With the weather forecast promising a cold morning and clear blue skies I booked a day off work in the hope of getting a taste of winter which so far had eluded us this year. To increase my chances of cold weather I drove the few miles up the road to Priddy Mineries high on the Mendip Hills.
Priddy Mineries is a Frost Hollow. Following a warm day the skies can remain clear overnight. On these nights the lack of an insulating cloud cover causes the ground to leak heat out into space. On the top of North Hill this cold air near the ground flows down into the valley and collect around the ponds of Priddy Mineries. It cannot escape and so a hoar frost forms on the frozen ground and grasses.
A hoar frost forms when the temperature of the ground and grasses drops below the freezing point of water before the dew starts to form. When this happens tiny ice crystals form, instead of water droplets, creating a white feathery frost close to the ground.
Priddy Mineries is a former Somerset Wildlife Trust nature reserve. In the past the area had been mined for lead creating a series of ponds, mounds and hollows. The picture below shows the main path throw the site. This morning it was partially covered in a white hoar frost. I headed out to the first pond on the left.
The grasses around the edge of the pond were still covered in a hoar frost. The sun takes a while to reach the ground in the hollows – this bought me some extra time.
To photograph the frost I had to lie on the ground to get a low enough angle to look across the water. One lady was very surprised to find somebody lying next to the water. She was very relieved when I sat up to show her I was not dead.