Cloud inversions — where a temperature inversion causes low cloud to sit in the valleys while the summit stands clear above a white sea — are one of the most sought-after conditions for Snowdon photography. They occur most frequently in autumn, particularly in September and October, and they are impossible to guarantee but increasingly predictable with the right forecast reading.
A cloud inversion typically forms after a period of rain, when high pressure moves in rapidly overnight. The cold, moist air from the rainfall stays trapped in the valleys while the temperature above rises. The result, on a clear morning, is a mountain apparently floating above cloud.
To increase your chances: check the MWIS forecast for a predicted temperature inversion (they usually describe this explicitly); look for the cloud base to be settling at 400–700 metres the evening before; and plan to be at your chosen viewpoint at or before sunrise. The window can be short — the inversion often burns off by mid-morning as the sun warms the air.
The best viewpoints for inversions are Llynnau Mymbyr at Capel Curig (roadside, no walk required), Llyn Llydaw causeway on the Miners' Track (approximately 3 km from Pen y Pass), and the summit itself — which frequently provides the most dramatic perspective, standing above the cloud looking down at invisible valleys.