I have seen the question asked many times, usually from someone in a car or plane, “what is that white ‘smoke’ coming out of the air conditioning vents?” Since the appearance of smoke in a car or plane should be cause for concern, the typical follow up question is “why is no one doing or saying anything about it?” Thankfully, the phenomenon has nothing to do with actual smoke but is instead just water condensing out of the air and quickly re-evaporating.

The air in the car or plane when you first get in it is usually warm and humid with a relatively high dew point (assuming it’s a day you plan to use the A/C). For reference, the dew point is the temperature at which air reaches 100% humidity and water droplets form. To illustrate dew point (and why it’s called dew point), consider a summer morning where you notice water (dew) all over the grass even though it didn’t rain. Some time during the night, the air temperature fell below the dew point and some moisture condensed out of the air and onto the grass. As the air temperature warms up during the morning that water will evaporate back into the air.

Now back to our original scenario. The cold air coming out of the A/C vents mixes with the warm humid air in the cabin causing it to drop below the dew point. This forces some water out of the air and into vapor forming mist/fog/cloud in the space. It doesn’t last long because as the air continues to mix, it goes back above the dew point and the droplets re-evaporate. If the air stayed too cold, you would eventually see moisture start to build up on surfaces since the water wasn’t fully evaporating.

This process is obviously nothing to worry about and that answers the followup as to why no one is doing anything about it. If you are interested in more info on how airplanes air conditioner the air, be sure to check out our Airliner HVAC article. Finally, here’s a great video of the phenomenon along with everyone ignoring it.

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