'Rain-shadows' cast by mountains of west

 

'The rain in Ireland," according to William Bulfin, doughty scribe of Rambles in ╔irinn, a zesty travel memoir of three quarters of a century ago, "does not come heralded by dust and thunder, or accompanied by lightning or by roaring tempests, like rain in the tropics; nor does it wet you to the bones in five minutes.

"You scarcely know when it begins. It grows on you by degrees. It comes on the scene veiled in soft shadows and hazes, and maybe a silver mist."

It is also obvious that Irish rainfall varies considerably from place to place.

Most parts of the west of Ireland, for example, collect 1,000 to 1,200mm of rain in the average year, with around 2,000mm being the norm in the Kerry, Donegal and Galway mountains.

The eastern part of the country, on the other hand, experiences falls of between 750 and 1,000 mm, while a few places just to the north of Dublin have a little less. And 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 which is remarkable.

This suggests, firstly, that altitude plays a major part in rainfall. And so it does, producing an extra 50mm or so of rain for every additional 100ft 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.

But 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 elsewhere.

Very broadly speaking, the eastern half of Ireland lies in one big "rain-shadow".

The mountains of Kerry, Connemara and Donegal extract large quantities of moisture from the Atlantic winds so that the rain, as Bulfin puts it, "folds you in, and make of you a limp, sodden and unsightly thing for all its soft embraces". But much of the rest of the country benefits by getting less rain than it really ought to.