`A little fire," said Shakespeare, "is quickly trodden out, which being suffer'd rivers cannot quench." The truth of the bard's assertion has been vividly demonstrated in Los Alamos, New Mexico, in recent days, where a forest fire has ravaged nearly 20,000 acres of woodland, and caused millions of dollars worth of damage.
The specific causes of about 70 per cent of forest fires remain unknown, while data for the other 30 per cent show man to be the main initiator. Some are accidental, or may result from broken power cables, or are ignited by a stroke of lightning.
The sun can be a culprit if bright sunshine is focused to a point by broken glass to provide a tiny but effective solar furnace. A few are caused deliberately and some result from activities related to agriculture or, as appears to be the case in Los Alamos, from activities concerned with forest husbandry itself.
Whatever the spark, a forest fire is viable only when the weather at the time, and that of the recent past, combine to provide suitable conditions.
The flammability of wood depends critically on its water content. It will hardly burn at all when the moisture content exceeds 15 to 20 per cent, but will ignite almost as easily as petrol when it is only 1 or 2 per cent, and a fresh breeze combined with low relative humidity reduces the moisture content of the forest fuels to these values very quickly.
As one might expect, wind also affects the rate of spread of forest fires. For wood to ignite, it must first be heated to a temperature of about 300 degrees Celsius; since this is well above the boiling point of water, all the moisture in the fuel must first be vapourised before this ignition temperature can be reached.
Each active fire area provides the heat necessary to evaporate the water in adjacent fuel and then to raise its temperature to this critical ignition temperature. Wind greatly facilitates the process by bringing flames and heated columns of air into contact with adjacent trees, and in Los Alamos it was crucial.
The weather charts showed a frontal trough stretching from the Great Lakes down towards the valley of the Rio Grande, linking a depression over Canada with another near the Mexican border.
It was the northerly air-flow on the western flank of this later depression that provided winds of up to 45 m.p.h., with gusts to 60. Not only did these strong winds facilitate the spread of the fire in the way already described, but they also, of course, increased the supply of oxygen, thereby further enhancing the combustion process.