Interactive lesson

Evaporative cooling, step by step

Interactive lesson: wet hessian, a charcoal cooler, a wet bottle and a night draught — how to measure whether it will work and when evaporation does harm.

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The whole lesson as text

1. How it cools

Water that goes into the air takes the heat with it.

Water takes the heat — A wet cloth on a bottle cools the bottle, and a wet cloth over a basket cools the lettuce. Water uses heat from whatever it evaporates from.
Dry air and a draught — Dry air (20–50 % humidity), a draught and a warm afternoon cool the most. Humid air, fog and a still, enclosed corner cool little.
The limit: the wet bulb — Evaporation never goes below the wet-bulb temperature. On a dry day (30 °C, 30–40 % humidity) that is 18–20 °C; on a muggy one (70 %) only 25–26 °C.
Humidity rises — Evaporation also dampens the air. That is good for vegetables that must not wilt, and bad for bread, dried fruit, flour and salt.

Why: To turn into vapour, water needs about 2,400 kJ per litre. It takes that energy from the surface it evaporates from, so a wet cloth, hessian or sand cools down.

Evaporation is for vegetables, fruit, greens, water and drinks. Not for meat, milk or cooked food.
Wet surfaces must be dried and cleaned: mould, mosquitoes and stale water are the price of this method.

2. The psychrometer

Two thermometers, one with a wet wick, show in advance how much cooling you can get.

Two identical thermometers — Take two identical thermometers and check them in iced water. Set them side by side.
A wet wick — Put a wick of clean cotton, free of detergent and grease, over the bulb of one, with its end in a glass of water. Use rainwater or boiled water, not hard water.
Shade and a draught — Leave them in a shady draught for 5–10 minutes, or swing them for 2 minutes. The wick must always be wet.
Read and write it down — Measure when you need the cooling, usually 1–5 pm, where the cooler would stand. A difference below 4 °C: do not bother.

Why: The difference between the dry and the wet thermometer shows how dry the air is. So you measure first and build afterwards.

A difference of 8 °C or more: excellent. Below 3 °C: do not wet, air instead.
Cheap hygrometers are often 5–10 % out, so check them against a psychrometer now and then.