OK 95% of the time 96.3 is a blank frequency in my area as I've even checked during inversions. The other day I signed off my FM frequency @ 6PM EDT. Around midnight I decided to scan the frequency to see if there was any distant station during an inversion. This time I picked up a station 152 miles away in Jacksonville, NC. The moral of the story is if you know there is gonna be an inversion in temperature in the area where you operate your FM station its a good idea to quickly go quiet ever hr 1/2 to be sure your not jamming a distant station that could be coming in and quite clearly during an inversion. Remember that during these conditions your station also will travel quite further than it would travel normally and @ 500 mW you would normally travel 1/4 mile on a rubber duck antenna (yes I know this from personal experience). Now this in mind we can be good broadcasters by practicing this respectful act and playing nice on FM. Even if your pushing a little past part 15 you will avoid a NOUO by not jamming anything. Operating after midnight actually makes this chance greater than regular daytime hours. It could be why some have gotten a NOUO. We know there are jealous stations out there, but if we show them we don't intend on jamming a licensed station it also shows the FCC that they should allow part 15 FM to be 1 Watt without issuing any NOUO or NAL.
I am guessing you are talking about tropospheric ducting? During the summer this is a well known effect for VHF and UHF bands including the FM broadcast band.
It can be as fun as it can be a pain in the butt since it is neat being able to listen to distant FM stations or watch TV stations from far away that fade in for a short span of time, but can be detrimental to local broadcasts being overpowered temporarily from the distant ones.
My guess is that most pirates are not running enough power to really achieve tropo skip during the magic times, or even if they do it is so short lived and not to be a huge worry, especially to the licensed stations running thousands of watts.
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