In a word, persistence. Intermittent faults are the worst. Openreach engineers hate them. Customers hate them. The problem is that by the time the engineer actually arrives, the problem has usually gone. The water ingress can be anywhere, so it's a potential nightmare.
Yeah, we have the very same issue with the wind and rain, which causes issues on our line, TBH I have just got so fed up having to get in contact with BT for them to resolve the issue, they just tell me its up to the engineer if they decide on replacing copper lines due to issues and in most cases that hardly happens.
So what I end up doing now is when it gets bad to a point where we have lost 80 to 90% of our speed I just wait for the rain to stop and then unplug the router and wait 5 mins and then power back up and hope.
If I see the SNR has gone up I then phone BT to reset it and then 2 to 3 days later it reboots and we are back to normal, well until the next rain fall that is.
We are still having loud screeches/static noise on our phone line, strange thing is only we can hear it and the people the other end cannot.
Many years ago I do recall having an engineer arrive a few days after when the rain had stopped where they checked the line and did say there was some issue on the line and that they would need to come back later to confirm this, they never came back, and we still have this intermittent issue and this is on both of our lines not just one, so it would probably need for the whole wad of copper line to be replaced, which will never happen, probably be cheaper for them to install fibre
Roll on fibre.