Neil has just come in from a walk around his garden. The grass is covered with a heavy frost everywhere except under the trees.
Was the sky clear or overcast? How do you know?
The sky was definitely clear, and it is likely night or very early in the morning and the sun is very low.
Radiative heat transfer is a function of a surface at one temperature exposed (via line-of-sight) to a second surface at a different temperature. In this case the two surfaces are the sky (effectively outer space) and the lawn, and the exposed portion of the lawn (and anything else) will cool to below the air temperature.
As a result, frost can and will form on surfaces exposed to the clear night sky even though the air temperature is a few degrees above freezing. If the air temperature is more than a few degrees above freezing, then convective heat transfer from the air overwhelms the radiative cooling and there is no frost. Anything not directly exposed to the clear sky will not have this type of frost, and it doesn't happen if clouds block the sky exposure, as they are warmer than "outer space".
Once the sun is up, then there is radiative transfer from the sun that will quickly melt this type of frost.
We actually discussed this effect in a university level heat transfer class back in the day.
Edited on August 17, 2022, 7:28 am
Edited on August 17, 2022, 7:31 am
Edited on August 18, 2022, 12:10 pm
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Posted by Kenny M
on 2022-08-17 07:21:55 |
Solution, alternate explanation
