How Fires Spread

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One of the most obvious ways fire spreads is through direct ignition. This is where the flames of the fire actually ignite the fuel that it comes into contact with and this is typically seen when lighting a small bonfire. To build up the flames, you put small bits of kindling around the fire, and in turn, the flames ignite these new bits of kindling, causing the fire to grow. However, it does not necessarily have to be a flame that causes this, direct ignition can be caused by sparks or embers as well. This can even be done manually, for example when you strike a match and it lights, you then move the flame over to the fuel you want to burn, perhaps a cigarette or a candle.

Whilst direct ignition is probably the most common way that fire spreads, there are actually many more ways of it doing so. First of all, there is radiation. This is where the heat of the fire heats up the air particles around it. If you have ever been very close to a fire, you will probably have felt the warm air radiating from it. This heat becomes the ignition source for the Fire Triangle, and if the other two factors are present, a fire will start. The easiest example of radiation is the sun, where its rays expand far beyond that which we can see. One of these ways is through infrared rays, which cause particles to heat up, and on an especially hot day, a fire can ignite. This can also be seen when you focus a magnifying glass onto a fuel source, eventually, a fire will start.

Another way of fire spreading is through what is called convection. Again, this is where particles from the fire heat up and these particles begin to vibrate more and more, which causes the air to expand and become less dense. The less-dense (and hotter) air will then rise up, which pushes the cooler air down towards the fire.  This air subsequently heats and rises above the warm air which by now has started to cool. This cycle repeats and repeats and eventually, the air becomes hot enough for another fire to start, perhaps if wind is blown towards a source of fuel.

The final main way that fire can spread is through conduction. Again, this is due to particles being heated, however unlike the other ways where the air heats up, conduction refers mainly to solids, but it can also occur in liquids and gases as well. When a solid heats up, its particles vibrate more and more and these particles then bash into each other and transfer heat through the object. This is why, for example, saucepan handles become hot when they are on a stove. If the object being heated is touching a piece of fuel, a flame could start.