Insights & News

Frost – Part 1: Bright Side…

6th December 2019

If you’ve ever looked a little closer on a clear, cold winter morning,
you might have noticed differences in frost patterns and formation:
Find out where, when – and why – you’ll find crystalline,
continuous and spiked frost manifestations…

 

Cold Contact

Whenever the surfaces of outdoor objects fall below freezing, the stage is set for some form of frost formation. As atmospheric moisture meets the sub-zero surfaces, it freezes.

This happens more readily than you might think. You see, heat is a representation of stored energy, and objects generally can’t hold onto that. The energy radiates away, at a rate proportional to the thermal conductivity of the object. So, objects tend to cool unless more energy is put into them.

Sometimes, even the most ordinary of surfaces can be transformed by frost & ice.

In many objects, especially most metals, this radiative cooling happens so much that their surface temperature can fall below that of the air around them. Grass is not as conductive but can still become several degrees colder than the air. That’s how ‘grass frost’ occurs, while the air temperature is 1-3°C above zero.

“Grass is not as conductive but can still become several degrees colder than the air. That’s how ‘grass frost’ occurs, while the air temperature is 1-3°C above zero.”

Dry Frost is Dull Frost…
Where’s it Most Magical?

Freezing surfaces aren’t going to produce visually striking results if the air is too dry.

Atmospheric moisture tends to be highest next to large bodies of water. Well inland, away from any lakes, there’s rarely much moisture to be found. Such places – e.g. Moscow – have a continental climate, which is much drier than the maritime one experienced by the UK, for example.

For this reason, frost there tends to be visually unspectacular.

Fun fact: The low atmospheric moisture also leads to most of their snowfall being very ‘powdery’!

To see the heaviest frost, you need to be somewhere that has a maritime climate yet sees overnight temperatures near-freezing. The places that see that most often tend to be within about 25-100 miles of a large water body.

Frost Has Many Faces…
Crystalline, Rime & Advection

In calm or near-calm conditions, frost tends to take a crystalline form, like in the example below-left.

However, if there’s a really high amount of moisture in the air, it can instead form a near-continuous ice layer. This is known as ‘rime frost’, of which I’ve also snapped an example (below-right).

Heavy frost on the left, rime frost on the right.

When it’s windy but still cold, you may instead see advection frost. That type takes the form of small ice spikes, which can become very dramatic in freezing, windy weather.

So that’s the pretty part… read on in a few days to discover its darker side…

 

James Peacock MSc

Head Meteorologist (…and frost enthusiast!) at MetSwift

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