The hydrologic cycle describes the continuous movement of water on, above, and below the Earth’s surface. This closed loop system means the total amount of water remains constant, though its location and physical form are always changing. Solar energy powers the entire cycle, providing the heat necessary to drive the various phase changes of water.
Water Vapor Enters the Atmosphere (Evaporation and Transpiration)
The first stage begins with evaporation, the conversion of liquid water into a gaseous state. Solar radiation heats surface water in oceans, lakes, and rivers, giving water molecules enough energy to escape as vapor into the atmosphere. About 86% of global evaporation occurs directly from the surface of the oceans. A related process, transpiration, also contributes water vapor when plants absorb liquid water through their roots and release it through pores in their leaves called stomata. Evaporation and transpiration are often grouped together as evapotranspiration, representing the total flow of water from the Earth’s surface and vegetation back into the air.
Cloud Formation (Condensation)
As water vapor rises in the atmosphere, it encounters cooler temperatures and loses energy. This cooling triggers condensation, the phase change back from gas to liquid. The vapor reverts to microscopic liquid droplets or ice crystals. For this change of state to occur efficiently, water molecules must have a surface to condense upon. These surfaces are provided by tiny airborne particles known as cloud condensation nuclei (CCN). CCNs are typically about 0.2 micrometers in size and can be composed of natural substances like dust, pollen, and sea salt, or human-made aerosols like soot. The accumulation of billions of these droplets around condensation nuclei forms clouds.
Water Falls Back to Earth (Precipitation)
The water stored in the clouds eventually returns to the Earth’s surface through precipitation. As cloud droplets continue to condense and collide, they grow larger and heavier. When the accumulated water mass becomes too heavy for air currents to keep suspended, gravity pulls it toward the ground. Precipitation is the primary mechanism for transferring water from the atmosphere back to the land and oceans. This release takes on various forms depending on the temperature profile below the cloud, including liquid rain, frozen snow and hail, or partially frozen sleet.
Surface Movement and Storage (Collection)
Once precipitation reaches the ground, the final stage involves the collection and movement of water across and beneath the surface. Water that flows over the land is known as surface runoff. Driven by gravity, this runoff travels downhill into streams, rivers, and eventually, larger bodies of water like lakes and the ocean. A portion of the precipitation seeps into the soil and rock layers through infiltration. This water moves downward, contributing to soil moisture and replenishing underground reservoirs known as aquifers. Water stored in these groundwater reserves, glaciers, and large bodies of water completes the storage aspect of the cycle until it is drawn back into the cycle through evaporation, transpiration, or discharge into surface water systems.