The weather is being peculiar. Here at the end of October, it’s been ludicrously warm, interspersed with high winds and lashing rain. All of which conspires to give me a problem: windscreen fogging. The warmth means that as soon as it rains, and the outside temperature plummets, the residual warm air in the car (and this is with the heater on cool) hits the cold glass and - POW! - instant loss of visibility. Usual response - to put the blower on the windscreen - fails miserably, only causing more fogging. I try turning the heat up, as I do in winter - it just gets worse.
I try to approach this logically.
Fogging is caused by condensation - warm, moisture laden air passing over a cold surface, the water condenses out onto the surface. It’s meteorological.
I need to equalise the temperature somehow.
To clear the rear window, you put the inbuilt heating element on - this heats up the glass and stops the condensation. Heating up the windscreen - by using the hot air blower - does not have the same effect. It doesn’t prevent the condensation, it makes it worse, except for a thin strip at the bottom, which clears a bit. Hmmm. OK - so there’s even more moisture in the air now, with it being warmer. (And I’m melting…) So I need to take the moisture out of the air - dry it out. I stick on the air conditioning and the air recycling, whizzing the moist warm air through the drier, in effect, and not letting any new damp stuff in. This isn’t very effective, but does reach halfway up the windscreen. Something is working, I’m just not sure what.
SO - if heating up the screen fails, try the opposite! Cool the air down.
Turn down the heater, and blow cold air across the screen. This still fails to give me visibility. Take it down a notch - I stick on the air conditioning, turn up the blower, and wheee! The screen clears almost immediately. I freeze, but I can now see where I’m going.
So the answer, at least in warm-ish weather, is to cool down the air that’s actually hitting the windscreen - effectively putting a thin layer of icy air between the screen and the moist air. Now that’s quite logical, but given the meteorology involved, it leaves me with one question. When the moist warm air hits the icy layer, why doesn’t it condense out? My guess is that the cold air is moving too much - but I still wanna know - why aren’t there clouds forming just above my head?