Question

Earth Science: The Water Cycle.

Answer 1

The water cycle is nature's way of recycling water.

The water cycle is at work continuously on earth's surface. This is how water moves through living and nonliving parts of the environment. Water moves from bodies of water, land, and living things on earth's surface to the atmosphere, and then back to the surface of earth.It is important to remember that the water cycle is just that, a cycle. It does not continue in the same path every time. Let's learn about what paths water takes.Without the sun's energy, water would not move from the earth's surface to the atmosphere and back again. Let's start with the sun heating up earth's surface. As the sun's radiation heats the surface, the water molecules in surface waters—oceans, lakes, ponds, or rivers—absorb the radiation. Surface water molecules will continue to do so until they absorb enough energy to change from liquid to gas. This is the process of evaporation . Evaporation moves water from earth's surface to the atmosphere. Water passes from the liquid state to the gaseous state during evaporation. Water enters the atmosphere in other ways, too.

Water evaporates from large bodies of water—such as oceans or large lakes—from small puddles, from the soil, and even from your skin as you sweat. But, evaporation is not the only way water enters the atmosphere in the water cycle. Water also enters the atmosphere through the process of transpiration . It is one more way that water is added to the atmosphere. The leaves of a plant have a large surface area that is exposed to the air. Plants lose water by way of evaporation through the surfaces of their leaves. That's transpiration.The water in the atmosphere now needs a way to return to the surface.

After changing to a gaseous state and evaporating into the atmosphere, a water molecule is carried on warm air. As the water molecule rises, the temperature drops. This may all sound familiar to you by now. Cooler air cannot hold as much water vapor as warmer air. Therefore, some of the water vapor cools and changes state again. This is the process of condensation . These water droplets, now in the liquid state, may clump together around a dust particle or some other condensation nuclei . This leads to the formation of clouds. What goes up must come down.

So how does the water get back to earth? You have just seen how water vapor condenses in the atmosphere and forms clouds. For the water cycle to continue, water tied up in the clouds must return to the surface. As more and more water vapor condenses around the condensation nuclei that initially formed a cloud , the cloud becomes so heavy that some of the water begins to fall back to earth. It falls in the form of precipitation.You have learned about different types of clouds as well as the forms of precipitation that fall to earth. Precipitation is part of the ongoing water cycle.Most precipitation falls on the oceans.

This isn't really that surprising when you realize that the oceans are the largest reservoir of water on earth and cover so much of earth's surface. Nearly 97 percent of the water on earth is found in the oceans. Evaporation and precipitation are very active processes over and near the worlds' oceans.

A discussion of the water cycle cannot be complete without a brief discussion about freezing. Ice sheets, glaciers, and icebergs are the largest reservoir of freshwater. Nearly 2 percent of all the water in earth's water cycle is stored in ice. Freezing is the process of changing liquid water to ice. This occurs near the poles and at higher elevations.Precipitation falls on the surface.

Several things can happen to precipitation that falls on land. It all depends on where the precipitation falls and onto what type of surface.

Some water that falls onto the surface evaporates right away. Some moves along the surface as runoff. It can move as runoff in rivers, streams, or small gullies that form during rainstorms. In this form, the water may again evaporate back into the atmosphere, or it could eventually make it to the ocean. Some precipitation that falls to the ground sinks into the soil and becomes groundwater. Groundwater moves underground and also eventually reaches a larger body of water, such as the ocean.The infiltration of water will purify it.

Precipitation seeps into the ground and becomes groundwater through a process called infiltration . Water that circulates on land as groundwater tends to be filtered and cleaned. Water that falls to the surface and becomes part of the surface runoff system will not likely be purified. Surface runoff moves fast, while water that passes underground moves slowly as it makes its way through a filter of rock and soil.

Water that falls to the surface and enters into the evaporation or transpiration cycles also tends to be purified. Evaporation changes the water into a different state, leaving behind the other substances that might be in the water. Plants will also release only water to the atmosphere, leaving behind any other particles. Water moves from the liquid to the gaseous to the liquid (and sometimes solid) states in the water cycle.

As you have seen, water moves from the atmosphere to earth's surface, and back to the atmosphere in the water cycle. Water can pass from the vapor state to a liquid state—and sometimes to a solid state—throughout the cycle. The water can also reside in lakes or oceans, in the atmosphere, in clouds, in living organisms, or even underground.