13 Nov 2013

Air frost and ground frost – what’s the difference?

Today, most of us woke up to a cold, frosty start with scrapers at the ready to remove the ice from car windscreens – never a pleasant chore first thing in the morning.

Considering that it’s mid-November, we’ve seen relatively few frosts so far, although that is likely to change into next week, as cold Arctic air spreads down from the north.

In order for frost to form, there are a few vital ingredients – chilly air, clear skies and light winds. These combined provide the perfect frosty recipe.

frosty_tree_g_wp

What causes frost to form?

Chilly air is the obvious ingredient, because if the air is inherently warm, then it doesn’t matter how little cloud or how light the winds are, it won’t cool sufficiently.

clear_moon_g_wpClear skies allow the heat from the ground to escape into space. This process is particularly effective in autumn and winter because the nights are longer than the days. This results in more hours of the day when heat can escape and thus more cooling.

Light winds are crucial to avoid mixing the air at the close to the ground with the air just above. As the air cools from the ground up, any mixing from above will bring relatively warmer air towards the ground – stopping the temperature from falling further.

So that’s how frost forms, but did you know that there are notable differences between an air frost and a ground frost?

Ground frost

A ground frost occurs when ice forms on the ground, objects or trees, where the surfaces have a temperature of freezing or below, causing water to freeze.

Weather stations measure the ground temperature with a thermometer that’s 5cm from the ground. When the temperature at this point hits 0C, then a ground frost has been recorded.

However, because the ground cools quicker than the air around a metre above, it is possible for a ground frost to occur without an air frost.

There’s also something called a grass frost, which is where natural surfaces such as grass freeze when man-made surfaces such as tarmac and concrete don’t. This type of frost is of most interest to gardeners.

Air frost

An air frost occurs when the depth of air above the ground has increased to the level at which weather stations measure air temperature – which is 1.25 metres in the UK.

So, effectively the layer of cold air near the ground hitting 0C or below has become thicker and thicker as more heat energy is lost to space and the temperature falls.

Hoar frost

This is another type of frost that we typically associate with a frosty morning and has a white appearance.

frosty_leaf_g_wpA white frost, consisting of little blobs of ice, occurs when dew has formed and then subsequently frozen when the temperature hits 0C.

A feathery frost, consisting of feathers and needles, occurs when the temperature has already reached 0C and then dew forms.

Don’t forget, you can keep track of the next frosty night on the Channel 4 Weather website. You can also send me your frosty weather pictures on Twitter – @liamdutton

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2 reader comments

  1. David Reeks says:

    Liam, you say “This process is particularly effective in autumn and winter because the days are longer than the nights.” Surely it is the opposite way round? Nights are longer than days.

  2. Roger Hammond says:

    I have just noted how a defined section of ground has been affected by a ground-frost.

    From my house I look over a very shallow valley which falls gradually to one end.
    There is a hedge on either side of the valley.
    At the lower end of the valley there are trees across the width
    As the mist gradually escapes, it reveals the ground from the higher part of the valley which is positively ‘white-over’ with what will be ice particles.

    Now the sun is rising and the dispersal of mist is increased.
    The sun falls on the valley floor and whilst I write this the green of the grass is revealed again.
    I think this is the first time I have witnessed this process

    At age 71, I am reminded of my early 20’s when my studies captured my imagination as I learned of something which I now fully appreciate.

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