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.