Ask Copilot) A sea of flames in an instant. The terror of deadly fires: 'Flashover' and 'Backdraft'

News about fires, such as the large-scale fire in Oita City at the end of last year, never seems to stop. I recently watched an excerpt from the movie 'Backdraft' on a YouTube video. I believe it's gone now, but USJ also used to have an attraction based on this movie.
With snow falling in the city center yesterday, it is a time of year when it is getting cold nationwide. Because of that, fires are scary, so I looked into the two phenomena mentioned in the title.

Q: I think the two scary phenomena at a fire scene are flashover and backdraft. Please tell me the difference between the two. Also, are there any other phenomena that lead to major accidents when a fire occurs?

A: Below, I will explain the difference between flashover and backdraft in an easy-to-understand way based on the Fire and Disaster Management Agency's explanation, and also introduce other serious fire phenomena.

🔥 Differences between Flashover and Backdraft

🧨 Flashover

A phenomenon where the entire room suddenly bursts into flames.

• A localized fire spreads throughout the room in a short time (a few seconds to several tens of seconds).

• The high-temperature smoke layer accumulated near the ceiling and the ceiling/walls radiate heat, causing combustible materials throughout the room to ignite simultaneously.

• Occurs when there is a sufficient supply of air.

• The temperature at which it occurs is generally 500–600°C or higher.

👉 Feature: 'Rapid spread of fire' that occurs during the process of a fire growing.

💥 Backdraft

A phenomenon where explosive combustion occurs when air suddenly enters an oxygen-deprived enclosed space.

• A fire occurs in a highly airtight space, and combustion weakens due to a lack of oxygen.

• When fresh air flows in, such as by opening a door, unburned gas ignites all at once, and explosive flames and airflow burst out

• The name comes from 'back' (pushed back) and 'draft' (airflow).

👉 Feature: Lack of oxygen → sudden influx of air → explosive combustion.

🔍 Comparison Table

$$
\begin{array}{|c|c|c|c|}
\hline
\textbf{Phenomenon} & \textbf{Occurrence Conditions} & \textbf{Mechanism} & \textbf{Direction of Danger} \\
\hline
\text{Flashover} & \text{Air supply present, fire growing} & \text{Simultaneous ignition of combustibles due to hot smoke/radiant heat} & \text{Entire room becomes a sea of flames instantly} \\
\hline
\text{Backdraft} & \text{Oxygen-deprived enclosed space} & \text{Explosive combustion of unburned gas due to air influx} & \text{Explosive flames burst outward from doors/windows} \\
\hline
\end{array}
$$

🔥 Other Serious Fire Phenomena

In fire science, besides flashover and backdraft, there are other phenomena that lead to major accidents that are known.

1. 💣 Smoke Explosion

• A phenomenon where flammable gas accumulates in a space and ignites at a distant fire source, causing an explosive combustion event

• Similar to a backdraft, but it is not caused by oxygen deficiency which is the difference

• It is treated as one of the three major dangerous phenomena at fire scenes

2. 🔥 Rollover (Ceiling Flame)

• Unburned gas near the ceiling partially ignites, and flames race across the ceiling

• Important as a precursor to flashover

• If this is seen, it is judged that a flashover is imminent

3. 🌫 Collapse of High-Temperature Smoke Layer (Smoke Layer Collapse)

• High-temperature smoke accumulated near the ceiling suddenly descends, causing the room temperature to rise rapidly

• Risk of people who failed to escape receiving fatal damage from the heat

4. 🧯 Rapid Fire Intensity Change due to Ventilation

• Ventilation operations by firefighters, such as breaking windows or opening doors, cause the fire intensity to increase rapidly

• Depending on the stage of the fire, it can induce a flashover or backdraft

🔎 Summary

• Flashover: The fire grows in the presence of air, and the entire room ignites at once

• Backdraft: Explosive combustion occurs when air enters an oxygen-deficient space

• In addition,

  • smoke explosion

  • rollover (flame-over)

  • smoke layer collapse

  • rapid fire growth due to ventilation
    can lead to serious accidents.

Fire scenes are full of 'invisible dangers,' and understanding fire science is the key to protecting lives.

~The air is especially dry in winter. The forest fire in Ofunato, Iwate Prefecture, last year is still fresh in our memories. Please be careful when handling fire.

Thank you for reading until the end.
*The title image was borrowed from the 'Minna no Photo Gallery'.

References