Mountain Weather
Information Service

Temperature and humidity curiosities this morning

I spotted something very unusual in this morning's weather observations. 

As expected, under this high pressure, the mountain summit observations are above an inversion - higher temperatures than locations in valleys, and the air far from saturated.

But on this (very rare) occasion, the inversion has sunk, in some areas to practically sea level. Most valley locations are frosty, but there were some notable exceptions.

At 7am - Aviemore temperature was +4.4C. (Lossiemouth on the Moray coast was -3.0C)

The most striking example was recorded in north Wales - at Capel Curig (location of Plas-y-Brenin in Snowdonia), it was -1.1C at 5am, with relative humidity of 91%, but by 6am it had risen dramatically to +7.2C, with relative humidity of 25%. (An exceptionally low dew point of -11.8C). By 8am humidity was just 21%.

Meanwhile, nearby at sea level, Caernarvon aerodrome was +7C, dew point -9C, humidity 30%.

The explanation for these unusual events was that a strengthening southeasterly wind was blowing down off the hills this morning under stable atmospheric conditions. This aided the descent of the warm dry air existing above the inversion down into the valleys and in some places ultimately to sea level.