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The Influence of Geography

	Ground level depression geographic features, such as valleys, can laterally confine the fire. Confinement concentrates the heat in a smaller airspace, increasing combustion and fire spread potential.
	Cleared land geographic features, such as wide rivers, cultivated land, or clearings, can be natural fire breaks. They can be a barrier to fire spread because the fire cannot leap the fuel-deficient span. However, this effect is not absolute; wind can lift burning particles over these natural fire breaks onto a new fuel load.
	The topography of slope, both angle and orientation (to or away from the sun), can significantly affect fire spread. Slope increases flame contact with uphill fuels, thereby more rapidly heating them and increasing combustion potential. Therefore, the fire burns more intensely and quickly. Uphill winds accelerate this process, as do slopes that face the drying warmth of the sun.

http://www.interfire.org/features/wildfires.asp

http://www.idahoptv.org/dialogue4kids/season3/wildfires/facts.html

http://www.howstuffworks.com/lightning.htm

Fire Spread
Fires spread in three general patterns: ground fires, surface fires and crown fires.

Ground fires burn organic material in the soil beneath the litter on the surface. They burn by glowing combustion.

Surface Fires have a flaming front and burn leaf litter, fallen branches and other materials on the ground.

Crown fires are the hottest and most intense. They are often difficult to control, need strong winds, steep slopes and lots of fuel to keep burning. Crown fires burn the top layer of foliage on the tree. http://www.idahoptv.org/dialogue4kids/season3/wildfires/facts.html