Yesterday the gremlins were at work again. Among the lies, damned lies and statistics promulgated by "Weather Eye" was the bizarre suggestion that there are currently some 60,000 major wildfires burning in the western United States. A more realistic figure, you may well have guessed, is 60; thankfully not all the north American continent is ablaze. But let's have another look at the phenomenon which sets these fires alight: dry lightning.
Lightning is nearly always associated with a thundercloud, although meteorologists are not entirely sure of the precise mechanisms by which it comes about. It may be that the build-up of electricity is the result of raindrops moving up and down in currents of air through a small initial electrical charge; or it may be that the cause is friction between ice crystals contained within the cloud.
Whatever the reason, very large electrical charges of opposite polarity develop in different regions of a thundercloud, or even more commonly between regions of the cloud and Earth.
There is a strong tendency for an electric current to flow from one to the other to redress the balance.
If this electrical tension becomes high enough, the pent-up energy is released in a sudden surge and burns a path through the intervening air. The incandescent channel, no more than an inch or two in diameter, is heated by the electricity passing through it to a temperature of 30,000 degrees or more - and yes, this time it is really 30,000, not just 30.
A popular distinction is often made between "forked" and "sheet" lightning, but this is, in fact, a distinction that contains no difference. The former is a stroke of lightning whose path is clearly visible; the latter is the diffuse glow that can be seen if the lightning flash itself is obscured by cloud or rain.
Essential to all this theory is the presence of a thunderstorm, and a thunderstorm, as we know, nearly always means a downpour of very heavy rain.
But not invariably. Sometimes when the air under a mature thundercloud is exceptionally dry, the raindrops evaporate as they fall earthwards but before they reach the ground. The rain can then often be seen in the sky as streamers hanging obliquely from a thundercloud cloud - phenomena known to meteorologists as virga.
Here in Ireland, virga are often seen hanging from an ordinary shower cloud. The atmosphere is rarely dry enough, however, for the big raindrops from a thunderstorm not to survive the journey to the ground. But in the western United States it is common to have thunder with no surface rain - and abundant flashes of what is therefore aptly called "dry lightning".