I love a snow day. Who doesn’t? They’re usually filled with cosy pyjamas, warm mugs of hot chocolate and sledging down the local hills (which are few and far between in low-lying Essex). We might usually be blessed with a day or two of snow in February, March or April, but this year, many of us Essexonians have enjoyed snow in December. It has been bone-shatteringly, teeth-chatteringly cold and I, for one, have made the most of the Christmassy chill.

Before the snow there were days and days of sub-zero temperatures, painting the gelid landscape in crystal white. Splashes of robin red zipped across the muted canvas and landed on branches, fluffing up their feathers to keep warm. On one of these crystalline days, someone described the ice as a ‘hoar frost’, which got me thinking: what are the different types of frost?

Air Frost

According to the Met Office, ‘an air frost occurs when the air temperature falls to or below the freezing point of water.’ This is usually defined as the air temperature being below zero at a height of at least one metre above the ground.

Ground Frost

A ground frost is when ice forms on the surface – as well as objects or trees. These surfaces have to reach a temperature below the freezing point of water and, because the ground can often cool quicker than the air, a ground frost can occur without an air frost.

There is another un-official type called a ‘grass frost’ which takes place when other surfaces like concrete or tarmac don’t experience a frost as they are better at holding onto their warmth. Gardeners have to be aware of grass frost in particular as it can occur into late spring and even early summer, after the risk of more wide-spread frosts have disappeared.

Hoar Frost

A hoar frost is composed of tiny ice crystals and formed by the same process as dew but when the surface temperature is below freezing point. It can have a ‘feathery’ (or even ‘fluffy’) appearance which forms when the surface temperature reaches freezing point before dew begins to form. When dew forms first and then freezes, the ice looks more globular – like frozen droplets – which is called a ‘white’ frost. If the night is foggy rather than clear then the formation of hoar frost is less likely because fog prevents surfaces from cooling as quickly.

Glaze and Rime

Glaze or rime is sometimes confused with frost. I had never heard of either! According to the Met Office, ‘Rime is a rough white ice deposit which forms on vertical surfaces exposed to the wind. It is formed by supercooled water droplets of fog freezing on contact with a surface it drifts past.’ However, ‘Glaze can only form when supercooled rain or drizzle comes into contact with the ground, or non-supercooled liquid may produce glaze if the ground is well below zero degrees Celsius. Glaze is a clear ice deposit that can be mistaken for a wet surface and can be highly dangerous.’

If you’d like to learn more about frost and ice then check out this article from the Met Office about ice pancakes – a phenomenon where spectacular discs of ice form in very cold oceans and lakes. As these are usually observed in the Baltic Sea and around Antarctica, we’re unlikely to see them in Essex!

Although it can be a dangerous slippery customer, frost is undoubtedly a natural wonder. Drawing open the curtains on a winter’s morning, it glistens in the golden sunrise and sings of the joys to come that day. It sees us through the darkest time of year and ensures one truth: in the bleak midwinter, there is magic to be found.

Jack Frost

I saw him once, that boy of ice,
December was the time of year.
He looked around and through the trees,
Checking that the coast was clear.
From his bag he took a brush,
With a set of paints and palette, too.
“Spring rains will make leaves green again”,
Then he painted autumn colours through.

Silver on birch, gold on oak,
Between trees he left a trail of snow
Which sparkled against the burning leaves,
He smiled at me, then turned to go.
“Jack Frost’s the name!” he said in cheer,
He left the wood, sliding into the street.
Frost’s voice was icily sharp and clear.
His evening’s work was incomplete.

On every house, like a game,
He painted all the windowpanes
In frost. When he got to mine, he wrote:
“Jack. Keeper of the wintertide”.

Now every year he visits us, and speaks to us in icy ways,
The crunch of glass, the clink of glass, and snowball fights on snowy days.
I ask you now, in warmer months, on winters past you must recall:
Who started the fun?
Who brought the snow?
Who threw the very first snowball?

Andrew Millham

Essex Wildlife Trust Ambassador

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