> What atmospheric conditions cause FM to travel farther than line-of sight...
OK, very complex question -- one that can only be touched upon here. That being said, a basic primer:
FM broadcast, like all VHF signals, can be bent in the troposphere, the lowest layer of the atmosphere. What generally causes this is a temperature inversion, a basic reversal in the usual lapse rate in the atmosphere. Hobbyists call this tropospheric ducting. And, it's cool. Nice ducts can go for literally hundreds of miles.
Look for cool, dry air at the surface, and more warm, juicy air above. When the vertical boundary between the two layers becomes sharp, nice bending or refraction can occur. And, yes... to some extent, this can be predicted.
Bill Hepburn, a meteorologist and hobbyist in Canada, releases prediction maps daily. They are experimental, and results vary, but, they can be useful. A link:
http://www.dxinfocentre.com/tropo.html
There is also a real-time map that shows ducts actually happening. It works off 144 MHz Ham radio data signals, called APRS. Again, it's very cool.
http://www.mountainlake.k12.mn.us/ham/aprs/path.cgi?map=na
Again, this is all just a starting point, but will begin to give you a feel for what is happening in the troposphere. But, the troposphere is not the endpoint to all this -- the ionosphere does occasionally refract FM broadcast. It's mainly seasonal in the early to mid-Summer. Look for openings from late May, peaking in early July. Signals from as far as 1300 miles away, or more, are not uncommon. From here in Memphis, I have listened to FM from Wyoming, Florida, the Eastern Seaboard and even Cuba while just driving around town -- the ionospheric stuff can get pretty strong.
There is a club of folks who follow this stuff. There is much information there. Again, a link:
http://www.anarc.org/wtfda/
Hope this helps.
DE
