A frost hollow is a local low point in a landscape where cold dense air collects overnight. It is also known as a ‘frost pocket’.
Under clear skies, the heat radiates out of the land. This creates a very cold layer of air near the ground.
Cold air is denser than warm air. As soon as it forms, it flows slowly downhill, like treacle.
This cold heavy air flows downhill with gravity until it reaches the lowest points in the landscape. And this is a frost hollow. It is where the cold air sits, freezing anything it touches.
Frost hollows are very local.
In the photo above the air has flowed down off the hillside, but then stopped as it hits the raised bank at the edge of a road.
Some frost hollows are tiny. I went for a walk on a cold morning in Yorkshire recently.
As I crossed a large field, I only saw one fully frost-covered leaf. It was in the lowest spot in the field, a frost hollow that was only a few inches wide.
From the book, The Secret World of Weather:
Land is never perfectly flat, and its shape makes a huge difference
to frost.
Convex landscapes, like hills, are frost-unfriendly, and concave landscapes, like valleys, welcome it. There are two main reasons for this.
The first is that hills are exposed to the wind, which dries
the ground and mixes the warmer, higher air with the very cold air
below, making frost less likely. But the main reason is that cold air is
denser than warm air and will flow downhill, like molasses.
Let us imagine a hill and a neighboring valley during a very
cold starry night. The heat radiates out of both in similar ways.
We now find that we have a layer of very cold air just above the
hilltop and the valley. Gravity pulls the cold dense air downhill.
The cold air in the valley has nowhere to go and sits there, but
the cold air on the hilltop will flow down toward the valley. This
has two effects: It leaves the hilltop with warmer air and adds
another layer of cold air to the valley, sandwiching that in. This is
why some areas are prone to severe frosts and are known as frost
pockets or hollows. It is also the reason that hilltops are often a
lot warmer than valley bottoms at the start of the day after clear
skies.
A Frost Hollow Cold Case Study
My thanks to Lex Harrison for writing with his own interesting observations about a local frost hollow:
Hi Tristan,
We were really inspired by your talk yesterday at Dell Quay Sailing Club and my son Samuel is now set on the journey to become a natural navigator!
When you signed our books I briefly mentioned a strange phenomenon creating a cold spot on the Downs nearby my home and you asked me to send you more details:
I live in Bury, at the NW corner of the ‘crossroads’ of the South Downs and the River Arun. I used to cycle daily to Amberley Station on my Brompton folding bike and had a strange occurrence on two occasions around ten or so years ago during cold winters where my brake cables froze solid. The location is a tight ‘S’ bend where the road dips to a very low altitude (6m), immediately at the base of the Downs, below a woodland which forms what I understand is known as a ‘hangar’ (a sort of crescent shaped bowl in the hillside – a local Sussex word I am guessing).
On both occasions I braked as I approached the S-bend (heading south), but when I released the levers both brakes stayed firmly on and I had to stop and force the callipers apart by hand. I could also physically feel a significant temperature drop at that section of road and then rise again – perhaps for only 30m, across the lowest point. Thus my conclusion was that moisture between the inner and outer brake cables had rapidly frozen due to a sudden drop of temperature.
I’m guessing it was something to do with cold air being funnelled by the shape of the hangar and rolling down to the lowest point, but I’d love to know your thoughts. You probably know the steep road section of the A29 (Bury Hill) which runs across the top of it. I’ve been cycling for years and have never known this happen, other than due to mud/crud in the cable, but these were spotlessly clean at the time.
Below is a map of the location and a 3d visualization from the OS Maps app. The 3d isn’t especially accurate but gives some idea of the topography – it’s looking ESE.
Hi Lex,
Fascinating example. I think your theory is right, this sounds to me like a ‘frost hollow’, where cold dense air pools at the base of a slope (see p.87 ‘White contours’ section of The Secret World of Weather for more).
Frost hollows are most extreme when there is something blocking the cold air from continuing its journey, eg. an embankment or other sharp upturn in the gradient. I’ll check next time I’m there, but can you recall if there was something like that on the opposite side of the steep woodland above the road?
Thanks Tristan, I’ll read up on that. There is a hedge on the downward side of the road, and now I think about it the field beyond does have a deep hedge then a small incline before it drops away again towards the river which may well be blocking the cold air . All makes sense! Thank you for opening my eyes further!
Lex
You might also enjoy:
Which Way Are We Looking – Frosty Basketball Edition
The Secret World of Weather – The Book