The hydrologic cycle — also known as the water cycle — is the continuous exchange of water between Earth's surface and the atmosphere. As the planet's natural mechanism for transporting and recycling water, the hydrologic cycle is critical for maintaining conditions on Earth. There are five basic steps within the water cycle: condensation, precipitation, infiltration, runoff, and evapotranspiration.

Gaseous water vapor in the atmosphere condenses to form clouds, which can produce precipitation. Rain, snow, and sleet return water from the atmosphere to Earth's surface. On the ground, the water cycle continues with infiltration, the process in which surface water seeps into the soil where it can become groundwater. The amount of water that infiltrates into the ground depends on many factors, such as soil type and rock type. Topography also influences infiltration — a steeper slope forces the water to run off more quickly, preventing much infiltration. In addition, if the soil is already saturated with water, it cannot absorb much more, which leads to an increase in runoff. Land cover, such as vegetation or man-made surfaces, also affects the movement and infiltration of water.

Water that doesn't infiltrate the ground is called surface runoff. When water flows over land, it follows a path to the lowest point, running down hills to creeks, streams, and rivers until it eventually reaches a sea or ocean. Rainwater and melted snow and ice that move too quickly to infiltrate the ground become runoff. Runoff replenishes the water on Earth's surface and helps to continue the water cycle.

The next step in the cycle — evapotranspiration — returns water to the atmosphere. The Sun causes evaporation by heating liquid water on Earth's surface. Transpiration — the evaporation of water from pores in the leaves of plants — also releases water vapor into the atmosphere. Water vapor in the air is invisible; visible clouds and steam are actually millions of tiny droplets of liquid water or ice that form when water vapor molecules condense around small particles in the air. However, as more water molecules collect on the cloud droplets, the drops get too heavy and fall from the cloud back to Earth's surface as precipitation.

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