On Mon, 3 May 2010 12:00:02 -0700 (PDT), Dave Hill wrote:

Have you noticed how some of the forecasters now talk about a Grass
frost rather than a ground frost?

Presumably they are just being more accurate about the figure they
are reporting. The stations measure "grass" temperature and
"concrete" temperature. The grass temp is that at the top of short
(2" long IIRC) mown grass. The concrete thermometer has it's bulb
just in contact the the surface of a concrete slab.

--
Cheers
Dave.