"David" wrote :
On Oct 10, 12:25 pm, "Philip Eden" wrote:
"whitehead1972" wrote:

Mornin' all (again). Today I recorded a minimum temperature of 8.2c on
my Oregon Scientific thermo. My Casella sheathed alcohol thermometer,
however, recorded a minimum of 9c. Is it possible that the grass
temperature can be higher than the air temperature?
The previous night's readings were 6.2c (air) and 5.5c (grass).
The site of the grass minimum is not Met Office standard (my garden's
not big enough) being situated close to a sheltered gravel board fence
and shrubs. The ground make up is imported loam over a clay base.

Yes, it can. In fog, for instance. And after the passage of a cold front
after sunrise but before 0900. Mine was on Wednesday morning.
It will happen more often (and your grass min will be rather higher
than it should be on radiation nights) if the thermometer's 'sky view'
is limited - by shrubs, a fence, or neighbours' trees, for example.

: I think I can count on the fingers of one hand the times it 's
: happened in the last 10 years. Yet Philip has had one this week! I
: must admit that the last 2 locations have not really been fog prone,
: which rules that option out. Has it happened many times with you
: Philip?
:
I guess no more than four or five times per year at most.

On Wednesday morning the cold front went through between 0400
and 0500, the temperature dropped rapidly from about 15-16°C
to 11°C and then more slowly to 8.8°C at 0800. The sky was
still overcast, with much moisture on the ground, so by the time the
grass temperature had hit its minimum of 9.3°C (my guess is
around 0700-0730) there was sufficient incoming radiation
(in spite of the cloud) to halt the cooling process.

Philip