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
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:
||source or sink
||falling of mass (1.97e11 g falling average of 207 m)
||crushing of concrete (9e10 g to 60 micron powder)
ignoring water vaporization
||heating of gasses (2e9 g air from 300 to 1020 K)
||heating of suspended concrete (9e10 g from 300 to 1020 K)
assuming water vaporization sink was not supply-limited
||vaporization of water (2.38e9 g water)
||heating of gasses (2e9 g air from 300 to 373 K)
||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
The towers also contained approximately
50,000 computers each made with four to twelve pounds of
and this does not take into account the five other buildings that were
destroyed. The tens of thousands of fluorescent light bulbs each contained
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
Gonzalez, Juan] The smoke detectors contained radioactive
( 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
page last modified: 2009-08-20