Water
How Is Water Cycled through the Environment?
Waterglossary term (opens in a new window) is essential for the survival of all living things on the planet. While it is true that water covers a large percentage of Earth’s surface, much of this water is not available for plants and animals that live on land, because it is either salt water or frozen in glaciers.
The Water Cycle
The water cycle is made up of various processes that move water through the atmosphere and biosphere. These processes are evaporation, condensationglossary term (opens in a new window), precipitation, and transpiration. All of these processes involve changes in the state of water.
Evaporation occurs when liquidglossary term (opens in a new window) water changes into its gaseous form: water vapor. As water on Earth’s surface—in lakes, rivers, oceans, and soil—warms, it evaporates. Water also evaporates from the leaves of plants in the process of transpiration. The water vapor rises into the atmosphere. As it rises, it cools. When it cools enough to change from a gas to a liquid, in a process called condensation, it collects in droplets. Clouds and fog are made up of many of these water droplets.
As the water droplets become larger, they may fall to the ground as rain. If the air is very cold, water in the air may freeze and fall as snow, hail, or sleet. Rain, hail, snow, and sleet are all forms of precipitation. Precipitation occurs when liquid or solid water falls from the atmosphere to Earth’s surface. Some of the precipitation falls into bodies of water. Some falls on land, where it either flows into a body of water or soaks into the ground and is used by plants and stored as groundwater.
Driving the Water Cycle
Since water’s state changes are crucial to the water cycle, and state changes occur when temperature of the water changes, it is important to recognize that this cycle needs an energy source. The main energy source for the water cycle is the sun; it is the sun’s energy that causes liquid water to evaporate into water vapor, and solid water (ice) to melt into liquid water.
Gravity is another driver of the water cycle. Earth’s gravitational force pulls precipitation to the ground. Once on Earth’s surface, gravity’s effects depend on the surface on which the precipitation falls. Some precipitation falls directly into bodies of water, and some falls on land. If rain falls on soil or sand, the water will soak into the ground, where it can be absorbed by the roots of plants. If rain falls on rocky areas, gravity pulls much of the water along the ground, so that the water flows downhill, creating systems of rivers and streams that provide water to animals and plants that live along the shore. Eventually, most of the water in rivers and streams will flow all the way to the ocean.
The sun’s energy is also the cause of weather systems that bring precipitation to almost all of the parts of Earth—even those far from the ocean. Sunlight is absorbed by water, land, and the air around Earth. But not all of Earth receives the same amount of sunlight, causing Earth to heat unevenly. Since heat flows from areas of greater warmth to cooler areas, moving air—wind—moves Earth’s atmosphere in currents. Air masses, driven by convection, are a powerful force that moves water across the planet. As an air mass warms, it becomes less dense. The warmer air mass will be pushed upward by the cooler air below it. As the rising air cools, it begins to sink. The rising and falling air creates convection currents that continuously circulate air as wind.
Teacher Note: Connections
In this item, students will use their understanding of how energy drives the cycling of water within the water cycle to identify the energy source or energy type most directly responsible for different ways in which water moves. Students may not recognize that gravity “moves” water through the transformation of gravitational potential energy into kinetic energy. You might extend this activity by further exploring the energy transformations that occur within the water cycle.
