Why does my car automatically switch to "outside air" whenever I try to defrost my front windshield?

Whenever I want to defrost my front windshield, the car changes from %26quot;circulate interior air%26quot; to %26quot;outside air%26quot; by itself. I can force it to circulate interior air as long as I hold down the button that changes my air options while I defrost. The defroster still works just fine either way! This seems to be the same for any car I'm in, whats the point?Why does my car automatically switch to %26quot;outside air%26quot; whenever I try to defrost my front windshield?Humidity. Outside air has much less of it, which means it will be more effective in evaporating the moisture that has condensed on the inside of the glass. The inside air is more humid because of you, your breath in fact. Water vapor saturates the interior, which is why you need a defroster in the first place. They make the switch automatically so you do not have to remember to do it. The defroster does NOT work fine either way. Yes, it gets WARMER either way, but no, it does not defrost as well with the more humid interior air as it does with the dryer outside air. The person above me who said the interior air is dryer is wrong. The object is to warm the interior surface to evaporate the moisture which has condensed there and dryer air is more effective than humid air when it comes to evaporating water.
Why does my car automatically switch to %26quot;outside air%26quot; whenever I try to defrost my front windshield?
The quickest way to defrost (defog) a windshield is to have the two sides as close as possible in temperature and especially in humidity. (The moisture content of the air hitting the glass surface.) Since the recirculating air is being heated or cooled in a closed environment, it tends to become drier than the outside air. Switching to %26quot;fresh%26quot; air helps match the air better. Remember that the defroster (defogger) setting is considered a safety setting where all other settings are merely comfort settings. Part of the pretrip inspection of a commercial vehicle is making sure the defroster works properly. The heater/AC settings don't matter.
Why does my car automatically switch to %26quot;outside air%26quot; whenever I try to defrost my front windshield?
Thats what there designed to do.
cause if you close the vents, it's slower to defog. and sometimes leaving your vents close actually causes it to fog up.



in your case, your melting ice, but the main purpose is for it to remove fog. and it works best with the vents open.
Additionally, cars equipped with A/C automatically blend heater air with 10% A/C air to lower the humidity and defog the windows faster.