I live in the Chicago suburbs. Every once in a while, particularly yesterday, my FM radio reception is really strange. I can’t get any local stations clearly, but I can get places like Davenport Iowa, Dearborn Michigan, or less impressive, Madison Wisconsin, clear as a bell instead. Anyone know why? I haven’t seen any correlation with the weather. I know there was a day in August that did the same when it was 98 and humid. Yesterday was 80 and dry.
Wierdness in the Heaviside layer:
Cool. Thanks for the info. 
Still, I didn’t think FM radio could travel that (as suggested in the OP) but only AM radio could. I know I got a Boise, Idaho, AM radio station in Phoenix, Arizona, one night. However, I don’t think I’ve ever gotten an FM station more than 80 miles from its source (rough guess). The atmosphere is transparent to FM frequencies (or so I thought) so the signal takes off into outer space. AM can be bent in the fashion described above.
Still, if chrisk got that Iowa station then who am I to second guess this.
FM radio uses similar frequencies as TV, but much higher in the radio spectrum than AM radio. AM radio’s signals can bounce back and forth between ionosphere and earth pretty readily, although the total distance traveled is subject to changing factors and the power of the transmitting station.
FM radio/TV is nominally line-of-sight, and does not bounce much compared to AM. But it does exhibit atmospheric effects on occasion. Here, north of Green Bay, Wisconsin, I have a tall antenna mounted in a pine tree (it gets taller every year!) which, 80% of the time, picks up Green Bay TV stations very well, but Milwaukee and Chicago ones very little. But last night – perhaps the same night as **chrisk72[/p] observed – the local (50 miles away) Green Bay stations faded out and the Milwaukee (150 miles), Chicago (250 miles) and even Muskegon, Michigan (100 miles) stations came in quite clearly.
It’s a good argument to have cable, but that wouldn’t be as exciting.
Supposedly FM radio and other EM signals in that frequency range are unaffected by the Heaviside Layer and so are restricted to line of sight. However, we don’t know everything there is to know just yet and strange things do happen.
For instance, over the ocean there is a phenomenon called “ducting.” In this case there is a layer of air that has considerable moisuture as compared to the surrounding air so it has a quite different refractive index. At the junction of the air layers it is possible to get total reflection of a signal and the humid layer acts like a virtual waveguide and radar signals can travel surprising distances following the curvature of the earth.
It is possible that in the vicinity of Lake Michigan and with tall buildings around you get weird atmospheric effects and reflections that result in funny propagation anomolies.
FM can definitely experience skip transmission from the ionosphere, as well as tropospheric ducting. There are a lot of “DX” hobbyists out there with a lot to say about it, like this:
Note: the E layer or E region is a synonym for the Heaviside layer.
Summer is a prime time for this, as thunderstorm activity tends to create the right conditions, but as that article notes, it isn’t neccesarily a thunderstorm where you are - so you won’t notice any correlation to your local weather. Chances are good that there is a storm somewhere between you and the unusually distant station you are receiving.