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The temperature last night in my suburb and in Montreal was 4 Celsius at 9 P.M. . The snow and rain of the day had melted and the roads were damp but not icy. This morning at 5 A.M, the temperature was 2 celsius. There was no moment during the night when the temperature dropped below zero. There was a south-west wind of 40 km with gusts at 60-70 km/h. And, yet , the roads had iced up . How could it be ?

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  • $\begingroup$ Had the temperature been below/near zero for an extended period of time prior to last night/today? $\endgroup$ – tpg2114 Feb 26 '18 at 16:33
  • $\begingroup$ This is often due to the way - or rather the location - those temperatures were measured. The temperature must have been below zero at road-level for ice to form. But the stated temperature might follow a standard and be measured in a certain height over the ground, maybe 1 metre. Up there the temperature can be higher. $\endgroup$ – Steeven Feb 26 '18 at 16:41
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The temperature measured in the weather reports is the temperature of the air (at 2 meter height). But a surface can become much colder. Especially on clear nights. These surfaces are radiating as black bodies. The most important thing preventing surfaces to cool down to the 3 kelvin temperature of the universe may be radiation from the carbon dioxide in the upper atmosphere at 260 kelvin. Or warmer clouds at lower levels.

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    $\begingroup$ A neat experiment: Take a thermos bottle, put some water in the bottom, and leave the lid off. Put a small thermocouple probe in the water. Take it outside on a clear, still night above 0C. Watch the water temperature drop, and potentially freeze, even though the air is above freezing. Radiation at work! $\endgroup$ – Jon Custer Feb 26 '18 at 18:32
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This is something that happens pretty frequently. To see how it happens think of things from the point of view of the road surface & what determines its temperature. This is three things, one of which comes in two parts:

  • direct exchange of energy with everything below the surface;
  • direct exchange of energy with the atmosphere above the surface;
  • radiation to and from everything above the surface, which can be thought of in two parts
    • the atmosphere,
    • and everything above the atmosphere.

Generally the first of these will heat the surface (although it can be the case that the ground beneath the surface is much colder than the surface in the case of permafrost in the summer, for instance).

The second includes things like conduction between the surface and the atmosphere in contact with it, but also evaporative cooling if the surface is wet and there's a lot of wind.

The final part -- radiative balance -- depends on what the whole atmosphere above the surface looks like, and in particular how opaque it is at the frequencies of interest, as well as what is above the atmosphere. If it is cloudy, then the surface can't really 'see' anything above the base of the clouds, and so the radiative balance will occur between the surface, the atmosphere below the clouds and the base of the clouds (of course, you need to know the temperature of the base of the clouds to say much useful about this). If the sky is very clear then the radiative exchange between the atmosphere and the surface may be rather low, and the surface will 'see' what is above the atmosphere. If it is night then much of this will be at 2.7K, so the surface will lose energy to it.

So, now, there are two cases where superficially odd things can happen.

The common case: still, clear air at night. In this case the surface exchanges relatively little energy directly with the atmosphere above it, and when it does it often results in a boundary layer of cold air just above the surface which is rather stable. But because the atmosphere is very clear the surface can exchange radiation rather freely with everything above the atmosphere, and this is very cold on average. The result is that the temperature of the surface drops, and often drops well below the temperature of the air immediately above it.

A less common case: wind and rain/snow. If the surface is very wet, and there is a strong wind, then the second thing I described above can become very important: there is furious evaporation from the surface as the wind dries it, and this can cause the surface temperature to drop significantly as latent heat of evaporation gets stolen from it. This can continue until either the surface dries or its temperature drops far enough that evaporation slows significantly.

My guess is that what you have experienced is the second of these effects.

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