The Fires' Severity
How Intense and Extensive Were the Twin Towers' Fires?
The plane crashes resulted in significant fires in both Towers, at least for the first few minutes after the crashes. The fires in the North Tower were considerably more extensive than than those in the South Tower. As time progressed the fires in at least the South Tower appeared to diminish greatly in severity. This was probably due to most of the jet fuel being exhausted within a few minutes of the impacts. Since kerosene (jet fuel) has a low boiling point and a low flash point, most of it would have evaporated and caught fire quickly.
The Fires at Their Most Severe
How severe were the fires at their greatest extents?
- Fires in the North Tower covered extensive regions, at least near the perimeter walls, of about three floors. Fires in the South Tower also extended over about three floors, but were more localized to one side of the building.
- The fires were not hot enough to produce significant window breakage in either Tower. Window breakage is a common occurrence in large office fires, particularly when temperatures exceed 600° C.
- The flames mostly remained within the buildings. Significant emergence of flames from the buildings, another common feature of large office fires, was only observed in a limited region of the North Tower.
- The fires did not spread significantly beyond the impact region. With the exception of a region of fire about 10 floors above the crash zone in the North Tower, the fires remained around the impact zones.
- The fires did not cause parts of the building to glow. At temperatures above 700° C, steel glows red hot, a feature that is visible in daylight.
The Fires' Progression Over Time
|Most photographs of the South Tower show relatively dark smoke, and in much less quantity than from the North Tower. See photographs.|
Given that the vast majority of the volatile jet fuel was consumed inside five minutes of each crash, the fires subsequently dwindled, limited to the fuels of conventional office fires. The fires in both Towers diminished steadily until the South Tower's collapse. Seconds before, the remaining pockets of fire were visible only to the firefighters and victims in the crash zone. A thin veil of black smoke enveloped the Tower's top. In the wake of the South Tower's fall new areas of fire appeared in the North Tower.
This summary is supported by simple observations of the extent and brightness of the flames and the color and quantity of smoke, using the available photographic and video evidence.
- Visible flames diminished greatly over time.
Significant emergence of flames from the building
is only seen in a region of the North Tower 10 stories
above the impact zone.
- South Tower: Virtually no flames were visible at the time of its collapse.
- North Tower: Flames were visible in several areas at the time of its collapse. A region of flames on the 105th floor is seen after the South Tower collapse.
- The smoke darkened over time.
While the fires in both Towers emitted light gray smoke
during the first few minutes following the impacts,
the color of the smoke became darker.
- South Tower: Smoke from the fires was black by the time it collapsed. At that time it was only a small fraction of the volume of the smoke from the North Tower.
- North Tower: Smoke from the fires had become much darker by the time the South Tower was struck, 17 minutes after the fires were ignited. The smoke was nearly black when the South Tower collapsed. Thereafter the smoke appears to have lightened and emerged from the building at an accelerated rate.
|After the fall of the South Tower, the North Tower continued to produce prodigious quantities of smoke, and showed regions of active fires. See photographs.|
Dark smoke implies the presence of soot, which is composed of uncombusted hydrocarbons. Soot is produced when a fire is oxygen-starved, or has just been extinguished. Soot also has a high thermal capacity and may act to rob a fire of heat by carrying it away.
Evidence of fires within the buildings' cores is scant. NIST found only two core column specimens in a condition allowing paint-analysis inferences about temperatures reached, and those temperatures were below 250°C. It can be assumed that most of the fires were near the perimeters of the Towers where broken windows around the crash zone allowed them a supply of air. The cores were an average distance of about 70 feet from the nearest walls, and had much less flammable material than the surrounding offices. The impact gash in the North Tower provided a line of sight to the core. Available photographs and videos show the gash as consistently dark, showing no signs of fire in the building's core.
Dozens of people were observed to jump from floors of the North Tower above the impact zone. They may have jumped to escape painful deaths from inhalation of toxic smoke, or to escape unbearable heat. Note, however, that temperatures unbearable to a human, such as 100° C, are insignificant to the survivability of structural materials.
At least 18 survivors evacuated from above the crash zone of the South Tower through a stairwell that passed through the crash zone, and many more would have were it not for confusion in the evacuation process. None of the survivors reported great heat around the crash zone. An audiotape of firefighter communications revealed that firefighters had reached the 78th floor sky lobby of the South Tower and were enacting a plan to evacuate people and put out the "two pockets of fire" they found, just before the Tower was destroyed.