Chapter 7 of Genesis tells us how, many, many centuries ago when Noah was a mere stripling of a lad of 600 years of age, "all the fountains of the great deep were opened up; and the flood-gates of the heavens overflowed exceedingly and filled all the face of the earth; and the waters prevailed beyond measure". Nothing on the same scale happened for 3000 years, until on June 11th, 1963, there was what seemed to be a very similar occurrence on the southside of Dublin city. The most dramatic thunderstorm to affect our capital city took place 38 years ago today.
The most severe flooding occurred in the triangle bounded by Dundrum, Blackrock and Sandymount, and the most spectacular rainfall record was that from a rain-gauge in Mount Merrion at the very centre of the storm.
It was read at hourly intervals during the event, and showed that the total rainfall for the day in Mount Merrion was 184 millimetres - more than 7 inches - of which 80 mm, or 3 inches, had fallen in the hour between 2 and 3 o'clock in the early afternoon.
Let us put the cloudburst in perspective. In an average June, most parts of Dublin experience a total of between 50 and 60 millimetres of rain over the entire month, significantly less than the 80 millimetres which fell in that single hour 38 years ago.
Moreover, using rainfall data collected over the years, it is possible for meteorologists to use statistical methods to calculate for a particular place the "return period" of rainfall of a given intensity.
Such calculations suggest, for example, that a fall of 15 mm in a single hour could be expected to occur in Dublin about once every five years; 20 millimetres an hour might be expected every 10 years or so; and 35 mm of rain in a single hour should only happen once every 50 years. One might almost say that a downpour of 80 millimetres in an hour should never happen at all, but it did.
It is possible, of course that even heavier rain has occurred in Ireland from time to time, but passed into oblivion undocumented; accurate assessment of any heavy cloudburst depends, as happened fortuitously in Mount Merrion, upon on a rain-gauge being in operation at the right place and at the right time to provide precise figures. Many spectacular falls over the years may have escaped detailed analysis by climatologists, their torrential nature evident only to those unfortunate enough to have been caught beneath them, or to have become victims of the flooding which frequently ensues.