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Watch: River of clouds descends on Alpine valley

The effect is due to a meteorological phenomenon called 'inversion'
The effect is due to a meteorological phenomenon called 'inversion' -
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A river of clouds could be seen descending on Wednesday through the Grimsel Pass in the Swiss Alps and into a nearby valley.

The swells of vapour are the result of a meteorological phenomenon known as inversion.

Inversion takes place when the normal temperature profile with altitude is inverted – meaning air temperature increases with height as opposed to the usual decrease in temperature in higher altitudes. Warmer air then flows above cooler air, resulting in a "river of clouds".

Switzerland's Grimsel Pass connects the upper valley of the River Aare with that of the Rhône, thus crossing the continental divide between the North Sea and the Mediterranean Sea – at an elevation of 2,164 metres.