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The water or hydrologic cycle is the constant exchange of water in its various forms of liquid, solid (ice and snow), and gas (water vapor) between Earth's surface, the oceans, and the atmosphere. The hydrologic cycle constantly renews Earth's supply of freshwater.

The sun provides energy to drive the system as it heats Earth, causing evaporation of liquid water. Water evaporates from the surface of all the bodies of water on Earth. The water vapor rises with the less dense warm air. Water also evaporates from plants. As plants transport water from their roots, much of the water evaporates through the leaves into the atmosphere. This is called transpiration.

As the air containing water vapor moves farther away from Earth's surface, it cools. Cool air cannot hold as much water vapor as warm air. In cooler air, most of the water vapor condenses into droplets of water that form clouds. The temperature at which water vapor condenses is called the dew point. Clouds form as water vapor condenses on small particles (cloud condensation nuclei) in the atmosphere.

Precipitation falls toward Earth when the water droplets that form in clouds become too numerous and heavy to stay in the air. Depending on the temperature of the air near Earth's surface, the precipitation can be in the form of rain, snow, ice, or a combination.

When precipitation reaches Earth, it either evaporates or flows over the surface (known as runoff) where it may accumulate in ponds or lakes or eventually reach streams, rivers, and the ocean. Plants require water and absorb it. Water may also be absorbed into the ground and become groundwater or remain frozen on the surface as snow or ice.