You never can tell. My local airport is 12miles away on the other side of a long hill - most times I get a very weak signal from the ground frequency. Yesterday it hosed down with rain most of the day - the ground frequency and even aircraft on the ground boomed in. Now it's a fine clear morning and not a peep from either.
You see, RF is like some birds. Some birds don't like rain so they go sit it out somewhere. RF is the same way, when it rains they go get in an antenna somewhere. So, you seem to have birds/RF that dislike rain and are sitting it out in your antenna. ... Hand me another beer, would'ya?