Friday's storm in Strasbourg, in which nearly a dozen people lost their lives, was quite unlike those we are accustomed to in Ireland.
Irish storms come from vigorous mid-latitude depressions which deepen rapidly as they move eastwards across the north Atlantic. They are large-scale phenomena, typically a thousand miles or more in diameter, with a central barometric pressure of perhaps 980 hectopascals.
There was no deep depression of this kind in the vicinity of Strasbourg. The weather map showed a shallow area of low pressure covering eastern France, the south of England and the western part of Germany. The central pressure was in the region of 1,003 hectopascals, and it varied little over the whole area, with the pressure differences on their own being such as to cause little wind.
But the air was warm and humid, and this otherwise unremarkable low-pressure zone was an area of marked atmospheric instability. There was a tendency for local violent thunderstorms to form, and it was one of these that brought tragedy to Strasbourg.
Irish thunderstorms bear no resemblance in their viciousness to their continental counterparts. The strong winds associated with the latter have their origin, not in the local pressure pattern, but in the powerful "downbursts" that are characteristic of a thundercloud - strong jets of air that surge earthwards through the cloud's core and which, when they hit the ground, spread radially out in all directions like a jet of water from the tap when it encounters the bottom of the kitchen sink.
Such squalls, although much shorter lived, are fiercer and angrier than any of the winter gales one might experience in Donegal or Kerry. The rainswept trees, laden with their summer foliage, lean first together to one side, sometimes almost reaching the horizontal; then after several minutes the wind direction changes without abating, and the trees are bent the opposite way. It is not uncommon for such a storm to produce winds of quite unpredictable direction at speeds well in excess of 100 m.p.h.
And sometimes a violent thunderstorm like this may spawn a localised, short-lived tornado. Marked differences in wind speed with height sometimes give the vertical currents in a thundercloud an initial turn and, given a suitable thermal structure in the atmosphere, this turning motion may rapidly accelerate into a local but deadly maelstrom capable of lifting virtually anything in its path. This almost certainly occurred in Strasbourg.