I read this in Popular Mechanics. "Always use the fresh-air setting rather than the re-circulate setting when trying to remove moisture from the windshield even when it's pouring rain outside. On damp, rainy days there's probably enough moisture permeating the interior fabrics, the occupants' clothes and the carpets to keep the humidity in the car near 100% for a long time. But cool, fresh air, even at 100% relative humidity outside the car, will have far less relative humidity when it's heated up in the heater core and then cooled off in the a/c evaporator. The Recirc air does go through the same heating, cooling and moisture-removal process in the heater, but it picks up plenty of moisture inside the car before it gets sucked back in for another round. That's why some cars automatically use fresh air in Defrost mode, giving you no choice." Hope this helps.

And then there's always Rain-X for the inside of the windshield, but I've only used it a couple times and haven't noticed a huge difference.

 

The article is generally on point. One thing that is wrong is that air gets cooled in the A/C first to condense water out of it, then heated in the heater. On Civics before a certain year (1996?) you have to engage the A/C manually, later ones will do it automatically when defrost is selected.

When you engage the A/C the air leaving the A/C to the heater is always 40 F and 100% RH no matter what the source. Thus the design to cool first then heat, as heating moist air will reduce its RH even though the water content is the same. When the air outside the car is below 40 F you should use it because it has less water by mass than the likely saturated and warmer air from the A/C.

Whenever it is below 40 F the A/C will not engage anyway.

But there is another concern that when you first start the car there is little output from the heater because the engine is cold. So during this phase of operation it may be an advantage to use interior air because it is warmer.