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It is possible for a cold layer of air to get trapped at ground level, with a layer of warmer air above it. This is known as a temperature inversion because it is the opposite of the normal temperature pattern of air being warmer near the surface and cooler above. A temperature inversion can happen when cold air settles in valleys, perhaps forming valley fog, as warmer air drifts overhead. Eleuthera du Breuil (Member 40,742) spotted this inversion in Slovenia’s Triglav National Park, where Lake Bohinj is hidden beneath the dense fog. ‘The fog is there nearly every morning in winter,’ Eleuthera told us. ‘But it burns off by about 10am.’ Up above the temperature inversion, a thinner misty layer of cloud has appeared, perhaps caused by the cooling of air drifting at this level by patches of snow on the hillsides.