Harness your greatest fan
Follow the news and there’s much confusion about keeping cool. Today’s summary on the BBC recommends drawing your curtains and staying indoors, but didn’t mention the best way to keep cool – using an electric fan.
People often say that a fan’s no use, writes professor Brian J Ford, because it just moves hot air around but doesn’t cool anything. That’s profoundly wrong. The moving current of air evaporates water from your skin, and the latent heat of evaporation cools you down. Evaporation demands an energy supply. If you sit in a current of air, whether it’s from a breeze outside, or a fan indoors, water will evaporate from your skin – and the energy it must have is the heat from your circulating blood.
You will likely perspire around half a litre of water per hour. Evaporating it requires 330 Watts of energy. The metabolism running your body is normally rated at about 100 W, so you’re providing three times as much energy to evaporate water from your skin.
Humidity matters. Latent heat doesn’t alter much, but the rate of evaporation changes enormously. At a Relative Humidity (RH) of 20% water evaporates readily, whereas at 80% RH evaporation is greatly reduced, and sweat drips from the body, causing no cooling effect. In a normal room RH is usually around 50% so evaporation is rapid. You’d only find an RH around 70% in the bathroom of an older house during damp weather.
Medical advice was given on the BBC that fans make things worse above 35°C, ‘they just direct hot air towards the body’. Not so. In moderate humidity, evaporative cooling works fine. I have been in a sauna at 110°C, hot enough to cook meat, and the highest temperature experimentally experienced by humans is over 130°C. Natural evaporation cools you down.
Place your fan in an open window? Definitely not – you want to keep the warming air that’s outside, out. To keep cool? Open every door and window that you can during late evening or around breakfast time, when the temperature can be some 16°C. Fill your home with that sweet, cool air. Then keep strict discipline: doors and windows kept closed to keep that cool air inside.
Close the curtains if a window is facing the sun, to keep out the infra-red, and keep your oscillating fan running on low, wafting a gentle breeze around your room. That will cool you
down. Portable air-conditioning fans are a scam; they blow cool air, but it’s cooled by evaporating water from within the device so ambient humidity goes shooting up. Your body will provide all the cooling it needs; the moving air from your fan will evaporate just the right amount.
A suitable fan costs less than £20 and the energy to run it comes to about 2p an hour. So keep the hot air outside, and rely on keeping cool with – your biggest fan.
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