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WTC Attacks : The North Tower's Dust Cloud
Posted by webmaster on 2003/9/29 6:25:28 (4775 reads)

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

by Jim Hoffman, October 16th, 2003
[Version 3.0]


This paper uses photographic evidence -- primarily a reference photograph taken from FEMA's report -- to estimate the volume of the dust cloud that grew from the collapse of the North Tower at about 30 seconds after the commencement of the collapse. The paper then estimates the thermal energy required to produce the observed expansion in the volume of the dust cloud, based on the assumption that most of the gasses and suspended solids in the cloud originated from within the building.

The most recent version of the paper identifies two major mechanisms for the expansion -- thermodynamic expansion of gasses due to increases in temperature, and expansion due to the vaporization of water.Both represent vast energy sinks.Whatever the relative contributions of these mechanisms to the expansion,the required energy inputs far exceeds the energy available in theform of the gravitational potential energy due to the tower's elevated mass.

Previous versions of the paper did not consider expansion due to water vaporization, and considered only thermodynamic expansion of gasses present in the building at the time of collapse. That required average dust cloud temperatures of around 1000 K, a feature several people found implausible. The addition of the heat of water vaporization to the analysis changes the picture dramatically. The heat energy requirements are similar, but the temperatures need not have been anywhere near 1000 K, since the phase change of water to steam occurs at 100 C.

The paper shows a large disparity between the energy required to produce the observed expansion of the dust cloud and that available from the conversion of all the tower's gravitational potential energy to heat. It does not consider the possible energy source of the unlikely rapid combustion of the tower's contents during its collapse, but even the energy available from consuming all of the oxygen in the tower to burn hydrocarbons is far short of the estimated size of the energy sink of dust cloud expansion.

On September 11th, Both of the Twin Towers disintegrated into vast clouds of concrete and other materials, which blanketed Lower Manhattan. This paper shows that the energy required to produce theexpansion of the dust cloud observed immediately following the collapse of 1 World Trade Center (the North Tower) was much greater than the gravitational energyavailable from its elevated mass. It uses only basic physics.


Vast amounts of energy were released during the collapse of each of the Twin Towers in Lower Manhattan on September 11th, 2001. The accepted source of this energy was the gravitational potential energy of the towers, which was far greater than the energy released by the fires that preceded the collapses. The magnitude of that source cannot be determined with much precisionthanks to the secrecy surrounding details of the towers' construction. However, FEMA's Building Performance Assessment Report gives an estimate: "Construction of WTC 1 resulted in the storage of more than 4 x 10^11 joules of potential energy over the 1,368-foot height of the structure. "That is equal to about 111,000 KWH (kilowatt hours) per tower.

Of the many identifiable energy sinks in the collapses,one of the only ones that has been subjected to quantitative analysisis the thorough pulverization of the concrete in the towers. It is well documented that nearly all of the non-metallic constituents of the towers were pulverized into fine powder. The largest of these constituents by weight was the concrete that constituted the floor slabs of the towers. Jerry Russell estimated that the amount of energy required tocrush concrete to 60 micron powder is about 1.5 KWH/ton. (See paper incorrectly assumes there were 600,000 tons of concretein each tower, but Russell later provided a more accurate estimate of 90,000 tons of concrete per tower, based on FEMA's description of the towers' construction. That estimate implies the energy sink of concrete pulverizationwas on the order of 135,000 KWH per tower, which is already larger than the energy source of gravitational energy. However, the size of this sink is critically dependent on the fineness of the concrete powder, and on mechanical characteristics of the lightweight concretethought to have been used in the towers. Available statistics about particle sizes of the dust, such as the study by Paul J. Lioy, et al., characterize particle sizes of aggregate dust samples, not of its constituents, such as concrete, fiberglass, hydrocarbon soot, etc. Based on diverse evidence, 60 microns would appear to be a high estimate for average concrete particle size, suggesting 135,000 KWH is a conservative estimate for the magnitude of the sink.

A second energy sink, that has apparently been overlooked, was many times the magnitude of the gravitational energy: the energy needed to expand the dust cloudsto several times the volume of each tower within 30 seconds of the onset of their collapses. Note that the contents of the dust clouds had to come from building constituents --gases and materials inside of or intrinsic to the building --modulo any mixing with outside air. Given that the Twin Towers' dust clouds behaved like pyroclastic flows, with distinct boundaries and rapidly expanding frontiers(averaging perhaps 35 feet/second on the ground for the first 30 seconds), it is doubtful that mixing with ambient airaccounted for a significant fraction of their volume. Therefore the dust clouds' expansion must have been primarily due to an expansion of building constituents. Possible sources of expansion include:

  • thermodynamic expansion of gases
  • vaporization of liquids and solids
  • chemical reactions resulting in a net increase in gaseous phase molecules
The evidence does not support the idea that chemical reactions in the dust cloud liberated vast quantities of gases. That leaves increases in gas temperatures and vaporization of solids and liquids, primarily water, to drive the expansion.

How much heat energy was involved in expanding the dust clouds?To calculate the energy we need to answer three questions:

  1. What was the volume of the dust clouds from a collapse at some timesoon after it started(before the clouds began to diffuse)?
  2. How did the mixing of the dust cloud with ambient air contribute to its size,and how can this be factored out to obtain the volume occupied by gases and suspended materials originally inside the building?
  3. What is the ratio of that volume to the volume of the intact building?
  4. How much heat energy was required to produce that ratio of expansion?

Since I have better photographs for North Tower dust,I did the calculation for it.

1. Quantifying Dust Cloud Volume

To answer question 1, 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 the around 29 seconds. In this photograph, as in other ones taken around that time, the dust clouds still have distinct boundaries.

The WTC Dust Cloud
Figure 1. Photograph from Chapter 5 of FEMA's Building Performance Assessment Report

I used landmarks in this photo to make several approximate measurementsof the frontier of the dust cloud. The following table lists some of them.Measurements are in feet. The first column lists heights above the street,and the second lists distances from the vertical axis of the North Tower.





3 230 1011 west corner of 45 Park Place
5 228 729 top of south corner of building with stepped roof
6 204 658 east corner of Building 7, 30 stories below top
7 600 776 upwell towering over southeast end of Post Office
8 700 ? upwell slightly higher than the top of Building 7
11 190 870 top of west corner of 22 Cortland St tower
12 508 588 8 stories below top of face of WFC 3
13 498 517 3 stories below top of upper face of WFC 2

To approximate the volume I used a cylinder, coaxial with the vertical axis of the North Tower, with a radius of 800 feet, and a height of 200 feet.All the above reference points lie outside of this volume.Although the cylinder does not lie entirely within the dust cloud, there are large parts of the cloud outside of it, such as the 700 foot high upwelling column south of Building 7. The cylinder has a volume of:

pi * (800 feet)^2 * 200 feet = 402,000,000 feet^3.
I subtract about a quarter for volume occupied by other buildings,giving 300,000,000 feet^3.

2. Factoring out Mixing and Diffusion

To accurately answer question 2 would require detailed knowledge of the fluid dynamics involved. However it does appear that for at least a minute,the dust cloud behaved as a separate fluid from the ambient air, maintaining a distinct boundary. There are several pieces of evidence that support this:

  • The WTC dust clouds inexorably advanced down streets at around 25 MPH.This is far faster than can be explained by mixing and diffusion.
  • As the dust clouds advanced outward, features on their frontiersevolved relatively slowly compared to the clouds' rates of advance. This indicates that that clouds were expanding from within and that if surface turbulence was incorporating ambient air, it's contribution to expansion was minor.
  • The top surface of the clouds looked like the surface of a boiling viscous liquid - churning but not mixing with the air above. Sinking portions of the clouds were replaced by clear air, not a mixture of the cloud and air.
  • The dust clouds maintained distinct interfaces for well over a minute.Mixing and diffusion would have produced diffuse interfaces.
  • There are reports of people being picked up and carried distances by the South Tower dust cloud, which felt solid.New York Daily News photographer David Handschuh recalled:
    Instinctively I lifted the camera up,and something took over that probably saved my life. And that was [an urge] to run rather than take pictures. 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.
Initially the dust clouds must have been much heavier than air, given the mass of the concrete they carried and the distances they transported it. As time went on the cloud became more diffuse, but all of the photographsthat can be verified as being within the first minutes how opaque clouds with distinct boundaries, indicating the dominant mode of growth was expansion, not mixing or diffusion. It seems reasonable to assume that mixing with ambient air did not account for a significant fraction of the expansionin the volume of the dust cloud by 30 seconds of the startof the North Tower collapse. Nevertheless, I reduce the estimate of the dust cloud volume of building originto 200,000,000 feet^3,imagining that a third of the growth may have been due to assimilation of ambient air.

3. Computing the Expansion Ratio

The answer to question 3 is easy. The volume of a tower, with it's 207 foot width and 1368 foot height, is:

1368 feet * 207 feet * 207 feet = 58,617,432 feet^3.

So the ratio of the expanded gasses and suspended materials from the tower to the original volume of the tower is:

200,000,000 feet^3 / 58,617,432 feet^3 = 3.41.

4. Computing the Required Heat Input

Above I identified two energy sinks that could have driven expansion of the dust cloud: thermodynamic expansion of gases, and vaporization of liquids and solids. Since most constituents and contents of the building other than water would require very high temperatures to vaporize, I consider only the vaporization of water in evaluating the second sink.

It is clearly not possible to determine with any precision therelative contributions of these two sinks to the expansion of the dust cloud. If the cloud remained uniform in temperature and density for the first 30 seconds, then the expansion would consist of three distinct phases:

  • The temperature would increase to 100 C, accompanied by thermodynamic expansion.
  • The temperature would remain at 100 Cuntil all of the water was vaporized.
  • The temperature would increase above 100 C,again accompanied by thermodynamic expansion.
Since such uniform conditions were not present,I will first treat the two energy sinks separately,and will compute the energy requirements for each if it alone were responsible for the expansion.

4.1. The Thermodynamic Expansion Sink

The ideal gas law can be used to compute a lower boundfor the amount of heat energy required to induce the observed expansion of the dust cloud, assuming that the expansion was entirely due to thermodynamic expansion. That law states that the product of the volume and pressureof a parcel of a gas is proportional to absolute temperature. It is written PV = nRT, where:

P = pressure
V = volume
T = absolute temperature
n = molar quantity
R = constant
Absolute temperature is expressed in Kelvin (K),which is Celsius + 273. Applied to the tower collapse, the equation holds thatthe ratio of volumes of gasses from the building before and after expansionis roughly equal to the ratio of temperaturesof the gasses before and after heating. That allows us to compute the minimum energy needed to achieve a given expansion ratio knowing only the thermal mass of the gasses and theiraverage temperature before the collapse.

I say that the ideal gas law allows the computation of only the lower boundof the required energy input due to the following four factors.

  • The finite size of molecules leads to a slight departure fromthe ideal gas law wherein the expansion of a parcel of gasleads to a decrease in its temperature. This means that slightly more heat energy is needed to achievea given expansion ratio than is predicted by the ideal gas law.
  • The dust cloud at the time of the photograph used to estimate its volumehad not finished expanding. Videos show that it continued to expand well after the 1 minute mark.
  • The suspended dust in the cloud had many times the mass of the gasses. This increased the energy needed to expand the dust cloudsince it takes energy to lift and accelerate mass.
  • The suspended dust in the cloud had many times the thermal mass of the gasses. Increasing in temperature of the dust cloudto a level needed to induce the observed expansion entailed raising the temperature of the gasses and suspended solids by similar amounts. Since the solids had many times the thermal capacity of the gasses,this multiplied the energy requirements.

In this paper I examine only the fourth factor.Before considering its effect on energy requirements,I first consider the energy requirements of heating only the gassesin the clouds to the level needed to achieve the observed expansion.

According to the ideal gas law, expanding the gasses 3.4-fold requires raising their absolute temperature by the same ratio.If we assume the tower was at 300 degrees K before the collapse, then thetarget temperature would be 1020 degrees K, an increase of 720 degrees. Given a density of 36 g/foot^3 for air, the tower held about 2,000,000,000 g of air. Air has a specific heat of 0.24 (relative to 1 for water),so one calorie will raise one g of air 1 / 0.24 = 4.16 degrees.To raise 2,000,000,000 g by 720 degrees requires:

2,000,000,000 g * 720 degrees * 0.24 = 345,600,000,000 calories  = 399,500 KWH
To evaluate the energy requirements of the fourth factor, it is necessary to consider the composition of the dust cloud. The cloud was a suspension of fine particles of concrete and other solids in gasses consisting mostly of air. Since concrete was the dominant solid, I will ignore the others, which included glass, gypsum, asbestos, and various hydrocarbons.The small size of the particles, being in the 10-60 micron range, would assure rapid equalization between their temperature and that of the embedding air. Therefore any heat source acting to raise the temperature of the airwould have to raise the temperature of the suspended concreteby the same amount. Assuming all 90,000,000,000 g of concrete was raised 720 degrees (300 K to 1020 K), the necessary heat, given a specific heat of concrete of 0.15 is:
90,000,000,000 g * 720 degrees * 0.15 = 9,720,000,000,000 calories = 11,300,000 KWH.

If we assume that the water vaporization sink absorbed all available energy once temperatures reached water's boiling point, we can compute the size of the heat sink of thermodynamic expansion that was in play up to 100 C, or 373 K:

2,000,000,000 g * 73 degrees * 0.24 = 35,040,000,000 calories =  40,744 KWH
The associated sink of heating the suspended solids to this temperature would be:
90,000,000,000 g * 73 degrees * 0.15 = 985,500,000,000 calories = 1,145,000 KWH.

4.2. The Water Vaporization Sink

At 100 C at sea-level, water expands by a factor of 1680when converted to steam. Hence it is reasonable to expect that water in the building accounted for a significant part of the expansion. How much energy would be required to expand the volume of the cloudby the 3.41 ratio if water vaporization were entirely responsible for the expansion? Since water vaporization involves the introduction of volumes steam from comparatively negligible volumes of water, I assume that all the incremental volume was occupied by steam.The estimated 3.41 expansion ratio means that the incremental volume was:

200,000,000 feet^3 - 58,617,000 feet^3 = 141,383,000 feet^3 = 4,003,542,000 liters
Given the 1680 to 1 ratio between the volume steam and water,2,383,000 liters of water would have been required. The heat of vaporization of water is 540 calories/gram at 100 C. Therefore the heat energy required to produce the expansion is:
2,383,000,000 g * 540 = 1,286,820,000,000 calories  = 1,496,000 KWH

Was there enough water in the building for this sink to be anywhere near this large? That is a matter of great uncertainty. Even well-cured concrete has a significant moisture content. Assuming that the estimated 90,000 tons of concrete in the tower was 1 percent water by weight, that would have provided 900 tons of water or about 900,000 liters -- well short of the 2,383,000 liter estimate above. However, there is a large amount of uncertainty in thewater content of the concrete, which, like the rest of the remainsof the disaster was apparently disposed of with little or no examination. Moreover there were other sources of water in the building, such as the plumbing system, which could have accounted for tens of thousands of liters, and, gruesomely, people. The thousand victims never identified could have accounted for about 30,000 liters of water.

4.3. Which Energy Sink Was Dominant?

Both thermodynamic expansion and water vaporization have the capacity to produce vast expansion in gas volume given sufficient heat. Two major difference in the features of these sinks may help in understanding the relative contributions of each. First, thermodynamic expansion to the observed ratio requires very high temperatures, whereas vaporization-driven expansion occurs at a constant temperature of 100 C. Second, vaporization-driven expansion would be limited by the available supply of water.

If all the expansion was due to thermodynamic expansion, it would require that the dust cloud was heated to an average temperature of about 1020 K. Certainly the temperatures of the cloud near the ground were no-where near that high.Eyewitness reports show that the cloud's ground-level temperaturesmore than a few hundred feet away from its center were humanly survivable. Most of these reports are from the South Tower collapse,and it is unclear how similar the dust cloud temperatures following the two collapses were. Although serious fires raged in Buildings 4, 5, and 6, other nearby buildings that suffered extensive window breakage from the tower collapses, such as the Banker's Trust Building, and Word Financial CenterBuildings 1, 2, and 3, did not experience fires. Digital photographs and videos show a bright afterglow with a locus near the center of the cloud, commencing around 17 seconds after the onset of the North Tower's collapse. Once the afterglow started, the cloud developed large upwelling columns towering to over 600 feet, and the previously gray cloud appeared to glow with a reddish hue. This suggests that at lest the upper and central regions of the North Tower cloud reached very high temperatures, but the evidence is insufficient to draw even general quantitative conclusions about the ranges and distributions of temperatures.

If enough water was present for vaporization to drive most of the expansion,temperatures in much of the cloud would have remained around 100 C until most of the water had vaporized. Thermodynamic expansion would occur in regions with liquid phase wateruntil 100 C was reached, and again after the water was vaporized.

To the extent that thermodynamic expansion was the dominant factor driving the expansion, the distribution of concrete dust in the cloud, and its relationship to the temperature distribution in the cloud, would greatly affect the total energy requirements. Less energy would be required if the hotter portions of the cloudhad a lower density of dust. The density was probably greater toward the central portions of the cloud, which also seem to have experienced the most heating. On the other hand, much of the dust may have settled out by the 30 second mark. The violent churning of the cloud, and the opaque appearance of its frontier,suggest that most of the dust had not settled that early.


The dominant energy source assumed to be in play during the leveling of each of the Twin Towers was the gravitational energy due to its elevated mass, whereas the energy sinks included the pulverization of it's concrete, the vaporization of water, and the heating of the concrete and air 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 imbalance between sources and sinks is striking, no matter the relative shares of the thermodynamic and water vaporization sinks in accounting for the expansion. Moreover, it is very difficult to imagine how the gravitational energy released by falling mass could have contributed much to any of the sinks, since the vast majority of the tower's mass landed outside its footprint. The quantity for the crushing of concrete appears to be conservative since some reports indicate the average particle size was closer to 10 microns. The quantity for the heating of suspended concrete has a large amountof uncertainty, but the energy imbalances remain huge even when it is ignored entirely.All of these energy sink estimates are conservative in several respects.

  • It is based on an estimate of dust cloud volume at a timelong before the cloud stopped growing.
  • It uses a liberal estimate of the contribution of mixing to the volume.
  • It ignores thermal losses due to radiation.
The calculation also ignores the role the massof the suspended materials in impeding the expansion of cloudand thereby increasing the required energy.


The amount of energy required to expand the North Tower's dust cloudwas many times the entire potential energy of the tower's elevated mass due to gravity.The over 10-fold 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.

Revision History

The paper is now in its third version.A complete version history is archived here.

Version 2 adopts much smaller estimates of concrete and total building mass, and refines the argument that mixing could not have accounted for much of the expansion. Version 3 considers a source of expansion ignored in the earlier versions -- the vaporization of water.


I wish to thank Jerry Russell, proprietor of, for his work on the physics of the World Trade Center collapses, work which was invaluable in the development of my thermodynamics analysis.

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