On 2009-09-21 23:07:44 +0100, "jbm" said:

Air frost
When the temperature in the Stephenson screen reaches zero, there is said to
be an 'air frost'. Even with an air frost, the ground can sometimes stay
above freezing. This often happens in early autumn, when the soil still
retains some of its Summer heat. Normally though the temperature at ground
level will be significantly colder!

Ground frost
Sometimes the air temperature at night dips to 3 or 4 degrees, but the
forecaster still warns of a 'ground frost' and the need to de-ice your car
in the morning. This is because the ground can reach freezing while the air
temperature remains above.

Hoar frost
The white crystals seen on the grass on a cold morning are neither a ground
nor an air frost. This is a 'hoar frost', which forms when the air cools and
water condenses onto the grass.



Taken from http://www.bbc.co.uk/weather/feature...ng/frost.shtml

Hoar Frost = Grass Frost.

jim, Northampton

Thank you! I'm going to bookmark that.
--
Sacha