Reply #42: Around 20m/s.

[Make7]

I got just over that for the first video and just under for the second one. (Although it is difficult to make precise measurements on the quality of video usually obtained in the internet.) To reach these numbers I stepped through the videos frame by frame to get as close of an estimate as I could. Those are average speeds - the dust does seem to slow down at the end of its travel.

If one assumes that explosives were in the core area, the dust from a blast could have slowed down a great deal by the time it got to the perimeter of the building - and the louvers would have slowed it down even more I imagine. (BTW - I don't recall ever seeing any pictures of the louvers.) But if they were shaped charges, would the dust clouds even reach the perimeter? Or were there explosives placed near the perimeter?

In the first video, when the dust appears it is concentrated in the middle, then it spreads out horizontally and then the center section seems to continue moving out much more than the rest of the dust. It just doesn't look like the few explosive squibs I've seen - although the fact that the air movement from the debris falling above the mechanical floors is also having an influence on it might have a lot to do with why it doesn't look the same.

As far as the air handling system is concerned, I doubt if most buildings being demolished had running air conditioning systems when they collapsed. And I really don't know enough about air conditioning systems to know if this could account for the amount of dust seen. However, I do think it is possible.

This venting looks like it is occurring on the bottom mechanical floor to me. Now we just need to find someone that knows what the hell is on those mechanical floors. (I'm not gonna hold my breath on that one.)

- Make7