What are the steps in the water cycle?
Most science resources focus on four main steps of the water cycle—evaporation, condensation, precipitation, and collection—but some resources get more granular.
Here’s a breakdown:
1. Evaporation - Heat from the sun (or another source) comes into contact with water. The heat excites the molecules of water, spreading them out; as a result, the water loses density and evaporates, rising into the air as vapor.
This can happen at any temperature between water’s freezing and boiling point, but it occurs quickest as water approaches its boiling point of around 100 degrees Celsius (212 degrees Fahrenheit). If humidity and air pressure are low, the molecules need less energy to evaporate, since there’s substantially less pressure holding the molecules together in the first place.
Processes similar to but scientifically distinct from evaporation are sublimation (when ice or snow turns directly into water vapor) and transpiration (when plants release water vapor into the air).
As water vapor rises, it carries tiny particles of dust and dirt. This is important for the next stage of the water cycle.
2. Condensation - The water vapor rises and contacts cold air higher in the atmosphere, forming clouds. Gradually, the molecules in the water become less excited and begin to hold on to the particles of dust and dirt. This is called condensation.
Clouds consist of water vapor, droplets, and ice, along with the aforementioned dust and dirt. A typical cumulus cloud holds more than 500 tons of water. Upward movements of air called updrafts keep the water from falling out of the cloud...for a while.
3. Precipitation - As more molecules of water join together, they can become too dense to remain in the air, and eventually, updrafts disappear or weaken to the point where water can escape the cloud. The water falls to the ground as rain (or snow, or ice, depending on the temperature, air pressure, and other factors). This process is called precipitation.
4. Collection - Most water falls onto oceans, lakes, or other bodies of water, restarting the cycle. Some water falls to land, where it travels along the slope of the ground toward creeks, rivers, and streams; this is called runoff, and some consider this an additional step in the cycle.
Some water undergoes a process called infiltration, in which it seeps down into soil and rock.
Inevitably, water collects after falling to the ground, so scientists refer to this stage of the water cycle as collection, regardless of where water falls.
For most purposes, the four steps outlined above provide a sufficient explanation of the water cycle.
If you want to explain the basics to your friends, evaporation, condensation, precipitation, and collection are good places to start. If you really want to impress them, you can talk about sublimation, transpiration, runoff, and infiltration.
But why is the water cycle important? The water cycle provides a steady source of clean, usable water. It’s a natural recycling process, and because all living organisms depend on water to survive, the cycle is critical to life.