Written and compiled by Fred Jaicks
Water is a basic necessity for life on Earth. It is an odorless and tasteless molecule that exists in several forms. These forms include liquid, solid, and gas. Liquid water is the water that we drink and the water found in the oceans. Solid water is the water that makes up ice, hail, and snow. The gas phase of water is known as water vapor, and it is invisible. The term humidity refers to how much water vapor is found in the air.
Water covers nearly 75% of the surface of the Earth, and the majority is found in the oceans. The vast size and number of bodies of water covering the Earth make it appear that water is abundantly available. In reality, the Earth has a limited supply of water, and the water that we do have is recycled over and over via the water cycle. The water cycle, also known as the hydrologic cycle, is an endless water exchange between the atmosphere, groundwater, surface water, soil, and plants. The water cycle is made up of four stages: evaporation, condensation, precipitation, and collection.
Evaporation is the transformation of water from a liquid phase to a gas or vapor phase. In the water cycle, evaporation happens when water moves from bodies of water or from the ground into the overlying atmosphere. For evaporation to occur, heat is needed. This heat energy breaks the bonds holding the water molecules together. Solar radiation is the primary energy source for evaporation.
Evaporation includes the transpiration of water from plants and trees. Transpiration releases water vapor from the soil and the plants and trees into the air. Transpiration from plants occurs when moisture is carried from the roots up to the leaves. In small pores on the leaves, moisture is changed to vapor and gets released into the atmosphere. The rate of transpiration is affected by the type of plant, the availability of moisture in the soil, temperature, humidity, and air and wind movement.
Condensation is what happens when water vapor in the air becomes cold due to a decrease in temperature and is transformed into liquid water droplets. This process is the opposite of evaporation.
Water droplets formed from condensation will stay in the atmosphere if they are very small. Millions of water droplets suspended in the atmosphere come together to form clouds and ground-level fog. The clouds eventually become heavy and produce precipitation, a means for the water to return to the ground.
The majority of the fresh water on the planet gets deposited via precipitation. Precipitation is condensed water vapor, those millions of water droplets up in the atmosphere, that falls back to the Earth from the clouds. The majority of precipitation occurs in the form of rain. Other forms include snow, hail, sleet, and fog. Snow is formed when cold air freezes the water droplets into ice crystals. As water vapor increasingly collects on the crystal, it becomes heavy and eventually falls from the sky. As the ice crystal falls, it may come into contact with warmer air that causes it to melt a bit. This melting causes many ice crystals to bond together into larger flakes. This creates a fluffy snowflake.
Collection is the process by which water that has fallen in the form of precipitation is stored in the ground. This storage occurs in bodies of water (oceans, rivers, lakes), aquifers under the ground, and in snow and ice caps. The term infiltration describes the process of water on the ground soaking into the soil. This water moves underground and goes between rocks and the soil. Some of the water that moves underground will be soaked up by the roots of plants and trees, while some of the water will continue moving downward until it reaches a level that is saturated with water. This is called an aquifer. The top of this layer is called the water table. If an aquifer gets filled with water to the point that it overflows onto the surface of the land, a spring is formed.
Water that does not soak into the Earth’s surface will travel over the landscape to the nearest creek, stream, or river. This water is called surface runoff. Surface runoff occurs when the ground is saturated with water and will not absorb more. This excess water will flow into streams, then into larger rivers, and eventually into the oceans.
Round and Round It Goes! The Water Cycle
Discovery Science: Water Cycle
Monterey Institute: Water Cycle
Science Up Close: The Water Cycle
Iowa Public Television: Cycle with Water
The Water Cycle: Water on the Move
U.S. Geological Survey: The Water Cycle
The Evergreen Project: The Water Cycle
Earth Observatory: The Water Cycle
Looking at the Sea: The Water Cycle
Umbrella Stands for Market Umbrellas
Northwest River Forecast Center: Hydrologic Cycle
Conservation and the Water Cycle
Water Resources: The Global Water Cycle