The Water Cycle

El método, paso a paso

1. 1

   Evaporation lifts water into the air

   solar energy → liquid water becomes vapour

   The sun supplies the energy that drives the entire cycle. Most evaporation happens from the oceans, which cover the majority of the surface, though lakes, rivers and soil contribute as well.

2. 2

   Transpiration adds water from plants

   roots draw water up → released as vapour from leaves

   Plants move water from soil to atmosphere continuously. Combined with evaporation this is called evapotranspiration, and over vegetated land it accounts for a substantial share of the water entering the air.

3. 3

   Condensation forms clouds

   rising vapour cools → condenses onto tiny particles

   Air cools as it rises, and cooler air holds less vapour. The vapour condenses into droplets around dust particles, and enough droplets together form a cloud. Cooling is what triggers this, not any change in the water itself.

4. 4

   Precipitation returns water to the surface

   droplets merge until too heavy to stay aloft → rain, snow, hail

   Which form falls depends on temperature through the layers of air the droplets pass. The same cloud can produce rain or snow depending on conditions between it and the ground.

5. 5

   Collection and runoff return water to the sea

   surface runoff and groundwater flow → rivers → ocean

   Water that falls on land either flows over the surface, soaks in to become groundwater, or is taken up by plants. Eventually most returns to the ocean, where it can evaporate again — closing the cycle.

A cycle has no starting point

Diagrams usually begin with evaporation from the sea, which makes it look like the first stage. It is not — the cycle runs continuously and every stage is happening everywhere at once. A question asking where the cycle begins is testing whether a student has understood that.

It is also a closed system in terms of quantity. The water on Earth is essentially the same water that has always been here, moving between states and locations rather than being created or destroyed.

• Evaporation — sun turns liquid water to vapour
• Transpiration — plants release vapour from leaves
• Condensation — cooling vapour forms cloud droplets
• Precipitation — rain, snow or hail falls
• Collection — runoff and groundwater return to the sea

The sun and gravity

Two forces drive the whole cycle. Solar energy lifts water into the atmosphere by evaporation, and gravity brings it back down as precipitation and moves it downhill through rivers and groundwater. Every stage is one or the other.

Naming these makes the cycle explicable rather than merely describable. A student who can say that the sun supplies the energy and gravity does the returning can answer questions about why the cycle continues, which a labelled diagram alone does not prepare them for.

The part that is easy to forget

Not all precipitation runs over the surface. A large proportion soaks into the ground, where it moves slowly through soil and rock as groundwater and may remain for years before emerging into a river or the sea.

This slow store matters practically — it supplies wells and springs and sustains rivers during dry periods. Diagrams that show only surface runoff give an incomplete picture, and questions about drought often turn on the groundwater component.

How we teach the water cycle

We connect it directly to states of matter, because it is the same physics in a different context. A student who understands evaporation and condensation at the particle level meets the water cycle as an application rather than as a new topic to memorise.

We also insist on naming the energy source. 'The sun drives the cycle' is a single sentence that converts a labelled diagram into an explanation, and explanation is what the higher-mark questions ask for.