echoesof.science · Ages 15–16
The water cycle: reservoirs, fluxes and latent heat
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echoesof.science/school/water-cycle/ages-15-16/
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The water cycle moves water between stores (oceans, ice, groundwater, lakes, the atmosphere, living things) through fluxes such as evaporation, transpiration, condensation, precipitation, runoff and infiltration. The cycle is driven by energy from the Sun and by gravity.
The stores are very unequal. The oceans hold about 96% of all Earth's water, while the atmosphere holds only about 0.001%. If all the water in the air fell at once it would cover the globe to a depth of only about 2.5 cm, yet the world receives on average about 980 mm of precipitation a year. So water in the atmosphere must be replaced many times a year.
About 90% of the water vapour in the atmosphere comes from evaporation from water bodies and about 10% from transpiration by plants. Of the water a plant takes up, only about 1% is used for growth; about 99% is transpired.
Experiment: Evaporative cooling: wet and dry thermometers
- Read both thermometers and check that they show the same room temperature.
- Wrap the bulb of one thermometer in cotton wool soaked in room-temperature water and hold it in place with thread. Leave the other bulb dry.
- Fan both thermometers equally with the card (or place them the same distance in front of the fan).
- Record both temperatures every 30 seconds for 5 minutes.
- Repeat with the water at the same start temperature but without fanning. Compare.
- Plot temperature against time for each case and find the largest temperature difference.
| Time (s) | Dry, fanned (°C) | Wet, fanned (°C) | Dry, still (°C) | Wet, still (°C) |
|---|---|---|---|---|
| 0 | ||||
| 60 | ||||
| 120 | ||||
| 180 | ||||
| 240 | ||||
| 300 |
Questions
Explain, using the idea of latent heat, why sweating cools you down.
Calculate the energy needed to evaporate 1 mm of water from 1 m² of ground (1 litre ≈ 1 kg), using Lv = 2256 kJ/kg.
The atmosphere holds water equal to about 2.5 cm of rain, but about 980 mm falls each year. Estimate how many times a year the water in the air is replaced, and roughly how many days a water molecule stays in the air.
Your experiment: the largest difference between the wet and dry thermometers was ____ °C. Give two reasons why it was bigger with the fan, and one source of error.
Sources for every fact on this sheet: echoesof.science/school/water-cycle/sources/