The Water Cycle
Image credit: Science Sparks
Water is essential to life. Every living creature depends on it for survival. Water is always moving above, on, and below the surface of the Earth. This natural movement is called the water cycle or the hydrologic cycle. During this cycle, water not only moves, but it also changes its state, or form.
Water can be in liquid form, like the water you’d swim in or drink.
It can also be in vapor form, which is when it becomes an invisible gas that floats in the air. If it’s a hot, sticky, humid day outside, you can be sure it’s because there is a lot of water vapor in the air.
When it’s really cold outside (below 32℉ or 0℃), water will freeze and become a solid form called ice.
These three different states are called phases, and during the water cycle, water will change its phase many times. It has been doing this for a very long time.
Water can be in liquid, vapor, or solid form. These are called phases. (Image Credits from left to right: mrjn Photography / Taylor Van Riper / Francis Nie)
If you can, grab a glass of water right now and study it for a moment. How old do you think the water is? The water you drink, bathe in, and swim in has been moving all across the planet and changing its phase for billions of years. The water in your glass could have been deep ocean water during the time of the dinosaurs, and then it could have been part of a frozen glacier during the Ice Age. Water is constantly “recycled” as it makes its way around the planet. Let's explore some parts of the water cycle below.
The water cycle on Earth has many parts that all work together. (Image credit: USGS)
The Sun and Water
The Sun
The water cycle wouldn’t work without the sun’s heat providing energy. The sun can be considered the most important part of the water cycle because its heat allows water to change phases.
The Different Water Sources
As we know, water can change phases from liquid to solid to vapor at any time. But not all water comes from one source, such as rain, nor does it stay as rain forever. The Earth’s water supply can be grouped into two main categories: freshwater and seawater.
Freshwater
Freshwater only makes up a very small percent of the water on our planet: about 2.5%. Freshwater sources are from lakes, ponds, swamps, rivers, streams, groundwater (water under the earth’s surface), glaciers, ice caps, ground ice, and permafrost. Some freshwater is also contained in the soil, atmosphere, and inside living things such as plants and animals.
Seawater
Most of the water supply on Earth (about 97.4%) is in the oceans and other saline (salt) waters. Humans can’t drink seawater, so they must rely on the small amount of freshwater that is available, mostly from rivers and lakes.
Most of the water on Earth is in the ocean. The rest is a small percentage of freshwater. (Image credit: USGS)
The water on Earth moves around through stages that are the power behind the water cycle. Ocean water may turn into vapor, rising high in the sky, only to be rained down on land, eventually reaching a lake. What are these stages?
Stages of the Water Cycle
Evaporation and Transpiration
As mentioned above, the sun is probably the most important part of the water cycle. Energy from the sun heats the Earth’s surface causing the water temperature to rise. Some of the water leaves its liquid form in lakes, oceans, and other water sources, and turns into a vapor/gas form in the air, or atmosphere. This process is called evaporation.
The water in plants and trees can also be lost to the atmosphere through their leaves. This process is called transpiration. It is partly because of evaporation and transpiration that you have to make sure your houseplants or garden vegetables are watered whenever they are dry.
To learn more about evaporation, click here!
Condensation
Condensation is sort of the opposite of evaporation. When liquid water turns into vapor, it usually rises high into the sky. Because of the cold air up there, it eventually condenses, or turns back into liquid in the form of clouds. So clouds are just floating water! Air currents high up in the atmosphere move clouds across the globe.
Condensation can also be seen when you pour yourself an ice cold glass of water on a hot summer day. After a few minutes of being in the hot air, the outside of the glass will start to collect water droplets. This water did not come from your glass, but from water vapor that was floating around in the air. It turned back into a liquid form due to the glass being colder than the surrounding air.
To learn more about condensation, click here!
Precipitation
We all know that when white clouds start to turn gray, it's usually a sign that rain is coming. Rain happens when water droplets in the clouds become too heavy for the air below to hold them afloat. They then fall back down to Earth in the form of rain, snow or hail. This process is called precipitation.
Fun fact: raindrops are not actually shaped like teardrops. They’re shaped more like hamburger buns! It is only when a raindrop has hit something like a windowpane, then it starts to look more like a tear shape.
To learn more about precipitation, click here!
Collection
After it rains, snows, or hails, the fallen precipitation is often collected into bodies of water such as oceans, lakes and rivers. Water can be collected on the land as well. Here are some ways in which collection happens:
Water can flow across the ground until it reaches a body of water (surface-runoff).
Water will fall on vegetation (plants and trees) and evaporate back into the atmosphere.
Water can trickle down into the soil only to be taken up by plant roots.
If it’s really cold outside, precipitation will fall on land and build up as snow, ice or glaciers. When it gets warmer, the snow and ice will melt and flow to a body of water or sink into the ground.
Usually, no matter where the precipitation fell, it will eventually make its way back to a body of water, and the cycle will start all over again.
Image credit: Anastasia Taioglou / Unsplash
There you have it! The basics of the water cycle! It’s our planet’s way of recycling water, and it has been happening for billions of years.
Want More Water Cycle Science?
If you’re looking to gain a deeper understanding of the water cycle, click below to visit the United States Geological Survey’s Water Science School website and view an interactive diagram that will tell you more about the many stages and parts of the water cycle!
Click below for:
In addition to the diagrams above, you can also watch some videos about the water cycle by visiting the links below.
“The Water Cycle” from The National Science Foundation (NSF)
“Earth’s Water Cycle” from NASA Goddard
By Megan Goldsmith
