The Rain-shadow Effect

Sheltered by mountains
by Philip Eden

"Rain shadow" is a phrase often used by meteorologists in their day-to-day work, and is occasionally mentioned by television weather forecasters. Some may regard it as a solecism, but it well describes the phenomenon whereby much less rain falls on the lee side of a mountain range compared with the windward side. Last week produced one of the best examples of the rain-shadow effect we have seen in the UK in recent years thanks to a persistent southwesterly airflow.

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 any 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 - that is, there is less weight of air above. The laws of physics tell us that when the density of a mass of air 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 again, 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.

Last week's rain shadow effect (see plot above) was very prominent in the shelter of the Scottish Highlands. During the week ending 0600 on Saturday (Jan 19) almost 100mm of rain was recorded in the Strath of Orchy in Argyllshire and an estimated 150mm fell in the upper parts of nearby Glen Strae and Glen Kinglass. By contrast no measurable rain at all fell at Kinloss in Morayshire, and only 2mm at Aberdeen. The effect was also well illustrated in northern England where Shap in Cumbria recorded 83mm of rain compared with 4mm at Newcastle, and also in Wales and the Midlands with 96mm at Capel Curig in Snowdonia but only 2mm at several sites in the Midlands.