Snowdonia downpour, early-February 2004

Britain broke records last week for both warmth and wetness. The maximum temperature of 17.9ºC recorded at Gravesend was the highest ever observed in the first week of February, but the rain that fell in many western districts, notably Snowdonia, was also exceptional for this time of the year.

"Orographic enhancement" and “rain shadow” are phrases often used by meteorologists in their day-to-day work. Some may regard both as solecisms (orographic is from the greek oros meaning ‘mountain’ and graphein, ‘to write’), but taken together they well describe the phenomenon whereby much more rain falls on the windward side of a mountain range compared with the leeward side. Last week produced the best example of these two complementary effects we have seen in the UK in recent years, thanks to a persistent southwesterly airflow delivering warm and moist air from tropical latitudes south the Azores.

It is best observed when the wind blows consistently from one direction, delivering very moist air following a long journey across the ocean. The air is forced to rise over a range of hills in its path, and as it rises it becomes less dense because the barometric pressure aloft is lower than it is at sea-level – there is less weight of air above. The laws of physics tell us that when the density of an air-mass decreases its temperature will also decrease, and the cooler it becomes the less moisture it can support. If the air is already saturated when it reaches the mountains, the excess moisture will condense into cloud-droplets, eventually producing rain, and if the airflow persists for several days large quantities of rain are likely to fall over these windward slopes.

Things are very different on the leeward side of the mountains. The air-mass has now lost much of its moisture, and as the winds descend the lee slope the air becomes denser and therefore warmer. As it warms up its capacity to hold moisture increases again, thus it is no longer saturated. The mechanism which produced the persistent rain on the windward slope is now switched off, the rain stops, and the clouds dissipate.

At Capel Curig, six miles ENE of Snowdon, 164.6mm of rain fell in the 24 hours between 9pm on Monday 2nd and 9pm on Tuesday 3rd. During the three days of most intense rainfall, a total of 273mm was collected between Monday 2nd and Wednesday 4th inclusive, while the six day total from last Friday 30th January week to Wednesday 4th February amounted to 417mm. This is 25 per cent more rain than fell in the entire year in parts of Essex in 2003.

Capel Curig is certainly not the wettest place in Snowdonia. Hard under the eastern flank of Snowdon itself, places like Cwm Dyli, Llyn Llydaw, Glaslyn and Crib Goch collect almost twice as much winter rainfall as Capel. Thus it is quite possible that at least 750mm of rain fell in these locations during those six wet days, with as much as 300mm during the wettest 24 hours.

© Philip Eden

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