EVERY year more than 1,000 acres of Irish trees are totally destroyed by fire. Although the recognised season for forest fires lasts all the way from February to October, it is during the months of April and May that by far the greatest number take place.
It is no coincidence that these, on average, are the driest two months of the year in most parts of Ireland, and the dangers are obviously exacerbated when March, too, has been very dry, as was the case this year.
A great many fires start, not on the forest floor itself, but in the dry vegetation at the sides of roads or railway lines or forest paths, and then spread from there to the plantation. The flammability of this environment varies with the season, being lowest in autumn and winter when it is saturated with moisture, and low also in mid summer when there is a fresh flush of green growth; it is high in spring when the vegetation is often dried up by cold easterly winds, and sometimes also in late summer if it has been scorched by the sun and drought.
Many forest fires have their origins in human activities of some kind, and are usually a consequence of carelessness.
Some, for example, are the accidental result of activities related to forestry itself, or undertaken in the course of agriculture; some result from broken power cables, and a few now and then are clearly deliberate, and malevolent in intent.
But the weather, too, may often play a part. Lightning is a very common source of ignition, and dry warm weather increases the risk of fire, through human negligence or lack of care, by encouraging more activity in wooded areas. Even the sun can be a culprit: if bright sunshine is focused to a point by broken glass, it provides a tiny but effective solar furnace.
It is by controlling the moisture content of the forest fuels, however, that the weather most effectively sets the stage for any such events that may take place. A dry spell, particularly if combined with a fresh breeze and low relative humidity, extracts the moisture from the vegetation very quickly.
A breeze also facilitates the spread of fire; each active fire area provides the heat necessary to raise the temperature of adjacent fuel to ignition temperature, and wind facilitates this transfer by deflecting flames and heated columns of air away from the vertical, and bringing them into contact with adjacent trees.