4 Water cycle
The Water Cycle
Earth’s water is naturally recycled through the water cycle. The water cycle is the continuous process by which water moves from Earth’s surface to the atmosphere and back. In the water cycle, water moves from bodies of water, land, and living things on Earth’s surface to the atmosphere and back to Earth’s surface. As shown in Figure 9, the water cycle has three major steps—evaporation, condensation, and precipitation. The cycle itself has no real beginning or end. But it is driven by an energy source—the sun.
Water Evaporates
As you learned earlier, evaporation is the process by which molecules at the surface of a liquid absorb enough energy to change to a gaseous state. Water is constantly evaporating from the surfaces of oceans and large lakes. Smaller amounts evaporate from the soil, puddles, and even from your skin. Plants play a role, too, in this step of the water cycle. Plants draw in water from the soil through their roots. Eventually the water is given off through the leaves as water vapor in a process called transpiration.
Condensation Forms Clouds
What happens after a water molecule evaporates? Warm air carries the water molecule upward. At higher altitudes, air tends to become much colder. Cold air cannot hold as much water vapor as warm air can. As a result, some of the water vapor cools and condenses into liquid water. Condensed droplets of water clump together around tiny dust particles in the air, forming clouds.
Water Falls As Precipitation
As more water vapor condenses, the water droplets in a cloud grow larger and larger. Eventually, they become so heavy that they fall back to Earth. Water that falls to Earth as rain, snow, hail, or sleet is called precipitation.
Most precipitation falls directly into the ocean. The precipitation that falls on land may evaporate immediately or run off the surface into rivers and lakes. From there, it may evaporate or flow back into the ocean. In addition, some water may trickle down into the ground. After a long time, this groundwater may reach a river, lake, or ocean and continue the cycle by evaporating again.
Precipitation is the source of all fresh water on and below Earth’s surface. The water cycle renews the usable supply of fresh water on Earth. For millions of years, the total amount of water on Earth has remained fairly constant—rates of evaporation and precipitation are balanced.
Earth’s water is naturally recycled through the water cycle. The water cycle is the continuous process by which water moves from Earth’s surface to the atmosphere and back. In the water cycle, water moves from bodies of water, land, and living things on Earth’s surface to the atmosphere and back to Earth’s surface. As shown in Figure 9, the water cycle has three major steps—evaporation, condensation, and precipitation. The cycle itself has no real beginning or end. But it is driven by an energy source—the sun.
Water Evaporates
As you learned earlier, evaporation is the process by which molecules at the surface of a liquid absorb enough energy to change to a gaseous state. Water is constantly evaporating from the surfaces of oceans and large lakes. Smaller amounts evaporate from the soil, puddles, and even from your skin. Plants play a role, too, in this step of the water cycle. Plants draw in water from the soil through their roots. Eventually the water is given off through the leaves as water vapor in a process called transpiration.
Condensation Forms Clouds
What happens after a water molecule evaporates? Warm air carries the water molecule upward. At higher altitudes, air tends to become much colder. Cold air cannot hold as much water vapor as warm air can. As a result, some of the water vapor cools and condenses into liquid water. Condensed droplets of water clump together around tiny dust particles in the air, forming clouds.
Water Falls As Precipitation
As more water vapor condenses, the water droplets in a cloud grow larger and larger. Eventually, they become so heavy that they fall back to Earth. Water that falls to Earth as rain, snow, hail, or sleet is called precipitation.
Most precipitation falls directly into the ocean. The precipitation that falls on land may evaporate immediately or run off the surface into rivers and lakes. From there, it may evaporate or flow back into the ocean. In addition, some water may trickle down into the ground. After a long time, this groundwater may reach a river, lake, or ocean and continue the cycle by evaporating again.
Precipitation is the source of all fresh water on and below Earth’s surface. The water cycle renews the usable supply of fresh water on Earth. For millions of years, the total amount of water on Earth has remained fairly constant—rates of evaporation and precipitation are balanced.
