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Water—the main reason for life on Earth—continuously circulates through one of Earth’s most powerful systems: the water cycle. Water flows endlessly between the ocean, atmosphere, and land. Earth’s water is finite, meaning that the amount of water in, on, and above our planet does not increase or decrease.
The water cycle describes how water evaporates from the Earth’s surface, rises into the atmosphere, cools, condenses to form clouds, and falls again to the surface as precipitation. About 75% of the energy (or heat) in the global atmosphere is transferred through the evaporation of water from the Earth’s surface. On land, water evaporates from the ground, mainly from soils, plants (i.e., transpiration), lakes, and streams. In fact, approximately 15% of the water entering the atmosphere is from evaporation from Earth’s land surfaces and evapotranspiration from plants. Such evaporation cools the Earth’s surface, cools the lower atmosphere, and provides water to the atmosphere to form clouds.
Of all the water that exists on our planet, roughly 97% is saltwater and less than 3% is freshwater. Most of Earth’s freshwater is frozen in glaciers, ice caps, or is deep underground in aquifers. Less than 1% of Earth’s water is freshwater that is easily accessible to us to meet our needs, and most of that water is replenished by precipitation—a vital component of the water cycle, affecting every living thing on Earth.
Precipitation is any product of the condensation of atmospheric water vapor that falls quickly from a cloud. The main forms of precipitation include drizzle, rain, sleet, snow, graupel (soft hail or snow pellets), and hail. While precipitation is the ultimate source of the freshwater we use in our daily lives, this essential natural resource is not distributed evenly across our planet. On land, some places are drenched with rain, such as temperate and tropical rainforests. Other locations receive little rain and snow and are so dry that communities, such as Las Vegas, Nevada, recycle water that has been used for bathing and cleaning—known as grey water— to water their gardens.
Understanding the role of precipitation in Earth’s water cycle and how it interacts with other Earth systems requires a global view. The distribution of water throughout the atmosphere and how it moves, changing between its solid, liquid, and gaseous forms, is a powerful vehicle for redistributing Earth’s energy and influences the behavior of the planet’s weather, climate, and other environmental systems.
