Bite-size answers to commonly asked questions from inquiring minds
Q. Why does a fan keep us cool?
Did you know?
Antarctica is the coldest place on Earth. In 2009, researchers identified ‘Ridge
A’ using satellite technology. At nearly 14,000 metres high, Ridge A has an average
winter temperature of -70 degrees Celsuis (-94 F). Brrrrrr. (Source: MSN Science.)
A. Why does a fan keep us cool? Well, an electric fan could actually make the room
hotter because the fan’s motor generates heat. Do not leave a fan on in an enclosed
room when no one is present – it could only make it warmer and cost you money in
electricity consumption. So how does a fan keep us cool?
What a fan does is create a ‘wind chill’ effect when you’re in the room. Moving air
cools skin faster than still air. When the weather forecaster talks about wind chill
what they are referring to is convection: how the wind (moving air) increases convective
heat loss. But what is convection?
Convection occurs when a gas or liquid gets hot – it rises (when you have finished
reading this answer you can click here to find out why it rises!). If, for example,
you have a mug of hot coffee it will heat a layer of the air directly above. That
air then rises because it is hotter than the surrounding air and cold air will fill
the empty space left. In turn this air will heat up and rise. One can accelerate
the process by blowing. If we didn’t blow on the coffee, it would stay hotter for
longer. In effect, convection is a transfer of heat in a gas or liquid - it takes
place when heated molecules move from one place to another taking heat with them.
So how does this tie in with a fan making us feel cooler? When we get hot, we perspire
(in fact, we sweat to one extent or another pretty much most of the time). By blowing
air, the fan increases convective heat loss in us and makes it easier for the air
to evaporate our sweat. Liquid evaporating from the skin has a cooling effect. When
liquid – in this case sweat – evaporates its molecules turn to vapour (driven by
heat) and, in order to escape as vapour, it must take heat energy with it. The heat
it takes with it comes from the surface (our skin) from which it has evaporated.