Frost

I’ve spent all year thinking about ice and ice ages. And so when I woke this morning to find that there was frost on the ground outside, I examined it with unusual interest.

And one thing I noticed was that while the frost had settled on some surfaces, it hadn’t settled on others. And in the garden outside my flat the frost had settled on the grass, but not on the tarmac paths that crossed the grass, or the tarmac roads adjoining them.

And I also noticed that the frost had settled on the windscreens of the cars parked in the roads, but had not settled on the bonnets or roofs of the cars.

Why was that?

My explanation for why the frost had settled on the grass was that the grass forms an insulating carpet through which heat doesn’t flow easily, and so even if the ground beneath the grass was quite warm, the insulating grass would prevent the frost from melting. But the tarmac roads and paths were not good insulators, and conducted heat more easily from the ground to the frost, and melted it.

I offered a similar explanation for the absence of frost on the car hoods. Cars are made of highly conductive steel, and this steel would readily conduct heat to any frost on its surface.

But what about the glass? Glass is not as conductive as steel, but it’s not a good insulator either. It probably has a similar conductivity as tarmac or stone. So why had the frost melted on the tarmac, but not on the glass windscreens of the cars?

But then I thought that the windscreen is transparent, and droplets of water on the surface of the windscreen would have radiated heat through the windscreen into the interior of the car. And during the night the metal cars would have got very cold. They were probably colder than anything else, because their metal bodies conducted heat quickly away. And this may be why cars are like iceboxes in winter, and ovens in summer.

So droplets of water on the windscreen would radiate heat through the windscreen into the cold interior of the car. The interior of the car would warm slightly, and the water drops would cool and freeze to form frost.

If this was the true explanation for the presence of frost on the car windscreens then I wondered if one way of preventing frost forming on windscreens might be to put a layer of reflective kitchen foil on the inside of the windscreen. This foil would act to reflect radiated heat from the water droplets back onto them, preventing them from cooling and freezing.

I should be able to find out, simply by taping a sheet of kitchen foil onto the inside of my car windscreen the night before a sharp frost. Maybe even a piece of white reflective card would do. If it works, the car windscreen should be covered in frost, except above the area of foil. But I’ll have to get up early in the morning to see it, because the sun usually melts any frost fairly quickly.

And if it doesn’t work, it’s back to the drawing board to try to find another explanation for why frost forms on car windscreens but not on their roofs.

It reminds me that, back in the 1950s, when I often slept in an unheated bedroom, the windows would often be covered in frost in the morning, perhaps because it was almost as cold inside as it was outside. But since I’ve been living in warmer houses, I seldom have seen such frost forming on windows.

About Frank Davis

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13 Responses to Frost

  1. RdM says:

    Actually glass is a pretty good insulator.
    Just search on ‘ heat conductivity of glass ‘.
    Reflect (no pun intended) on thermos bottles, etc.

    • Frank Davis says:

      I have looked:

      Glass is made of sand and other minerals that are melted together at very high temperatures to form a material that is ideal for a wide range of uses. Since it is an amorphous solid material, it has not high thermal conductivity. Its thermal conductivity is about k = 1 W/m.K

      By contrast the thermal conductivity of iron is 42 W/m K, and of expanded polyurethane 0.02 W/m K, and water 0.59 W/m K

      Thermos flasks have (I believe) mirrored vacuums inside them. The vacuum prevents conductive/convective heat flow. And the mirror reduces radiative heat loss. The glass itself only provides a minor part of the thermal resistance.

      I’m now wondering whether leaving a bucket of water inside a car might stop it freezing inside during cold nights.
      .

  2. RdM says:

    Just a brief initial 1:30 am thought though!

    The tarmac is thicker and has retained the days heat longer of course.
    And you’re right – glass may be a good electrical insulator, but still, although not conducting heat as well as metal (or diamond?) perhaps conducts some. At least retains it (insulator!).

    Hot tea in a glass. Those glassy cooking surfaces.
    So try your aluminium foil reflector, report results!
    I suspect though that simply the steel roof is just much better conductor than glass.

  3. wobbler2012 says:

    Haha the old “Christ almighty there is ice on the inside of the windows” thing. Kids these days will never have experienced that phenomenon!!

  4. Doonhamer says:

    Freezing bedrooms.
    Made sure you dressed quickly in the morning.
    No faffing about in pyjamas.

  5. Peter Carter says:

    Actually, all windscreens are laminated these days i.e. made of plastic sandwiched between two layers of glass.

  6. Mick Walker says:

    Frost on the windscreen, sometimes on the back window too, but only rarely on the side windows.
    All made of glass.
    Difference – the angle.
    Because the water runs off before it freezes?
    I’m told no, it’s because the vertical glass is radiating to nearby houses, but the windscreen is radiating to space.
    I’m not quite convinced.
    What do others think?

    • Frank Davis says:

      rarely on the side windows.

      True. But that could be because (I believe) the frost falls first as dew, as a kind of gentle rain. And because it falls, it lands on horizontal or sloping surfaces, and not on vertical ones (except if there is a wind).

      That said, it’s perfectly true that the sloping windscreens will be radiating more to space than to adjacent buildings. And on cloudless nights they’ll be radiating more heat than on cloudy ones. And since these days windscreens have a flatter and flatter slope, they tend to radiate more and more. Keep a look out for cars that are parked in narrow streets, with little sky above them, and see if they have less frost on their windscreens?

      But I think the dew/frost on the windscreens will not only be radiating to space, but also through the glass into the interior of the car because the windscreens are transparent. So I think that my silver foil idea might still work, by reflecting back this heat radiated inwards.
      .
      No frost this morning though. Yesterday was a short cold snap.

      • Rose says:

        Frost flows downhill so it probably flows more slowly on the windscreen, more rapidly on the vertical side windows. With gardening the trick is to get the frost to form on the top of the covering and not to freeze directly on the plant as the water in the plant cells would expand and fracture them.
        A hardy plant has already solved this problem and doesn’t need the protection.

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