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# Unit 1: The Changing Fire Environment

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Environment   ::  Heat Transfer  ::   Behavior   ::   Predictions   ::   Summary   ::   Exercises

## Heat Transfer

To better understand when and how ignition and combustion occur in a wildfire, we need to discuss the physical processes involved. Note that heat transfer refers to the physical processes by which heat energy moves to and through unburned fuel.

Heat transfer refers to the physical process by which heat energy moves from one area to another.

The Heat Transfer Methods figure above illustrates various heat transfer methods. Branches above the fire are receiving heat by convection and radiation. Tree trunks and shrubs are receiving heat by radiation from the fire. Fuels on the ground are being preheated by conduction and radiation. Preheating of fuels may be occurring by all of these methods at the same time, depending on the arrangement or loading of the fuels.
We've stressed the importance of radiant heat transfer in the preheating of fuels and spread of the fire. How much heat will be received by fuels ahead of the fire? Well, this depends on the fire intensity and the distance, but how much?

## Common Methods of Heat Transfer

### Conduction

The first common method of heat transfer is conduction. Conduction is the transfer of heat from one molecule of matter to another. An example of this is fire smoldering through a solid piece of fuel. Since wood is generally a poor conductor of heat, conduction is the least important method of the three.

### Convection

This is the transfer of heat resulting from the motion of air (or fluid). It is the natural buoyant rise of warm air over a heat source that induces an automatic circulation within an airmass. Examples of forced convection are fire spreading from surface fuels to aerial fuels, and columns of smoke rising high into the atmosphere. Convection also includes direct flame contact, a powerful heat transfer process, especially in a head fire.