vapor is changed back into liquid water. Condensation is very important to our weather and climate because it is what is responsible for cloud formation.

Without clouds, we would not get to the third phase, called precipitation, which we will talk about in a minute. Clouds form when water vapor condenses around small particles, like bits of dust or smoke in the air. Depending on the size of the drops, these particles may or may not be visible. Even on a clear, cloudless day, water vapor is always present in the atmosphere, but it does vary in amounts. We know it is present on a very humid day; it often feels like we need to swim through the air! Fog is condensation near the ground.

Fog forms when moist warmer air comes in contact with cooler air near the surface. Just like when the bathroom mirror gets all foggy during a shower because of condensation, fog also forms because of this warm air contacting a cooler air mass. The fog forms drops in the air rather than on the surface of your mirror.

Multiple Choice

Cloud formation is an example of condensation because _____.

• water vapor condenses around particles in the atmosphere

• liquid water is converted into its gaseous state

• water is absorbed into the ground

• liquid water falls as rain, sleet, snow, or ice

Precipitation

Precipitation, the next phase of the water cycle, is water that falls from the atmosphere in the form of rain, sleet, snow, hail, or freezing rain. Clouds are required for precipitation because the raindrops are the drops of the clouds that have condensed enough water to begin falling. The cloud particles do not have enough mass to fall, but as condensation continues to add water to those particles, gravity eventually pulls them towards the Earth as precipitation.

According to the USGS, millions of cloud particles must coalesce in order to form one raindrop. Temperature determines the type of precipitation that falls from the clouds. Cooler weather means more ice and snow and also more hazardous driving. Yikes!

Other parts of the Water Cycle

Evaporation, condensation, and precipitation are the three main parts of the water cycle, but there are some other stages that water can cycle through. These different things occur mainly after water has fallen on the Earth. We know that water can become runoff when it flows along the surface into lakes, rivers, and streams.

It could absorb into the ground and become groundwater or get absorbed into plants. Groundwater is any water under the ground, usually collecting in the aquifers that contain most of the liquid fresh water we mentioned earlier.

Plants use water to help make energy, and they also lose some water to the surrounding air through the process called transpiration. Transpiration is a plant losing water through its leaves. It is just like we humans lose water to the air through our breath. Eventually, most surface water will either be absorbed as groundwater, evaporate from the surface or make its way to the ocean to eventually evaporate.

Multiple Choice

Transpiration is the process that occurs when _____.

• humans absorb water by breathing

• water vapor is converted to a liquid state

• plants lose water through their leaves

• water is absorbed into the ground

Multiple Choice

What are the three main parts of the water cycle?

• Absorption, infiltration and transpiration

• Evaporation, condensation and cloud formation

• Precipitation, infiltration and condensation

• Evaporation, condensation and precipitation