We are used to feeling the warmth from the sun on our faces. Heat radiates millions of miles across space, through our atmosphere to warm us. This type of radiating heat is easy to imagine and feel.
It is much harder to appreciate that everything around us is also radiating heat. Literally everything above absolute zero (0 degrees Kelvin, -273.15 degrees Celsius, or at -460 degrees Fahrenheit), which basically means everything. The trees, the rocks, the clouds, the ground… everything is losing heat as it radiates away.
Clouds act as blankets, stopping the ground from radiating much heat out to space. But under clear skies there is no blanket and the heat escapes easily and quickly.
The ground grows much cooler than the air. If there is little or no wind then the air that is in contact with the ground is also cooled by it. At this point water vapour (water in gas form) can condense to form liquid water droplets suspended in the air – what we call mist or fog. (Mist and fog are the same phenomenon, the difference is just one of visibility.)
In this case, because of the way it has been formed by heat loss through radiation, it is called radiation fog. It can form at any time of year in theory, but is most common in autumn and winter.
It is very common at the start of the day after a clear night. The rising sun or any wind usually mean it doesn’t last long.
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The Secret World of Weather – The Book