We saw a dramatic cloud inversion on the Aberfeldy to Acharn walk I led for the Drovers’ Tryst walking festival. Our high level route took us above the mist filling Strath Tay and we had wonderful views over a sea of fluffy white cloud to many Munros in a sweep from Ben Lawers to Beinn a’Ghlo. This phenomenon was caused by a temperature inversion that trapped cold, moist air below a warmer, drier atmosphere. Usually the air temperature drops as altitude increases.
The image of Schiehallion, due north of us, was particularly striking. One of Scotland’s best loved mountains, it forms an elegant cone, rising to 1083m (3,547ft). Beds of limestone outcrop around the lower slopes of the mountain and are riddled with a network of water-eroded caves. Its name, which translates from the Gaelic as “fairy hill of the Caledonians” probably derives from the association of these caves with supernatural spirits.
Schiehallion played an important role in the development of scientific knowledge. In 1774 the astronomer royal, Nevil Maskelyne, used it to calculate the mass of the earth (remarkably accurately as later proved). He placed pendulums around the mountain to measure the gravitational force exerted by its mass, so he could multiply this up to a body the size of the earth, whose circumference had already been calculated. One remaining problem was to calculate the volume of Schiehallion and this was solved by Charles Hutton who developed the concept of mapping mountains by means of contour lines. This method of showing the shape of landforms was adopted the world over and determines the appearance of Ordnance Survey maps we use today.
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