Fog/Mist coming from ac vent. Air is cold. What is going on?

If it happens only briefly, and there's no smell, it's just humidity condensing out of the air. Nothing to worry about.

If it's continuous, and there's a sweet smell, it's antifreeze, coming from a leaking heater core. That's a problem that will leave you stranded by the side of the road with a fried engine before the summer is over.

It's normal to have some slight fog or misting from the vents on humid days as very cold AC air meets warm, humid interior air or outside air. Setting the controls to recirculate mode may help remove more moisture from the car interior and reduce fog effects. Or leave windows slightly open when parked to help vent out interior moisture/humidity. In some cases though, the HVAC condensate drain at the bottom of the heater /AC ductwork case may be clogged which backs up water/condensate which then may be blown out of vents as a mist or reduces the temperature of AC system airflow. Check to make sure the HVAC drain is not clogged. You should usually see a puddle or drips of water on the ground under the car on humid days when AC is on/ idling in Park...that is the condensate draining from the AC system.

If the mist smells sweet and sticks to vent areas or windows/windshield, that would be a heater core leak...best to fix that right away.

http://www.driveaccord.net/forums/51-8th-generatio...

Check to make sure that the drain tube is not blocked or restricted. If the condensation cannot drain, it can create a situation where there is mist / fog coming out of the vents