Brendan McWilliams Weather analysisFlash floods, as the term is most commonly understood, are peculiar to regions that enjoy long periods of very dry weather, but that are also prone to occasional heavy thunderstorms.
A parched landscape is temporarily impervious to a sudden deluge, and dry river beds are quickly choked by a torrent of water which sweeps along the surface for miles and miles on end, advancing at great speed into neighbouring areas that may not have experienced any rain at all.
Flash floods of this kind are particularly common in parts of the United States, where they claim an average of 160 lives a year - more than any other natural hazard - and they are also common in a zone along southern Europe that extends from northern Italy, through southern France and into Spain.
But as we have seen recently in Cornwall, an arid landscape is not the only terrain that can be unreceptive to a sudden deluge.
Another reason why an area may react quickly and potentially lethally to a sudden downpour may be that it is already saturated, so that additional rainfall, rather than being absorbed, runs off immediately from the soaked surface. It is in the frequent falls of rain in the preceding days, therefore, combined with one additional torrential downpour, that we find the key to the recent inundation in Boscastle, Cornwall.
Whatever their origins, flash floods are treacherous phenomena. It is not the water alone which provides their great destructive power; as the flood courses down the valleys and gullies, it accumulates soil and sand and mud until the mixture forms a thick porridge which surges along as an irresistible force with its accumulated debris.
The kind of flooding we are most familiar with in this country, however, occur when relatively steady and continuous rain over an extended period gradually drowns the countryside over a wide area.
The low gradient of the flat midlands of Ireland makes for slow and ponderous rivers with a poor carrying capacity, and when there is an excess of rainfall, they sometimes cannot cope.
Urban flooding, on the other hand, like that experienced recently in Cork and Derry, may sometimes be an engineering problem; locally intense rainfall may simply be beyond the design capacity of the sewered drainage system, or building development may have taken place on naturally flood-prone land.
And although it was not the case as regards the present episodes, in urban coastal zones severe flooding may occur when very high spring tides are combined with strong, onshore winds.
Be all that as it may, the immediate cause of the intense rainfall on these islands has been visible for some days on the weather map as a very large area of low pressure hovering near Ireland and Britain and extending over most of western Europe.
Such a visitor at this time of the year is not unusual.
Indeed August, somewhat surprisingly for what is perceived to be the height of summer, is often a very wet month, and at a few places around Ireland, it turns out to be the wettest month, on average, of the entire year. The main reason is that rain, when it does come, is often heavy and thundery, with large amounts falling in short periods, as has been happening in recent days.
As a consequence, some of the worst examples of flooding on record in this country have occurred during the month of August.
Probably the worst such event was on August 25th, 1905. So heavy was the rain that it was calculated that if it had spread over the whole country, with no run-off, Ireland would have found itself under two feet of water by month's end.
By coincidence, flooding of almost equal proportions occurred on exactly the same date in 1986, in association with an infamously deep depression remembered as Hurricane Charley.
And who can forget that another Charley caused havoc in Florida a week ago?
It was last heard of as a dissipating low in the vicinity of Nova Scotia, but its surviving residue of warm, moist air, if channelled in the right - or wrong - direction, could still provide the fuel to revitalise our present downpours and cause history to repeat itself.