[TheORKINMan]

I am curious as to how quickly extremely dense fog can form on an interstate in Florida. There have been reports picked up in state and national papers about an 18 car pile up that caused 10 deaths on I-75 due to what is described as a mix of fog and smoke from a fire. Highway Patrol had closed the highway due to weather conditions for three hours and opened the highway up at 3:30 AM. The pile up happened 15 minutes later. Emergency responders have stated that the fog/smoke mix was so thick that they had to find people by sound as you could not see anything even immediately in front of you.

Now Highway Patrol is stating that visibility was clear at 3:30 when they opened the highway. Is it possible for fog to form and become that thick within 15 minutes so as to cause this accidents or is it likely highway patrol misjudged the visibility when they opened the road?

[Bazmundo]

I'd imagine the key factor here would be the smoke and how it moved. Fog occurs when the air is saturated, so humidity is very high, though the air can be fairly clear in these circumstances. It takes an additional environmental factor, such as cooling in the evening or nightime, to cause the water vapour to condense into droplets and reduce the visibility. However, even in warm conditions with little temp change smoke particles can give the water vapour somewhere to condense like any natural aerosol will do, again reducing the visibility. If the air was only misty, then the smoke would have increased that to a fog; if it were already a fog then you get dense fog or smog. Assuming something blew the smoke away in a different direction (or the source was extinguished) then the air would simply be saturated and misty but good enough visibility.

Alternatively, a different kind of air could have been intruding and could have raised the level at which this action was occurring, again increasing visibility at road level?

We had a similar incident to this last Nov in SW England, a fireworks display and bonfire near to a motorway caused a dense bank of smog in already patchy fog conditions. Witnesses described a wall of white/black fog that they couldn't see through until they crashed. It was still foggy for sometime afterwards while the rescue and recovery effort was underway, but at the time of the incident a witness about 10 metres lower than the motorway said it was fairly clear.

Welcome to UKww by the way.