Why does defrosting a windscreen with both hot and cold air work?

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Shouldn’t only one work and the other make it worse?

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Blowing plain vent air will work, but it will take a long time.

Blowing air-conditioned vent air will work better, because one of the effects of air conditioning is that it removes moisture from the air. So you’re blowing cool, dry air.

Blowing warmed-up air-conditioned air works best. The air conditioning dries it out, and the heat warms it up without adding any more moisture, so now you’re blowing warm dry air.

Your windshield is actually two panes of glass stuck together. It fogs up when there’s a difference in temperature between the two. So either cold or hot air works by making both the same temperature.

The biggest factor of defrosting is the humidity of the air used. As in, more than 80% of the effectiveness. When the designers of the air conditioning realized this, their designs started to focus on first drying out the air used before any other considerations. Warm air has more potential to carry moisture away and is more comfortable if it’s literally de’frost’ing and not just defogging so it is a secondary concern, but cool dry air is still more effective than warm moist air.

The air is warmer that frozen water, either by temp, or the friction of the air rubbing against the surface of the screen or ice, which technically warms it.

It has to to do with the a/c running when the defoggers turn on. A/c draws the moisture out of the air whether the temp is on hot or cold