Metman2012 wrote:

On 02/12/2020 10:55, John Hall wrote:
In message , Vidcapper
writes
On 01/12/2020 09:46, Bernard Burton wrote:
"Keith (Southend)" wrote in message
...
This November I only had two 5 minute logs (10 minutes) of 0.0°C, is
this classed as an air frost or should it be -0.1°C?

My thinking is not, so no air frost reported for me for the whole
month.

Keith (Southend)

Yes Keith, temperature needs to be below zero to warrant a frost


Try convincing the local car owners of that, though!


The temperature of car windscreens, and even more so car roofs, is I think
often a degree or so lower than the air temperature though in clear
conditions, as they are radiating heat directly out into space. So if the
car owners are covering their cars with tarpaulins, they may have a point.
But Keith is talking about an air frost. If you have a grass minimum
thermometer you can report the ground temperature and so have a ground frost
when teh air temp is above zero. On clear calm nights this can be many
degrees below the air temp measured at ~4feet. On windy nights it will be
very similar to the air temp. Cars are not good places to shelter in a cold
calm night - they radiate faster than the ground does - as I think John is
saying.

I had an interesting one yesterday morning. The minimum air temp was +0.3C but
the garden pond was completely frozen over. There was a Force 2-3 breeze
blowing so I suspect that the surface of the pond cooled below 0C due to
evaporation. A good example of physics at work.

--
Norman Lynagh
Tideswell, Derbyshire
303m a.s.l.
https://peakdistrictweather.org
Twitter: @TideswellWeathr