On Wed, 1 Oct 2003 08:37:01 +0100, Mike Tullett
wrote:

That wouldn't surprise me at all. Don't forget there is a very large heat
source only an inch or so beneath the grass minimum thermometer - the still
warm soil, especially after such a warm summer. At night, there will be a
considerable flux of heat upwards to the suraface and the soil will be
radiating more LW radiation than the screen top (there will be an added
supply of heat by simple conduction from the soil to the overlying air).
Some of this radiation will be intercepted by the grass min thermometer and
it will react accordingly.

....which reinforces your earlier point that frost tends to form on
exposed car roofs and upward facing glass surfaces before the ground.
The car's upper bodywork is a relatively small heat sink; the car
structure and the air inside it tend to insulate the car roof from the
ground heat source.

--
Dave