Although it may be hard at present to believe it, the amount of water available to fall as rain is not unlimited. Indeed if all the moisture in the atmosphere at any given time were to be extracted and poured as water on the ground, it would form a layer only one inch, or 25 mm, deep around the world. So where, one might wonder, has all this water come from?
To begin with, fallen rain is not distributed evenly around the world. Some regions get more than their share, like Ireland and Britain in the last few days; others, like the Sahara, and India at certain times of year, have hardly any. If rainfall is averaged over the whole world we find that the annual global average is about 1000 mm, which strangely enough, is just slightly less than Ireland's annual average for the country as a whole of about 1150 mm.
This latter figure means, as we know, that if the island were entirely flat and leak-proof, and bone-dry on January 1st, and if none of the water falling on it in succeeding months were allowed to escape or to evaporate, then by the end of December the entire country would be inundated to a depth of 1150 mm or slightly less than 4 ft. Some might think that this year this has been achieved already, in November. This global average rainfall of 1000 mm is about 40 times the 25 mm of water present in the atmosphere at any one time. To put it another way, the atmosphere contains about one fortieth of the amount of water needed for the annual global rainfall, or enough for about nine days of rain. Obviously, therefore, there must be continuous replenishment by evaporation to provide the raw material for the world's downpours. Moreover, in a warmer greenhouse world, the atmosphere could hold more moisture, and the resulting increase in evaporation would absorb several millimetres of sea level and cause a significant increase, in certain places, in the average annual rainfall.
Over the past few days, parts of the country have experienced over 100 mm of rain in less than a 24-hour period. Many of our present records for high daily rainfall were established on August 25th, 1986, when Hurricane Charlie visited our shores - among them the all-time high of 200 millimetres measured at Kilcoole in Co Wicklow on that day; twenty-four hour totals of 70 to 100 millimetres were commonplace over large areas of Dublin and Wicklow.
It remains to be seen if the 200 mm daily barrier will have been breached again during the recent onslaught.