As The Chief used to explain...
As The Chief Engineer used to explain -- back when there WAS one -- this is seasonal atmospheric "inversion."
Not just FMs. This used to play hob with the VHF 2-ways when I was at WTOP. We had to call WCAU/Philadelphis, which shared the channel, and negotiate when-in-the-hour our respective traffic guys could feed.
Also on VHF: I'm hearing guys from all over the place on Marine channels, as usual this time of year. Important to switch to low power on transceivers. If I hail the harbor launch here on Block Island @ high power from the top of my mast (52'), it'll sound like I'm in New London to receivers in New London. Easy does it.
Similar deal with FM.
There's a stretch here on Block Island, on Corn Neck Road, north/south between Fred Benson Town Beach and Scotch Beach, where, momentarily, New Jersey 101.5 comes in on-top-of WWBB...but only in the summer. First reported by my client, amigo, and fellow uniformed BlockIsland.TV Fourth of July Parade street team member Eric Johnson. A funster.
Still, as we learned on The History Channel, extraterrestrials in distant galaxies are NOT hearing us light years in the future. Apparently, RF dissipates within our Solar System.
STILL, keep one finger on that Dump button, eh? There are children in the car...