Frosty Nights And Cloud

[WhiteFox]

A relatively simple question considering some of the more complicated teleconnections!

Anyway, when I were a lad, I used to do a milk round on Saturdays and during the week on school holidays. This meant starting work around 3am and working through to around 9.30. During this time (around 4 years) there were of course many frosty nights. I remember temperatures down to -10 and the milk freezing in the bottles. What I was wondering was this: why, on a frosty night when the temperature gets down to around -3 and there is a heavy frost, does the temperature rise significantly above freezing when a cloud layer comes across? I'm not talking about an approaching frontal system with warmer air, but rather patchy cloud which will pass over for an hour and then clear. This melts the frost, especially on cars, and then refreezes when the skies clear again. It's not a massively common phenomena, even now, and is noticeable when scraping the car in the morning by the fact that it's coated in ice rather than hoar frost.

I suspect it may be that the upper air temperature is above freezing, but my understanding is that cold air is denser than warm air, so I would have thought that a cloud layer would not promote warm air at the ground level? I fully understand why we get frosty nights under clear skies when upper temperatures are above freezing, but I've never understood the warming effect of transient clouds!

[Backtrack]

As far as I understand, it's not that it promotes warmer air at ground level, it's down to the radiation that clouds emit. That's why frost melts when it clouds over, although I've noticed they have less effect in the day.

Perhaps someone could explain better than me, all I know is that the radiation warms the air, but there are other factors that do this too, such as wind.

[casparjack]

Backtrack is right, clouds emit infrared radiation (in this instance described as downwelling infrared radiation - though they emit it in all directions), it is efficiently absorbed by the ground and whilst the passing cloud is present has the effect of raising the temperature of the air nearest to the ground, this is obviously particularly noticable on a frosty night.

On the subject of wind, the simple explanation here is that it mixes up the air and the cooler air nearer the ground (cold air is dense and tries to find the lowest point) is mixed with the slightly warmer air aloft, the windier it is, the less effect the radiaitve cooling will have and accordingly the minimum temperature will be higher than it would have been on an 'ideal' perfectly calm night (though in reality these are almost non-existent as air will always be subject to eddies due to uneven terrain and therefore is never totally still).

Hope this helps...

[WhiteFox]

Thanks Guys. I'm comfortable with wind affecting frosty conditions, but I didn't realise that clouds radiated heat! I guess that's why thin, high cloud doesn't cause the frost to melt whereas lower clouds will. Interesting.

So, to take it to a more advanced level, how low does a cloud have to be in order to melt frost? Assuming temperatures at the 850 level are just above freezing (say around 2oC as an example) and no wind. I'm certain there is a formula for calculating this, and I bet it's not that straightforward (encompassing humidity, water vapour etc.).

[casparjack]

It really isn't a case of how low in terms of there being an absolute limit, there are too many other variables to consider, though obviously the lower and denser they are the greater (and quicker) the heating effect.

The difference is that often high cloud heralds a change in air mass so often it might have a marked effect, though sometimes high wispy cirrus could pass through, but actually have little impact on the temperature, maybe stopping it falling rather than actually raising it. Whereas patches of Cumulus (perhaps former shower clouds) drifting through on a Pm air mass might raise the temperature back a couple of degrees above freezing, then once they have cleared it falls back below 0c again.

There are various formulae that one could consider, but that gets into the realms of NWP; you need to remember that on a clear, relatively still night there will be a temperature inversion, with the air closest to the ground being the coldest then rising to perhaps 1,500 ft, then beginning to fall again as one would usually expect in relation to the altitude.