The transitional phase in a fire when the radiant heat ignites the flammable gases with in an enclosed structure. This looks like; the fire has spread quickly across the thick smoke on the ceiling, and at this point whatever the flame touches catches fire in a matter of seconds. The only warning sign other than the incredible heat, at the source of the fire; the plumes of smoke start to ignite and this is called rollover. These rollovers can exist for a few seconds before flashover.

Firefighters try to prevent a flashover from occurring by cooling the surrounding air with bursts of water mist sprayed into the air above the fire. The burst allows for the most amount of water to be converted into vapor, cooling a wider area than the spray off the nossle.

When this phenomenon occurs outside, it's referred to as a firestorm.

Flashover is the near-simultaneous autoignition of all combustible materials in an enclosed space. This is what a textbook, and Wikipedia, will tell you.

A firefighter in full PPE has between two and five seconds to exit a space undergoing flashover before death. This is why, when exploring a burning structure, one never enters an enclosed space more than can be recovered in two to five seconds. Temperatures in a flashing room approach 1000°F. Flashover is not a survivable event. This is what a fire instructor will tell you.

The phenomenon of flashover owes itself to the same processes that give human beings body heat. The compounds released by the flames in a flashing room are much the same as those released by breathing. Humans and fire share several lines in the poetry of science. This is what grace will tell you.

Solids and liquids do not burn. Only gases burn.

Fill a test tube with matches. Top the tube with a rubber stopper containing a spout. Hold a torch to the bottom of the tube. Eventually, you will see what appears to be smoke escaping from the spout at the top. If you hold a match to the escaping gases, they will ignite.

When sufficiently heated, hydrocarbons will begin to vaporize. With most hydrocarbons (read: paper, wood, plastic) this temperature requirement is relatively high. With hydrocarbons considered 'flammable' (read: gasoline, kerosene, propane), the temperature requirement is relatively low. Vaporized hydrocarbons are replete with free radicals, tragic little souls with empty outer valences.

Hit up Google and pull an image of the Periodic Table. Assuming you have zero knowledge of chemistry, you will notice that the elements run left to right. Among many things, an element's position on the table denotes how many electrons occupy its outermost shell, described alternatively by the term "valence." (I know: quantum this, probability that. I know.) Electrons like to travel in even numbers. Electrons like symmetry.

Take sodium, on the left. It's on the left because its outer valence denotes only one electron. This valence is desperate to bond with other compounds, thereby evening itself out. When this reaction takes place, electrons shift and energy is released as heat. Sodium is so reactive that it hisses when exposed to the moisture of the open air.

This is the nature of the free radical. Now imagine a room filled with them. Talk to a firefighter who's been on the job a decade or two, and you will likely find that he has kept a melted helmet as a trophy.

Oxidation is, intuitively, the process by which substances bond with oxygen.

Mitochondria oxidize energy-rich substances. Oxygen introduced into the bloodstream by the lungs combines with glucose retrieved from food by the digestive tract. The chemical reaction produces energy, and the byproducts are expelled back through the lungs as carbon dioxide, heat, and water.

In fire, Flick's law draws oxygen into the reaction zone. The heat catylizes the exchange of electrons between hydrocarbons and oxygen. Fire is a sustained chain-reaction of rapid oxidation. In a perfect burn — like that attempted by your car's engine — the by-products are carbon dioxide, heat, and water.

All things burn imperfectly: this is why there is smoke, resulting from an imperfect oxygen/fuel ratio. Smoke consists of incompletely-burned substances expelled from the fire by the convection column: tars, hydrogen sulfides, carbon monoxides, hydrogen cyanides.

Smoke burns. I toured a facility in Beaumont, California which disposes of waste by first burning it, then enclosing the smoke in a chamber which recreates the conditions of flashover. The resulting explosion of heat is converted to electrical energy which is then released onto the grid.

Autoignition can be defined as the ignition of matter without direct flame involvement. Spontaneous combustion would be a good analogy, but with an actual, verifiable cause.

The flashpoint of any given material is the temperature at which said material will autoignite.

Many of the things we interact with are hydrocarbons. More obviously, gasoline. Less obviously, carpet, wood, plastic, polypropylene. Many of these everyday substances have similar flashpoints.

All things absorb heat before re-radiating heat, vaporizing and igniting. In hydrocarbons, this is largely dependent on the porousness and surface area of the material relative to its mass.

Try to light a solid oak log with a match. It will not happen. The log will absorb the heat without a second (first) thought and you will burn your fingers.

Now, imagine that same log as a pile of wood chips. The same quantity of matter with many times more surface area. Blammo.

When a fire burns in an enclosed space — say, a bedroom — a certain amount of the heat is absorbed into the walls. Eventually, the walls radiate the heat back into the enclosed space. This is flashover. All hydrocarbons reach flashpoint simultaneously and ignite — this is flashover. The smoke itself, thrown off from what was once a weak curtain fire, ignites — this is flashover. Free radicals find each other in the dark and produce enough heat to soften steel. Imagine that you are there, that chemistry dictates your breath is the same substance billowing out the doorways, the windows. Firefighters call the licks of flame which signal the beginning of flashover "angel fingers."

Excellent video footage of flashover can be found here. As well as the visual stimuli, you will be provided with a good timeline/scale for the stages of an enclosed residential blaze.

Be also aware that the term "rich flashover" is interchangable with "backdraft." Firefighters tend to differentiate flashover and backdraft rather stubbornly, as backdraft is caused by the sudden introduction of oxygen rather than simultaneous autoignition of everything in an enclosed space.