This is one of those questions that seems so natural that we forget to ask them. But that’s what I’m here for: to make you wonder about things you take for granted, and then to replace your complacency with the smugness of knowing.
The water in your hair or clothes must first be converted from liquid into vapor before it can be spirited off by a stream of air. Blowing away liquid water isn’t easy, as you can tell by the hurricane-force winds they have to use to dry your car at the car wash.
Warming the liquid water in your hair or clothes, and that’s what the warm air does, speeds up the water’s molecules, so that more of them can fly off into the air. (Techspeak: Warmer water has a higher vapor pressure;.) The heat therefore speeds up evaporation of the water, and once it has evaporated into the form of vapor it can be swept away by the blowing air.
But how much vapor can the air-heated water produce? How fast can it evaporate?
Liquid water molecules can keep flying off and becoming vapor molecules only until the space above the liquid is so crowded with vapor molecules that just as many are bouncing back into the water as are flying out of it. (Techspeak: until the liquid and vapor are in equilibrium.) That’s where the blowing comes in. The moving air from the dryer blows away some of those water vapor molecules so that they can’t go back into the liquid. This “makes room” for more, and the evaporation continues.
That’s why clothes and hair dryers do both heating and blowing. One without the other wouldn’t do the job nearly as well. What if your hair dryer’s blower blew out, so that it only heated your hair, or if its heater blew out and it blew cold air?
If there is already a lot of water vapor in the air, for example, if the bathroom is already steamed up and humid from your shower, the water in your hair won’t be able to evaporate as fast.
It will require a much longer heating and blowing time to dry your tresses to that silky, sexy, slow-motion slinkiness that they show on TV.