My Octavia occasioally suffers from catastrophic "steaming up" mid drive. As in, you're driving along, ambient temp is about 3 degrees, climate control on auto and set to 20 degrees.... then BAM! You're blind. Whole care mists up in less than a minute. And takes a good 10 minutes with full fans etc to clear.

What's the problem? The only thing I can think of is maybe the climate control kicking on the air con for a sec or two to lower the temp?

Or could there be a coolant leak in the actual heater? I had to top-up the coolant the other day, even though the car was serviced at the end of Sept (btw, no leakage or puddles, and no blown head gasket symptoms). There is a mild copper like smell after the mist clears.

I hate the way my car has all these little annoying problems!

At 3c the aircon compressor is disabled, so it could be the lack of A/C triggering the steam up. Once the climate gets to 20c, it will slow the fans and this could be when the steam forms as the airflow is insufficent to prevent moisture buildup.

Could be a leaky heater matrix?

Ditto. Had this problem on the M6. All of a sudden, couldnt see a thing, it turned out to be a a pin hole in the heater matrix sending a nice spray of steam up the ventilator pipe work.

When the air con is running, it dries the air as well as cooling it. With the climate control set to AUTO, the air con will be engaged all the time as long as the outside temperature is above (I think) 5 degrees C.

Could it be that you're driving into a bank of cooler air and the air con is being switched off?

Cheers guys. Where is the heater matrix if I want to look for a leak?

My money is on that, esp with coolant levels dropping.

why the moisture is rising, dunno but to the question why it deposits on your wind screen would be because they are dirty. Clean them with vinegar or amonium then apply a product that stops water from condensing