Why does a fog come out of your car vents when you turn on the air conditioner?

Smoke, Mist or Fog will come out of your A/C vents on hot, humid days. The reason for this normal phenomenom is the cold, dry air coming from your A/C vents is lower than the dewpoint of the interior cabin. Basically what this means is that cold, dry air is mixing with hot, humid air and causing the smoke. This is similar to when you open your freezer door and see the fog coming from the freezer. This is a normal condition and will eventually dissipate once the temperatures in the interior cabin and the A/C vents start to balance out. One way to quickly balance the temperature out is to keep the A/C system on recirculated intake air. Hope this answers your question.