system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do I sometimes have to use cold air to defrost my windshield and at other times hot air?
gpt
There are a couple things going on here. First, hot air can hold more moisture than cold air. The reason condensation forms in the first place is that the glass in the windshield is cold enough that when it comes into contact with warm, moist air, the moisture comes out of the air and condenses onto the glass.

Your ultimate goal to prevent having a foggy windshield should be to warm up the glass so that it’s above the condensation point. How warm that is depends on the temperature of the air in your car. When you start blowing warm air over the glass it may make it a little worse initially (since the glass is still cold and moisture from the hot air you’re blowing over it will also condense). However, once you’ve warmed the glass up enough the moisture will disappear and stay in the air like you want it to. This is how most rear defrosters work – they just have heating elements that heat the glass up. No air involved.

The A/C sometimes helps because A/Cs put out cold, dry air. If the air is dry it will be able to absorb some of the moisture from the windshield. However, if you blow cold air over the windshield you will keep the glass cold so it will keep fogging up. 

The best thing you can do is to use both at once. Put the defroster on to blow hot air onto the windshield and blow the A/C in the car. That way you have cooler air with less moisture in the car so there’s less moisture that can condense on the glass and you are warming up the glass past the condensation point so there won’t be any more fog at all.