Answer to Question 1
ANSWER: Radiation, convection, and firebrands (or burning embers) allow a fire to ignite and progress. Radiant energy decreases with the square of the distance from a heat source, that is, at twice the distance the energy on a surface drops by four times. With only radiant heatingthe energy transferred by thermal radiation, without air movement or contact with flames50 seconds of intense crown-fire burn is sufficient to ignite dry fuels at a distance of 20 to 25 meters. Convective transfer of heat is more direct and efficient; it involves direct movement of heated air or flames to ignite fuel. Fire also spreads when burning embers, called firebrands, are carried with the wind and then deposited, igniting spot fires, which burn ahead of the main fire but can quickly spread.
Answer to Question 2
ANSWER: Canyons can funnel air, accelerate the fire, and cause more rapid spreading; fires move upslope faster than they move downslope. At a ridge top, fire often slows dramatically because the rising flames run out of fuel. A fire reaching a ridge crest can create an updraft of air from the far side that fans the flames, but the same updraft can reduce the chance that the fire will move down the other side.
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