Not a record – lowest ever relative humidity reading discounted

"Scotland drier than the Sahara Desert" ran the television news trail on Tuesday evening. It sounds like a good story; sadly it was based on a single dubious meteorological observation. A relative humidity of 1.6 per cent at Altnaharra in Sutherland was reported at 10 o'clock that morning, and this, had it been confirmed, would have been the lowest humidity recorded anywhere in the UK for 118 years.

In a way it was no bad thing that this doubtful reading attracted such interest. It drew attention to the fact that the air covering the UK at the beginning of the week was exceptionally dry and relative humidity on Monday and Tuesday dropped below 25 per cent over much of Scotland. That is why the Altnaharra report was not immediately self-evidently incorrect. The weather-recording station there is an un-manned electronic one. In the end it was a suspicious mind that flagged up the reading, and a fairly detailed analysis of the output of the station's data-logger which revealed three or four reasons why that ten o'clock observation was probably wrong.

The very dry air over Scotland was the result of an unusual coincidence of circumstances. The large anticyclone which has now controlled Britain's weather for almost a fortnight migrated to southern Scandinavia last weekend. As the wind blows in a clockwise sense around the centre of a 'high', this allowed clear, continental air to sweep across the British Isles. Pressure is high in an anticyclone because the air in its circulation is gently sinking, as it sinks it is compressed because there is a greater weight of air above it, and this makes it both warmer and drier. However, this subsided air does not normally reach ground level because it is not as dense as the lowest layer of air which is cooled in winter by contact with the Earth's surface.

Around the periphery of the anticyclone the wind can blow quite strongly, and when such an air-mass is forced to climb over a mountain-range the resulting turbulence can bring the warm, dry air down to ground-level. This mechanism caused the high temperature and low humidity observed in northern Scotland early last week.

So why is the Sahara a desert and Scotland isn't? Over the Sahara the sinking air of a high pressure system prevails on 350-360 days per year while in northwest Scotland there may be just 20 such days in a good year.

© Philip Eden

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