I've noted changes in the local amers here depending on the rainfall. If it's been a few weeks or so since the last good soaker, the sig levels are down across the board - not much but noticeably so and consistent, if it rained recently sig levels are back at peak levels. I wonder if the ground plane conductivity at the tx site is more to do with this effect than the rx site.
That could very well be the case. We have been in a pattern of generally lots of rain, but the occasional day or two will go by with no rain. Going to be dry the next few days, I'll check their signal and see how it behaves. My guess is this station is roughly ten miles away, so it is certainly all ground wave propagation.
As I've mentioned before, it is one of my test case stations / beacons of sorts, for checking out antenna designs and tweaks. I like to use ultra weak / just audible stations for this. I have a pair of AWOS stations on the VHF aircraft band I use the same way, one is about ten miles away and puts in a weak but generally audible signal, the other is maybe 25 or so miles away and is just barely above the noise floor.