The Twin Towers' Dust CloudsThe shredding of each of the Twin Towers was hidden behind dense clouds of concrete dust that billowed out from the moment each towers' top began its plunge. These clouds first emerged from the towers from around the crash zones, and grew rapidly as they descended. The clouds grew to several times each tower's intact volume before they even reached the ground, and continued to grow after each of the towers had vanished.
From across the Hudson, people watched the burgeoning North Tower dust cloud engulf the 500-foot tall World Financial Center buildings.
The monstrous dust clouds helped to give the attack on Manhattan the feeling of a Hollywood movie, despite the carnage being all too real.
The dust clouds provide important clues about how the towers were destroyed. Researcher Jim Hoffman used estimates of the volume of the dust cloud from the North Tower to compute a lower bound of the energy involved in the destruction. His argument is that, barring the involvement of explosives, and after factoring out possible contributions of mixing, the expansion of the dust cloud early in the event required the input of vast quantities of heat energy, either to expand the gases thermodynamically, or to vaporize moisture. He estimates that the minimum energy required was on the order of 1.5 gigawatt-hours.