On 19 Sep, 18:01, John Hall wrote:

The narrowest of ground frosts here this morning: -0.1°C. Screen min
3.4°C. First ground frost of the winter, about a week earlier than
normal.

I'd imagine that what is regarded as "normal" would be greatly affected
by how many years your record covers, as it's my impression that the
first ground frost (and air frost too, for that matter) now tends to be
a good deal later than was the case in the 1960s and 1970s.

I have records of daily grass minimum temperatures for four local
sites south of Reading back to 1959, except for the period Jun 1998 to
Nov 2003. There are inevitable differences between the sites, but all
are within 6 km and since 1987 I have used my own records, from two
sites within just 1 km of each other. My 'normal' period referred to
this 20 year observation period, but with the caveat 'about' as the
date of the first ground frost varies much more than that of the first
air frost and the averages can fluctuate considerably, particularly
after a single 'early' event..

For the record, the average dates of the first ground frost here by
decade are as follows:

1961-70 8 October
1971-80 22 October
1981-90 5 October
1991-97 1 October
2003-07 30 September

1988-2007 30 September - this was the average period used in my
original post.

The latest 'first ground frost' was on 20 Nov 1978, the earliest
(taken as the first ground frost after 31 July) was on 1 August 1986.
The only other August ground frost was on 20 Aug 1964.

Contrary to your comments above, the earliest ground frost between
these two dates (1964 and 1986) was 26 Sept, in 1972, and the decade
1970s had by far the latest 'first ground frost' in the near 50 year
series. The sharpest 'first ground frost' was in fact relatively
recently, on 24 Sept 2003, at -5.9°C (air min -1.5°C). It may be that
the increased frequency of warmer, drier summers in the last 20 years
or so - at least until recently - may establish more favourable
conditions (drier soil) for earlier ground frosts than was the case in
the generally cooler and wetter summers of the 1960s and 1970s (some
notable exceptions, of course).

--
Stephen Burt
Stratfield Mortimer, Berkshire