77 Fire Is Fearsome

People often speak of fire as though it were a living creature—it grows, dances, needs oxygen, feeds on whatever it can find, and then dies. And when a forest fire rages out of control, threatening human lives and homes, it must be fought like a “wild animal.” The fight is often desperate, since firefighters’ best efforts may be dwarfed by the fury of a large fire. But the fire's own traits can be used against it.

The heated air above a fire rises in a pillar of smoke and burnt gases, pulling fresh air in from the sides to replace it. Firefighters use this fact when they “fight fire with fire.” They start a fire well in front of the one that they are fighting. Instead of traveling on in front of the inferno, the smaller fire is pulled back toward it by the updrafts of the larger blaze. As it travels back to meet the large fire, the smaller backfire burns away the fuel that the forest fire needs to survive.

Even when a backfire has been well set, however, the fire may still win the struggle. The wind that the firefighters used to help them may now become their enemy. When the backfire meets the main fire, before both die for lack of fuel, there is tremendous flame, great heat, and turbulent winds. A strong gust may blow the fire into the treetops beyond the area, giving the fire new fuel and a new life.

77 火灾令人畏惧

人们常把火说得像活物一样——它会蔓延、“舞动”，需要氧气，吞噬一切能找到的东西，然后熄灭。当森林大火失控肆虐，威胁到人类的生命和家园时，人们必须像对付“野兽”一样与之搏斗。这场搏斗往往是殊死的，因为大火的狂暴可能会让消防员的最大努力相形见绌。但火自身的特性可以被用来对付它。

火上方的热空气会随着烟柱和燃烧产生的气体上升，同时从侧面吸入新鲜空气来补充。消防员在“以火攻火”时会利用这一点：他们在正在扑救的大火前方点燃一处小火。这处小火不会在大火前方蔓延，反而会被大火的上升气流吸向大火。当这处回燃的小火向大火靠近时，会烧光森林大火赖以生存的燃料。

然而，即使回燃的火布置得当，大火仍可能赢得这场较量。之前帮了消防员忙的风，现在可能变成他们的敌人。当回燃的火与主火相遇时，在两者因燃料耗尽而熄灭之前，会产生巨大的火焰、高温和紊乱的风。一阵强风可能会把火吹到该区域外的树梢上，给大火提供新的燃料，让它重获生机。