School › Water cycle › Ages 15–16
The water cycle: reservoirs, fluxes and latent heat
Where Earth's water is stored, how it moves between stores, and the energy involved in evaporation and condensation. Calculations with Q = m·L and an evaporative-cooling experiment.
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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.[1]
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.[1][3][5]
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.[3][5]
Changing state takes energy. To evaporate a mass m of water you must supply Q = m·Lv, where Lv is the latent heat of vaporisation: 2256 kJ for 1 kg of water at 100 °C, and about 2428 kJ/kg at body temperature. Melting ice needs Q = m·Lf with Lf = 334 kJ/kg.[6]
Because evaporation takes energy from its surroundings, it cools them: evaporating sweat draws heat from the skin. The reverse is also true: when water vapour condenses, the same large amount of energy is released as heat. This heat warms the air in clouds and helps power storms such as hurricanes.[6][5]
Evaporation leaves dissolved salts behind. Lakes with no outlet lose water only by evaporation, so their salt becomes concentrated, as in the Dead Sea and the Great Salt Lake. This is also why rain is fresh water even though much of it evaporated from the salty ocean.[5]
Try it: Evaporative cooling: wet and dry thermometers
You need
- 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.
What to do
- 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.
What happens, and why?
Water evaporating from the wet cotton takes the latent heat it needs from the thermometer bulb, so the wet thermometer cools below the dry one. Moving air carries the water vapour away, so evaporation and cooling are faster. The same effect explains why sweating cools the skin.[6][5]
Play: Takes energy or gives energy?
Does this change of state take in energy from the surroundings, or release energy to them? Some cards are calculations: work out the energy.
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Quiz
Other ages: Ages 6–7 · Ages 10–11