a record–lowest ever relative humidity reading
"Scotland drier than the SaharaDesert" 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 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 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.
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