"Rain", said Samuel Johnson to his pal Jim Boswell, "is good for the vegetables, and for the animals who eat those vegetables, and for the animals who eat those animals".
It is difficult to fault the relentless progress of his train of thought, and yet, as we all know, much too much of a good thing is not good at all.
In a typical year, if the precipitation landing on Ireland were to be spread evenly over the whole country, the national average rainfall would about 1150 millimetres.
This means if the island was entirely flat and leak-proof, if it were bone-dry on January 1st and if none of the water falling on it in the 12 succeeding months was allowed to escape or to evaporate, by the end of December the country would be evenly inundated to a depth of 1150 mm.
The country, however, is by no means flat and leak-proof, and evaporation we have always with us. In addition, it is obvious that rainfall varies considerably from place to place. Most parts of the west, for example, collect 1,000 mm to 1,200 mm of rain in the average year, with about 2,000 mm being the norm in the Kerry, Donegal and Galway mountains.
The east experiences falls of rain between 750 mm and 1,000 mm, while a few places just to the north of Dublin have a little less. If you take a chart which shows this pattern, and compare it with a map of Ireland showing the physical features, you will notice a similarity that is quite remarkable.
This suggests that altitude plays a major part in rainfall - and so it does, producing an extra 50 mm or so of rain for every additional 100 ft above sea level. Higher-than-average rainfall occurs near high ground because of the forced ascent of moist air; the resultant cooling causes the moisture in the air to condense, producing clouds and rain.
This surfeit of rain occurs only on the windward side of high ground and near the summit, a phenomenon which has a beneficial outcome for low-lying areas on the sheltered side of the hill.
If enough water has been extracted from the air in the form of rain as it passes over the ridge, there will often be insufficient moisture for clouds or rain by the time it reaches low ground on the far side. The end result is a region of relatively light rainfall on the eastern side of high ground - a "rain-shadow" where it often remains dry when it is raining steadily everywhere.