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Background Attack Aftermath Evidence Misinformation Analysis Memorial

Volume of Dust

Volume of Dust Clouds Proves Demolition

Both of the Twin Towers exploded into vast clouds of dust. That the clouds expanded to five times the volume of the towers within 30 seconds of the initiation of their collapses is a conservative estimate.

If the collapses were merely gravity-driven, then any clouds of debris produced in the immediate aftermath should have occupied about the same amount of space as the intact towers before they had time to significantly mix into the surrounding air. The bulk of the clouds could only come from the expulsion of gases in the buildings as they collapsed, and the mixing of ambient air into the clouds. The contribution of mixing increases over time, and is unbounded. However, the dust clouds appear to expand more rapidly than can be accounted for by mixing. This implies that heat energy was being added to the clouds in order to cause the gases to expand, and/or water to vaporize.

Could the known energy sources have accounted for the pre-mixing expansion? This question is treated in some quantitative detail in the paper: The North Tower's Dust Cloud: Analysis of Energy Requirements for the Expansion of the Dust Cloud Following the Collapse of 1 World Trade Center. This paper estimates the dust cloud volume of the North Tower and shows that, even allowing for expansion due to mixing, the heat sink is many times greater than the tower's gravitational energy. Even without such quantitative analysis, it is clear that the gravitational energy of the towers alone could not have driven the dust clouds' expansion, given limitations on conversion of that energy to heat and the apparent absence of extensive mixing early in the clouds' development.

The expansion of the dust clouds presents problems for the gravity collapse theory that are evident without quantitative analysis. Here we consider the role of the two main factors that could have worked to expand the dust clouds.

  • Heating of the air due to friction of the collapse
  • Mixing of the cloud's gases and suspended solids with ambient air

Did Friction Multiply the Clouds' Volumes?

Suppose that nearly all of the gravitational energy of the towers was converted into friction and therefore heat. Would that have been sufficient to expand the dust clouds? A clue is that in a typical demolition, the volume of the dust cloud grows to only slightly larger than the intact building's volume immediately following the collapse. Even if the gravitational potential energy of the towers was great enough to drive the expansion, it is highly doubtful that much of it would be converted into heat in the dust clouds, for several reasons.

  • Rubble falling through the air would not generate much heat energy until it hit the ground, and then most of the energy would be converted to ground movement and the finer breakup of the rubble rather than heat.
  • Rubble crushing the building would convert much of its kinetic energy to friction in the steel frame in the process of shredding it. The steel frame would not have enough surface area to transfer much heat to the gases during the split second in which the building around any given piece of steel was crushed, so most of the heat would have ended up in the rubble pile.
  • If much of the gravitational energy was converted to heat through friction, it would have necessitated longer collapse times than were observed.
At least one
academic paper has attempted to explain the rapidity of the collapses by promoting a questionably applicable mathematical model alleged to predict a nearly frictionless total collapse. Since that model has each tower neatly pulling itself down at near the speed of free-fall, there would be very little heat produced to drive the dust cloud expansion.

Did Mixing Expand the Clouds?

Mixing of building air with ambient air could not account for the rapidity of the expansion of the dust clouds, nor their appearance. Mixing of gases can occur through diffusion or convection. Diffusion is not relevant, since it is the space occupied by suspended particles that defines the volume of the cloud. Convection could only expand the cloud if there was a high degree of turbulence on the cloud's boundary, and would have produced a diffuse boundary. That does not appear to have occurred in the early stages of the Twin Towers' dust clouds. The clouds maintained well-defined interfaces as they expanded to many times the buildings' volumes. Moreover, features on the surface of the clouds evolved slowly relative to the movement of large portions of the cloud. The distinct boundaries and persistent shapes mean the clouds were expanding primarily by pushing aside the ambient air, not by assimilating it.

page last modified: 2011-09-08