Mountain Weather - A Guide to Understanding the UK's weather

• Description
• Contents
• Introduction
• Sample Route
• Reviews
• Further Reading

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21. Dull Top

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Many clouds over the British Isles have bases below 1,200m, so it is no wonder that our mountain tops are often in or near cloud, the more so in winter. But, even so, days when the forecast speaks of broken or little cloud over open country can be dull and sunless over the hills for hours on end. The reason is that mountains can make their own clouds. As we saw in Sections 5 and 6, most clouds are made by the lifting of moist air - in the tumbling eddies of strong winds, or the buoyant masses of warm air on sunny days, or the widespread gentle lifting near lows and fronts. But even if clouds are not forming by these kinds of lifting, other clouds can form when air rises over a mountain barrier.

Air rising up the windward side of a ridge can give cloud if it is moist enough. You can watch clouds forming there in more or less the same place for hours on end. They start at the base as small shreds, rise, grow, and join to form a mass that seems to sit on or over the highest ground (21A). On the leeward side, where the air is sinking, the cloud breaks up because it is being compressed as it sinks (21B). (An example of air being warmed as it is compressed is the pumping up of a cycle tyre.) The cloud may be just a small cap of stratus on an isolated hill (21C, 21D), or a vast grey shroud of stratus or stratocumulus over a broad highland, beneath which the deep valleys run like gloomy tunnels (21E).

Watch out for dull tops whenever a moist wind is forecast to blow over the mountains - as when the weather map shows the wind has come across the sea from low latitudes (5B, 12F, and 15E), or there has been rain for several hours (3D, 14C and 27A). Expect the cloud base to rise, even to above the tops, if the wind becomes drier, as it may well do if a fitful sun makes the day warmer, or if the wind direction changes to blow from higher latitudes (say, after a cold front has passed - 3E, 12F, and 15E), or from a warmer country in summer.