In message , Metman2012
writes
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.

Indeed. Which is why Paul's local car owners may be justified in
covering their cars, even when the minimum air temperature is forecast
to be a couple of degrees above freezing.
--
John Hall "Hard work often pays off after time, but laziness always
pays off now." Anon