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.

"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.

Philip

On Oct 10, 12:25*pm, "Philip Eden" philipATweatherHYPHENukDOTcom
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.

Philip

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?

David Mitchell. Langtoft. E Riding.

Although, as Philip says, it is theoretically possible for the grass min to
read higher than the air min read over the same period, if it happens it is
normally a result of the presence of fog. When radiation fog forms, the
minimum temperature in the lowest few tens of metres moves up from the
ground to the fog top, along with the outgoing radiation. Outgoing long wave
radiation from the ground then is absorbed by the fog and reradiated back to
the surface, the two effects tending to lower the temperature at the fog top
and increase it at the ground. Once the fog is established and persists for
some time, the screen temperature will be lower than that registered by the
grass thermometer.
There are, though, other considerations. In your case it sounds as if you
are taking measurements with different types of instruments, which may have
an effect on the result when comparing values from each.
If you wish to see a continuous record of air/grass temperature difference,
graphs of the data from the Wokingham AWS can be found at:
http://www.woksat.info/wwp6.html

My experience is that, while grass min can be a couple of tenths above air
min, the occurrence is extremely rare when comparing data taken over the
same period. A value as large as you report is probably not due to a natural
cause.

Bernard Burton
Wokingham, Berkshire, UK.

Satellite images at:
www.woksat.info/wwp.html

----- Original Message -----
From: "whitehead1972"
Newsgroups: uk.sci.weather
Sent: Saturday, October 10, 2009 10:46 AM
Subject: can the grass minimum temperature be greater than the air minimum
temp?


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.

"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

On Sat, 10 Oct 2009 at 07:34:30, David
wrote in uk.sci.weather :

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 find it happens mostly when it's cloudy/wet overnight.
--
Paul Hyett, Cheltenham (change 'invalid83261' to 'blueyonder' to email me)

It happens here occasionally, and without fog. I have been putting it down
to the fact that the grass has been allowed to grow too long and then people
have been pushing the bulb down into the grass instead of having the bulb
just touching the tips of the blades of grass, which should be short.
Pushing the bulb into too long grass shelters the bulb, while it is the top
of the grass which takes the radiation cooling.

There was a talk by David Attenborough on Radio 4 this morning in which he
told of a species of ant which is very sensitive to heat. Normally, in this
particular field where the colony of these ants was, sheep had been keeping
the grass short, but when the sheep were excluded from the field and the
grass grew longer as a result, the ants overheated and the colony died out.

There are probably several possible causes of this phenomenon, eg fog, as
has been mentioned.

Ian Bingham,
Inchmarlo, Aberdeenshire.

"whitehead1972" wrote in message
...
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.