Hi Peter,
Weather does affect ground conductivity, but as you say, it won't affect the Mod. unless SWR goes really out
Orford was built to run at high power, so earthing systems on both antennas for 648, would be done really well
with no expense spared. This is not a Hospital Radio type site, where SWR goes out when it rains !

The Term "Earthing" refers to soil, which is a poor conductor, If the 648 Earthing was poor, at the sort of power
that the world Service used to run, worms would have come up round a poor earthing system
In fact if you are a Fisherman that needs worms, around 1Kw into a single stake at 50hz, will give you lots!
(Disclaimer, Don't try this at home)

Because Soil is such a poor conductor, proper sites have lots of wire buried to reduce the Soil loss
Offshore Radio had the big advantage that No soil was involved, hence better antenna efficiency

Rod Watts