The water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the Earth’s surface. This global process ensures the availability of water for all living organisms and plays a significant role in regulating weather patterns across the planet. Water exists in three states—liquid, solid (ice), and gas (water vapor)—and constantly transforms between these forms as it circulates.
Evaporation
Evaporation is the process where liquid water transforms into water vapor and rises into the atmosphere. This transformation is primarily driven by solar energy. This process occurs extensively from large bodies of water like oceans, lakes, and rivers, as well as from soil moisture.
A related process, transpiration, also contributes significantly to atmospheric water vapor. Plants absorb water through their roots, and a large portion of this water is then released as vapor from their leaves through tiny pores called stomata. Both evaporation and transpiration collectively move water from the Earth’s surface into the gaseous phase within the atmosphere.
Condensation
As water vapor rises higher into the atmosphere, it encounters cooler temperatures. This cooling causes the water vapor to change back into tiny liquid water droplets or ice crystals, a process known as condensation. These droplets or crystals form around airborne particles, such as dust or salt, which are known as condensation nuclei.
The accumulation of these water droplets or ice crystals around condensation nuclei leads to the formation of clouds. Air reaches a saturation point where it can no longer hold all the water vapor, causing the excess to condense. Without these particles, water vapor would struggle to condense into visible clouds.
Precipitation
When water droplets or ice crystals within clouds grow large enough, Earth’s gravity causes them to fall back to the surface as precipitation. The form precipitation takes depends largely on the temperature and atmospheric conditions through which it falls.
Rain occurs when temperatures are above freezing, allowing liquid water droplets to fall. If temperatures are consistently below freezing, moisture in the air crystallizes directly into snow. Sleet forms when snowflakes partially melt and then refreeze into ice pellets before reaching the ground. Hail consists of ice balls that grow as they are carried by updrafts into cold regions of a cloud, becoming too heavy to be supported.
Collection and Runoff
After precipitation reaches the Earth’s surface, it collects. Some water falls directly into oceans, lakes, and rivers, replenishing these bodies of water. A portion of the water soaks into the ground through a process called infiltration, becoming soil moisture or groundwater.
Water that does not infiltrate the ground flows over the land as runoff. This runoff travels across fields, roads, and other landscapes, gathering into small streams and eventually flowing into larger rivers. These rivers then carry the water back to lakes or oceans, completing the water cycle.