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A thinning autumn mist in rural County Tyrone. This is caused by radiation in clear calm conditions. As the heat escapes into the air because of lack of cloud cover, the ground cools and cools the layer of air close to it. Since cold air cannot hold as much moisture as warm air, eventually the air may cool to the dew point temperature (with 100% relative humidity) and some of the water vapour will condense out to form visible water droplets. As the sun rises and heats the ground as is beginning here the process is reversed and the fog/mist will 'burn off'.