Flashover fires—such as the one that struck the Constellation bar in Crans-Montana on New Year’s Eve—are well known to firefighters but little understood by the public. A flashover is the moment at which a developing fire spreads suddenly and engulf an entire room.
The process typically unfolds in stages:
1. In the initial phase, one or more objects ignite. In Crans-Montana, investigators believe the ceiling was first affected.
2. The burning materials release hot combustion and pyrolysis gases—pyrolysis is the thermal decomposition of material caused by intense heat in the near-absence of oxygen.
3. If smoke and gases cannot escape, they accumulate near the ceiling, forming an increasingly hot layer. That heated layer radiates heat back into the room. Temperatures continue to rise.
4. Other combustible surfaces heat up, begin to decompose and release further pyrolysis gases.
5. At a critical point, those gases and surfaces ignite almost instantly. The room is rapidly engulfed in flames and temperatures can exceed 1,000°C, making survival virtually impossible.
If trapped smoke gases build up while oxygen remains available, ignition can trigger an explosive flash of flame across the smoke layer—a fireball that can spread rapidly through a space. Even fully equipped firefighters treat such conditions as extremely dangerous.
Basement fires are particularly perilous: anyone trying to escape will have to pass upward through a dense layer of smoke and heat.
The only reliable advice in such situations is: at the first sign of fire, leave immediately—do not linger or try to tackle the blaze.
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