I learned at school that warm air holds more water vapour than cold. So why does my car鈥檚 ventilation system demist the windscreen more quickly when I turn on the air cooling?
鈥� Air cooled by the car鈥檚 air-cooling system, cannot hold as much water as it did when it entered the system, so the excess condenses in the air-conditioning unit and runs out of the bottom of the car. The dryness of the air makes it more effective at clearing the windscreen.
鈥淐ondensation on the inside of a car windscreen occurs when warm, humid air meets the cool windscreen and the air can no longer hold the water vapour鈥�
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Malin Dixon, Nuneaton, Warwickshire, UK
Warm air can hold more water vapour than cool air. For example, 1 kilogram of air at 35 掳C can hold about 162 grams of water while air at 10 掳C can hold only about 86 grams of water.
Condensation on the inside of a car windscreen occurs when warm, humid air inside the car meets the cool windscreen. The temperature of the air on the windscreen drops, the air can no longer hold all the water vapour it is carrying, and condensation forms.
To demist the windscreen, the condensed water must evaporate 鈥� or be wiped 鈥� off the windscreen. The more water vapour air contains, the higher the water vapour pressure, and one of the main factors determining whether water evaporates off a surface is the water vapour pressure gradient. Evaporation increases the water vapour pressure of the adjacent air, and if the water vapour pressure of this air reaches the same level as its surroundings evaporation may cease.
When cool dry air is circulated by the car鈥檚 ventilation system, this point is rarely reached. The air sucked in by the air-conditioner becomes dry as it cools and this creates a favourable water vapour pressure gradient for evaporation from the windscreen. The windscreen clears more quickly when the fan is on high speed as the air near the windscreen is changed more frequently.
Peter Kamerman, University of the Witwatersrand Parktown, South Africa
Here鈥檚 the final word, which is not just a matter of semantics 鈥� Ed
鈥� The belief that warm air holds more water (the 鈥渨indbag argument鈥�) was disproved in 1802 when John Dalton found that water vapour pressure is almost the same in a vacuum as it is at normal ambient air pressures.
In other words, water vapour acts independently of the other gases in air. With an increase in temperature, evaporation increases, leading to more vapour and a higher vapour pressure.
At equilibrium, the evaporation rate and the condensation rate are the same. Rising temperatures increase the evaporation; falling temperatures increase the condensation rate (see Craig F. Bohren鈥檚 Clouds in a Glass of Beer, John Wiley and Sons, 1987).
Steuart Campbell, Edinburgh, UK