I had a friend who was with the FAA. I once asked him a very stupid question about why I couldn't hear a plane switching from HF to VHF to land at Pittsburgh but I could hear the Shannon flights calling St. Johns, NF like they were going over my house? His answer, "Well Pigmeat, the Shannon ones are way up over the Atlantic, height and salt water props. The ones coming into Pittsburgh are a couple of thousand ft. over trees and squirrels. You figure it out."