Driving up SR-18 near Tiger Mountain, my rear view mirrors suddenly fogged over...though there was no fog outside. Why did that happen?
When I flew airplanes, we occasionally encountered what we called 'splat icing'. That was caused by a very cold airplane flying into an area of moisture. The cold surface of the airplane caused rapid freezing of the moisture. In your case, I believe your cold car (you mentioned 27 degrees) drove into an area of increased moisture. The air in the valley wasn't cold enough to cause condensation of the moisture into fog, but the surface of your colder car probably caused enough additional cooling to condense the water vapor in the air into fog. Keep in mind that although the temperature was below freezing, water vapor can exist in a 'super-cooled' state...at temperatures below freezing. That's why it formed fog on your rear-view mirrors, and not ice.