The Water Cycle

Evaporation, Condensation, and Precipitation

The water cycle is the continuous movement of water through Earth's oceans, land, atmosphere, and living systems. It matters because it redistributes heat, shapes weather and climate, and supplies the fresh water needed for ecosystems and human life. Even though the total amount of water on Earth stays nearly constant, its location and state change constantly. Understanding this cycle helps explain rainfall, drought, floods, and groundwater supply.

Solar energy drives much of the water cycle by causing evaporation from oceans, lakes, and soil, while plants add water vapor through transpiration. As moist air rises and cools, condensation forms clouds, and precipitation returns water to the surface as rain, snow, or hail. Some water flows over land as runoff into rivers and oceans, while some infiltrates into the ground and becomes groundwater. These linked processes connect the atmosphere, hydrosphere, geosphere, and biosphere in one dynamic system.

Key Facts

Evaporation is the change of liquid water into water vapor due to added thermal energy.
Condensation is the change of water vapor into liquid droplets when air cools.
Precipitation includes rain, snow, sleet, and hail that fall from clouds to Earth.
Runoff is water that flows over the land surface into streams, rivers, lakes, and oceans.
Infiltration is the movement of water into soil, and percolation is its deeper movement through rock and sediment.
Transpiration + evaporation = evapotranspiration.

Vocabulary

Evaporation: Evaporation is the process in which liquid water gains energy and becomes water vapor.
Condensation: Condensation is the process in which water vapor cools and changes into liquid water droplets.
Precipitation: Precipitation is any form of water that falls from clouds to Earth's surface.
Groundwater: Groundwater is water stored beneath Earth's surface in soil and rock spaces.
Runoff: Runoff is water that moves across the land surface toward streams, rivers, lakes, or oceans.

Common Mistakes to Avoid

Thinking the water cycle only happens between the ocean and clouds, which is wrong because land, rivers, groundwater, ice, and plants are also major parts of the cycle.
Assuming evaporation happens only when water is boiling, which is wrong because liquid water can evaporate at many temperatures below its boiling point.
Confusing condensation with precipitation, which is wrong because condensation forms cloud droplets while precipitation is water falling from clouds to the ground.
Believing groundwater is separate from the water cycle, which is wrong because infiltrated water can be stored underground and later move into springs, rivers, or the ocean.

Practice Questions

1 A lake loses 18 mm of water in one sunny day due to evaporation. If the lake surface area is 2.5 x 10^6 m^2, what volume of water evaporated that day in cubic meters?
2 During a storm, 4.2 cm of rain falls on a field with area 800 m^2. If 65% infiltrates into the ground and the rest becomes runoff, how many cubic meters of water become runoff?
3 A region cuts down many trees but receives the same amount of sunlight as before. Explain how this change could affect transpiration, cloud formation, and local rainfall.