๐ Earth & Space Science: The Water Cycle in Motion
Explore how water continuously moves between Earth's surface and atmosphere through evaporation, condensation, and precipitation, and learn to quantify these processes.
Learning Goals
By the end of this lesson, you will be able to:
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Describe the main stages of the water cycle: evaporation, condensation, precipitation, and collection
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Explain how the Sun's energy drives the movement of water
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Calculate simple water quantities using basic rates and conservation of mass
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Relate water cycle processes to everyday weather you can observe
Before You Begin
Before this lesson, you should know:
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Understanding that water exists as a solid, liquid, and gas
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Knowing that heating adds energy and can change a state of matter
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Basic ability to add, subtract, and work with simple rates such as mm per hour
Notation & Symbols
Key notation and symbols used in this topic:
mm
Millimetre โ a unit used to measure rainfall depth
โC
Degrees Celsius โ unit of temperature
ฮ
Change in a quantity, e.g. ฮT is a change in temperature
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Approximately equal to
ร
Multiplication sign used in rate calculations
How the Water Cycle Works
The water cycle is the continuous movement of water between Earth's surface and atmosphere, powered mainly by energy from the Sun.
Evaporation: When the Sun heats water in oceans, lakes, and puddles, liquid water gains energy and changes into water vapour (gas) that rises into the air.
Condensation: As water vapour rises and cools, it changes back into tiny liquid droplets that gather to form clouds.
Precipitation: When droplets in clouds combine and become heavy enough, they fall as rain, snow, or hail.
Collection: Fallen water gathers in rivers, lakes, and oceans, or soaks into the ground, ready to evaporate again.
Conservation of water: The total amount of water on Earth stays roughly constant โ it is simply recycled, so water inโwater out.
Example
On a warm day at 30โC, a shallow puddle holding about 2 litres of water may fully evaporate within a few hours. The water is not destroyed โ it becomes vapour that later condenses into clouds and may return as rain somewhere else.
100%
Water evaporates from the ocean, condenses into clouds, and returns as rain in a continuous cycle driven by the Sun.
Worked Example
Basic
A town records 6 hours of rainfall. If the rain gauge collects 2mm each hour, how much total rainfall is collected?
Worked Example
Intermediate
During the water cycle, a puddle loses water by evaporation. A puddle starts with 500mL of water and evaporates at 25mL per hour on a warm day. How long until the puddle is completely dry?
Worked Example
Advanced
The Earth rotates once every 24 hours. Through how many degrees does it turn in 1 hour, and how many degrees in 20 minutes?
Why It Works
Understanding the reasoning behind the rule:
Why does the Sun appear to rise in the east and set in the west? It comes from the Earth's steady rotation, which we can express mathematically.
Your Turn
Try this one yourself โ use the hint if you get stuck:
A rain gauge collects 3mm of rain per hour during a storm. If the storm lasts 4 hours, then rains at 1mm per hour for 2 more hours, what is the total rainfall?
Watch Out For...
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Thinking evaporation only happens from oceans and large lakes.
Evaporation occurs from any water surface, including puddles, soil, wet clothes, and even leaves (transpiration). Any exposed water can gain enough energy to become vapour.
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Confusing evaporation with boiling.
Evaporation happens at any temperature at a water surface, while boiling needs the whole liquid to reach 100โC at sea level. The Sun's warmth drives everyday evaporation well below boiling point.
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Believing clouds form because water vapour rises and 'gets heavy'.
Clouds form because rising air cools, and cool air holds less vapour, so water condenses onto tiny particles. It is cooling, not weight, that triggers condensation.
In the Real World
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Weather forecasters track evaporation and condensation to predict rainfall, helping farmers decide when to plant or irrigate crops.
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Engineers design reservoirs and drainage systems knowing that roughly 90% of atmospheric moisture comes from ocean evaporation that later falls as precipitation.
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Dew on morning grass and fog in valleys are condensation happening right in front of you as warm air cools overnight.
Extended Practice
Test yourself with a mix of multiple choice and short-answer questions:
Q1.
Which process changes liquid water into water vapour?
MCQ
Q2.
Why do clouds form when moist air rises?
MCQ
Q3.
A puddle of 500mL dries up on a sunny day. In your own words, explain where the water went.
The Sun's heat gave the surface water molecules enough energy to escape as water vapour (evaporation). The water did not disappear โ it entered the air as an invisible gas.
Q4.
Name the process in which water falls from clouds back to Earth's surface.
MCQ
Extension Challenge
Advanced
A harder challenge that connects to the next skill:
On a warm day the Sun delivers energy to a shallow pond. If the pond loses 2mm of water depth to evaporation over 5 hours, estimate the average evaporation rate per hour and explain what drives it.
Key Takeaways
1.The water cycle moves water continuously between Earth's surface and atmosphere through evaporation, condensation, precipitation, and collection.
2.Energy from the Sun powers evaporation, while cooling of rising air drives condensation and cloud formation.
3.Evaporation can occur at any temperature at a water surface, unlike boiling which needs 100โC.
4.Water is never created or destroyed in the cycle โ it only changes state and location.
Printable Cutouts
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Ready to Practice?
Now that you've learned Earth & Space Science, put it to the test with a quick quiz!