Mirrors would fog up if i didnt turn the fan on

[nacho-man]

Hi guys,
Recently i had noticed that when i left the hot water in the shower on for a while, the mirrors would fog up if i didnt turn the fan on, so what I am wondering is how is it that the fan can prevent the windows from fogging up, I am assuming the windows fog up because of water vapor but I'm not to sure.
Thanks,
Nacho

 

[mgb_phys]

The mirrors fog up because they are below the temperature you have to cool water vapour to in order for it turn back into liquid water (called the dew point).
This depends on the temperature and the relativity humidty - at higher humidity you don't have to cool as much in order to get water forming.
The fan lowers the humidity by removing wet air and sucking in dry air from the rest of the house.

 

[nacho-man]

thanks mgb_phys