The hydrologic cycle, or the water cycle, is the continuous cycling of water through the atmosphere, ocean, land surface, cryosphere, and biosphere. The hydrologic cycle is mainly driven by energy from the sun. Major reservoirs (places where things are stored) of water on Earth are the ocean, lakes, rivers, wetlands, land and sea ice, snow, groundwater, and the atmosphere (see table below). Water can move between reservoirs by several different processes: evaporation, precipitation, melting, and by running downhill in rivers or underground. Evaporation is the process where water changes from a liquid to a gas (water vapor). Precipitation occurs when liquid or ice water particles in the atmosphere in clouds grow too big and they fall to the earth.
To illustrate the hydrologic cycle, one can follow a parcel of water on one possible path starting in the ocean. The water may evaporate (changes from liquid into water vapor) from the ocean surface and enter the atmosphere. Evaporation requires heat for the water to transition from a liquid to a gas. Once in the atmosphere, the water vapor rises and condenses (the process of changing from water vapor to liquid water and releasing heat), forming a cloud. The wind may blow this cloud over land and the water can precipitate as rain or snow. The water might then run over the Earth’s surface into a river or lake or seep into the ground to become groundwater. From the lake, river or groundwater, the water could flow into the ocean again. At any point in the example above, the water could have taken a different path. Much of the water that evaporates from the ocean precipitates right back into the ocean.