Temperature inversion layer above Fairfield taken from from St. Sunday Crag. This climate phenomenon is due to a layer of warmer air above the cold dense air mass closer to the ground. I believe that the principle reason for this formation is due to a combination of the high pressure air mass which was sat on top of the UK and the low angle of the winter sun. According to Wikipedia “An inversion is also produced whenever radiation from the surface of the earth is less than the amount of radiation received from the sun, which commonly occurs at night, or during the winter when the angle of the sun is very low in the sky.”
Conditions in the higher layer were extremely bright clear air with temperatures around freezing, maybe +1° Celsius. Visibility was in excess of 20 miles despite the haze and the wind was steady around 20 mph. Within the lower layer the temperature must have been closer to -8° Celsius and the wind significantly stronger and the humidity; judging by what most people call hoar frost, but is in fact advection frost, which formed on everything and every one; close to 100%. The visibility within the colder layer was down at around 15m and the transition between one layer and the next was very sudden, within around 20m.
There seems also to be some particular aspects of this specific fell and it’s surroundings that makes this dramatic formation occur regularly. I have seen and photographed it before on several occasions over the years.