On Saturday, November 29, 2014 10:58:05 PM UTC, Dave Liquorice wrote:
On Sat, 29 Nov 2014 18:02:29 +0000, Grimly Curmudgeon wrote:

Air temp approaching -10 C under clear skies and the windscreen
and
frost will be approaching -20 C.

What?

A clear night sky is at around -40 C. Thermo dynamics mean that the
different temperatures try to equalise but space is a bit big and
there isn't enough heat in the screen to warm up space. So the screen
just radiates heat into space and cools and cools and cools...

Bit of cloud cover reflects the radiation back keeping things warm.

It's perfectly possible to get a hefty ground frost with the air temp
remaining above zero.

--
Cheers
Dave.

Inversion. I've seen it flying through 500' on a frosty morning. Local coal fired power station was pumping out black smoke. When it reached the inversion it became trapped and spread across the sky like a river. Wish I'd had a camera with me