The statistics by now are well known. Some 20,000 firefighters have been struggling to contain more than 60,000 fires that have so far destroyed nearly four million acres of forest and woodland. Eleven American states have been affected, which together comprise almost the entire western half of the land area of the contiguous United States. It has been the worst such episode for 50 years, and no respite is in sight.
The weather sets the scene for wildfires by arranging for a deficit of moisture. For wood to ignite, it must first be heated to a temperature of about 300 degrees, and since this is well above the boiling point of water, all the moisture in the fuel must be vapourised before this ignition temperature can be reached, and the energy used in evaporating this water can use up a large proportion of any heat supplied to the wood.
But a long dry period, by reducing the moisture content of the available fuel, greatly enhances the potential viability of any embryonic wildfire.
As a forest fire spreads, each active fire area provides the heat necessary to raise the temperature of adjacent fuel to the critical ignition temperature. Wind facilitates this transfer of heat, by deflecting the flames and heated columns of air, and bringing them into contact with adjacent trees. Wind also increases the supply of oxygen, which further enhances the combustion process.
But the real villain of the piece on this occasion has been a phenomenon unknown, or almost so, in Ireland. A poet of the Rocky Mountains describes it very vividly:
There's danger comes this time of year, late in the after- noon. As thunderheads obscure the skies, it's not the rain that threatens me: It's that no rain will come.
As Zeus and Vulcan forge their bolts, look up, expect the water spill From blackness all around, only to see it disappear Before it hits the ground.
It's that which pierces all of this, that strikes my heart with fear; Dry lightning in my mountain land, presents a danger clear.
For the very reasons that have been so obvious on television screens in recent days, dry lightning brings terror to the hearts of those who live in regions susceptible to forest fires.
It comes from thunderclouds like any other lightning, but rather than being accompanied by a heavy downpour, it brings no rain at all. It happens when the air near the ground is exceptionally dry, and the falling rain drops evaporate before they reach the ground. In such circumstances, with dryness all around, the viability of any fire started by the lightning is virtually assured.