Why is there snow on that field only?

At this time of year in Cornwall, you may be suddenly startled to see what appears to be a field covered in snow. However, when looked at again you realise it is just another field being warmed up with plastic sheeting ready for the early, Cornish King new potatoes. If you look closely at this picture, the hill above Trefusis Point has such a field.

At least one or two fields in Cornwall

are honking with the devaststated remains of the cauliflowers that have been harvested; you can smell the stumps rotting from quite a distance.