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The Twin Towers' Dust Clouds

The 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 dust clouds reached the ground about 12 seconds after each tower started to collapse. Then they raced out in all directions. The explosion of the South Tower at 9:59 AM took bystanders by surprise, and many had to run for their lives. Some reported being picked up by the dense cloud. New York Daily News photographer David Handschuh recalled: "I got down to the end of the block and turned the corner when a wave-- a hot, solid, black wave of heat threw me down the block. It literally picked me up off my feet and I wound up about a block away." Others escaped into the temporary shelters of storefronts. All reported that there was complete darkness once the dust cloud had overtaken them.

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.

e x c e r p t
title: 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
authors: Jim Hoffman

... I made estimates based on photographs taken at approximately 30 seconds after the onset of the collapse. The photo in Figure 1 appears to have been taken around 30 seconds after the initiation of the collapse of the North Tower. The fact that the spire is visible directly behind Building 7 indicates the photo was not taken later than the 30 seconds, since video records show that the spire started to collapse at around 29 seconds. In this photograph, as in other ones taken around that time, the dust clouds still have distinct boundaries.

Figure 1. Photograph from Chapter 5 of FEMA's Building Performance Assessment Report.



The dominant energy source assumed to be in play during the leveling of each of the Twin Towers was the gravitational energy due to their elevated mass. The energy sinks included the thorough pulverization of each tower's concrete, the vaporization of water, and the heating of air and suspended concrete dust in the ensuing dust cloud. Estimates for these energies are:

energy, KWH source or sink
+ 111,000 falling of mass (1.97e11 g falling average of 207 m)
- 135,000 crushing of concrete (9e10 g to 60 micron powder)
ignoring water vaporization
- 400,000 heating of gasses (2e9 g air from 300 to 1020 K)
- 11,300,000 heating of suspended concrete (9e10 g from 300 to 1020 K)
assuming water vaporization sink was not supply-limited
- 1,496,000 vaporization of water (2.38e9 g water)
- 41,000 heating of gasses (2e9 g air from 300 to 373 K)
- 1,145,000 heating of suspended concrete (9e10 g from 300 to 373 K)



The amount of energy required to expand the North Tower's dust cloud was many times the entire potential energy of the tower's elevated mass due to gravity. The over-tenfold disparity between the most conservative estimate and the gravitational energy is not easily dismissed as reflecting uncertainties in quantitative assessments.

The official explanation that the Twin Tower collapses were gravity-driven events appears insufficient to account for the documented energy flows.

Hoffman admits that there is substantial uncertainty regarding several assumptions used to obtain the energy estimates -- in particular those regarding the contribution of turbulent mixing to the size of the dust cloud. The promised Version 4 of the paper will employ a more flexible approach for computing energy estimates.

Health and Environmental Disaster

The explosion and thorough pulverization of each of the Twin Towers created a health and environmental disaster without parallel in an urban environment.

e x c e r p t
title: WTCEO's Intro and Speech for the Conference
authors: Jenna Orkin

The towers also contained approximately 50,000 computers each made with four to twelve pounds of lead and this does not take into account the five other buildings that were destroyed. The tens of thousands of fluorescent light bulbs each contained enough mercury to contaminate a quarter of a city block. PCBs reached 75,000 times their previous record: ["PCBs were detected at high concentrations. The Toxic Equivalency (TEQ)... is 151pg/L. In previous harbor work...the highest observed PCB TEQ was 0.002pg.L." EPA Report, September 20, quoted in Fallout Gonzalez, Juan] The smoke detectors contained radioactive americium 241. ( EPA Policy Analyst Hugh Kaufman ) In early October, 2001, Dr. Thomas Cahill of the University of Davis at California found levels of very- and ultrafine particulates that were the highest he'd seen of 7000 samples taken around the world including at the burning Kuwaiti oil fields. Months after the disaster the EPA recorded hitherto unseen levels of dioxin.

page last modified: 2009-08-20
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The dust clouds were hundreds of feet deep.
The North Tower's dust cloud rose to the height of the World Financial Center towers, 400 feet to the west.
The dust clouds swallowed the financial district in Lower Manhattan.