Most of our Irish weather, as we know, approaches from the west, and the rain is typically of low intensity but of relatively long duration. Moreover, in this situation the mountains along our western seaboard coax much of its moisture from the air as it approaches, so that it has become significantly drier by the time it reaches areas further to the east. The rest of the country benefits from a "rain shadow" effect, and gets less rainfall than it really should.
But there is a striking difference between this scenario - what we might call our "normal" kind of rain - and the relatively short, heavy, sometimes quite spectacular interludes which often affect us during the summertime. At this time of year, the heaviest falls of rain are usually thundery by nature. The convective activity which results in the thunderstorms may be due to solar heating of the surface of the earth, which case the associated rainfall is distributed in more or less random "pockets" around the country.
But it often happens, too, that interludes of thundery rain in summer have their origin in a slow-moving area of low pressure which forms and persists for some considerable time just to the south of Ireland.
In this scenario, as happened at the weekend, the pressure pattern provides an easterly flow of air, the fronts associated with the depression settle over southern counties, and thundery troughs approach us from the east. When this happens, the normal rainfall pattern is reversed. It is the east and south which experience the worst and heaviest of the downpour, as thundery rain moves in towards Ireland from the Irish and the Celtic Seas; western and northern areas often escape comparatively unscathed.
A fall of rain of 100mm or 4 inches, typical of the cumulative effect of the recent downpours in the south, is the equivalent to about 400 tons of water on every acre of ground, so it is little wonder some of our rivers have found it difficult to cope. However, although it might have not have seemed so in recent days, the amount of water available to fall as rain is not unlimited. Indeed if all the water in the atmosphere at any time were to be extracted and poured upon the ground, it would form a layer only one inch deep around the world. Since this is equivalent to the average global precipitation for a period of only 10 days of thereabouts, it follows that there must be a continual re-charing of the atmosphere by evaporation to provide a steady supply of the raw material for Irish downpours.