Had this happen to me at Fort Ord, CA (now-defunct Army base near Monterey) when I was flying OH-58 helicopters for the Army. We took off one evening when the dew point and temp were very close, but there was zero fog. As we climbed away, I noticed a "blurring" of the airfield lights behind us, and made a slight turn to get a better view. There was a wall of fog rolling across the ramp in all directions away from our takeoff point. I quickly called Base Ops (tower closed at night) and told them we were scrubbing the mission due to fog. The duty officer thought I was crazy, because looking out his windows, which faced the far side of the airport, it was severe clear. I told him to look out the door on the other side. Talking with him later, he said it was the weirdest thing he'd ever seen – a solid wall of fog expanding outward like a mushroom cloud... Within minutes, the entire field was solid IFR, and the fog even spread 2-3 miles over to the adjacent town (Marina).
My hypothesis is that the air was perfect for forming fog, but there was no "nucleus" for the vapor to coalesce around until we began our takeoff and cleared the edge of the pavement, kicking up some dust. That provided the initial nucleus for the fog to form, and once it started forming, it just self-propagated. Like I said, one of the weirdest meteorological events I've ever witnessed.
