Teacher sheet · Ages 15–16 · about 45–50 minutes
The water cycle: reservoirs, fluxes and latent heat
Lesson page: echoesof.science/school/water-cycle/ages-15-16/ · Pupil worksheet: echoesof.science/school/water-cycle/ages-15-16/print/
Learning aims
- Describe the water cycle as stores and fluxes driven by solar energy and gravity.
- Use the USGS and NOAA figures to reason about the size of stores and fluxes.
- Use Q = m·Lv and Q = m·Lf.
- Explain evaporative cooling and the release of latent heat on condensation.
- Collect, plot and evaluate temperature-time data.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: why do you feel cold when you get out of a swimming pool on a windy day? |
| 10 min | Read the text; work through the 2 kg Q = m·Lv example. |
| 20 min | Wet and dry thermometer experiment in pairs; plot the graphs. |
| 5 min | Discuss: link the result to sweating and to latent heat in clouds. |
| 5 min | Quiz, or the game as homework. |
Materials for the experiment
- Two identical thermometers (or temperature probes and a data logger)
- Cotton wool or a strip of cloth
- Water at room temperature
- Rubber band or thread
- A piece of card or a small fan to move the air
- A stopwatch
Safety: Handle glass thermometers carefully and report breakages to the teacher; never use mercury thermometers. Keep water away from any mains-powered fan and its plug.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Evaporation adds heat to the surroundings. | Evaporation takes in energy and cools its surroundings; condensation releases energy. |
| Most of the world's water is in clouds and rivers. | The atmosphere holds only about 0.001%; about 96% is in the oceans. |
| Rain is salty because it comes from the sea. | Salts stay behind when water evaporates, so rain is fresh. |
Worksheet answers
- Sweat on the skin evaporates; changing from liquid to vapour needs latent heat (about 2428 kJ/kg), which is taken from the skin, so the skin cools.
- 1 mm × 1 m² = 0.001 m³ = 1 litre ≈ 1 kg. Q = 1 × 2256 = 2256 kJ (about 2.3 MJ).
- 980 mm ÷ 25 mm ≈ 39 times a year; 365 ÷ 39 ≈ 9 days on average (a rough estimate).
- Typical difference 2–8 °C. The fan removes vapour near the wet cotton and brings drier air, so evaporation is faster. Errors: thermometer reading precision, unequal fanning, water not at room temperature, timing.
Quiz answers
- How much energy does it take to evaporate 2 kg of water at 100 °C (Lv = 2256 kJ/kg)? 4512 kJ. Q = m·Lv = 2 × 2256 = 4512 kJ.[6]
- What fraction of Earth's water is in the atmosphere at any moment? About 0.001%. The atmosphere holds only about 0.001% of Earth's water.[3]
- When water vapour condenses in a cloud, energy is… released as heat. Condensation releases the latent heat taken in during evaporation.[6][5]
- Why does a wet thermometer read lower than a dry one in moving air? Evaporation takes latent heat from the bulb. Evaporation draws energy from its surroundings.[6]
- About what share of atmospheric water vapour comes from plant transpiration? About 10%. About 90% comes from evaporation and about 10% from transpiration.[3]
- Why is a lake with no outlet, like the Dead Sea, very salty? It loses water only by evaporation, which leaves the salts behind. Evaporation removes water but not the dissolved salts.[5]
Going further
On a hot day a person sweats 1 kg of water that fully evaporates. Using Lv = 2428 kJ/kg, how much energy is removed from the body? Then explain why sweating cools you less on a very humid day.
Sources used in this lesson
- [1] The Water Cycle (U.S. Geological Survey (USGS) Water Science School): reliability High
- [3] The Atmosphere and the Water Cycle (U.S. Geological Survey (USGS) Water Science School): reliability High
- [5] The Hydrologic Cycle (JetStream) (NOAA National Weather Service): reliability High
- [6] College Physics, 14.3 Phase Change and Latent Heat (OpenStax, Rice University): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/water-cycle/sources/
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