This subject comes up regularly, and the received wisdom
is that AWSs provide tremendous detail about the character
of rainfall events, but - especially for those guages in an
elevated position - catch less than a standard manually-read
gauge (127mm diameter copper gauge with its rim 30cm
above the ground.)

The Eden mantra is: "Always install a standard gauge
alongside your AWS".

Here are some comparisons (Manual/AWS):
At Luton the two sites are 1.2km distant so there will
occasionally be some geographical difference between
them, but the land is pretty flat and the difference in
altitude is only 2m. The AWS gauge is 1.6m above
the ground:

July 2007 1.04 (significant geog. difference on one day)
August 1.12
Septmbr 1.24 (significant geog. difference on one day)
October 1.14
Novmbr 1.15
Decmbr 1.08
Jan 2008 1.13
Mean 1.13

At Chesham, the two gauges are on the same site, but
the AWS gauge is 1.6m above the ground:
Oct 7 - Nov 6 1.04
Nov 7 - Dec 2 1.07
Dec 3 - Dec 17 1.07
Dec 18- Jan 4 1.06
Jan 5-28 1.05
Mean 1.06

Philip Eden