Using the AC button to direct the air to the screen the moisture on the glass will be evaporated very rapidly, long before a screen-heater has had a chance to warm up and will continue this advantage around the side glasses and on to the rear screen. For best effect use the AC button, perhaps in conjunction with the heater in winter and on the fresh air setting (not recirculation). AC is usually extremely good at de-misting and this safety aspect of good all round visibility, even when picking up a crowd of wet noisy kids in winter, is one of AC's strong points. It has to be admitted that very occasionally the climate can be so very damp on odd days that even with the AC working effectively the demisting function can work only slowly but this is very much in the minority - for most days the AC will clear the screen almost like magic.
Taken from this website which has lots of information about A/C http://www.airconditioningforcars.co.uk/ACpage01.htm