Providence, RI – Not long ago we talked about "tropospheric ducting" for FM and some TV signals. This time on the Engineer's Corner, we'll talk about its cousin for AM signals: skywave!
Certain layers of the Earth's atmosphere interact differently with radio transmissions at different frequencies. AM band frequencies, which are considered "medium wave" or MW, are reflected by the ionosphere, resulting in reception hundreds or even thousands of miles away. That's how WBZ 1030AM in Boston is famously known for being "heard in 38 states." Even if that only happens at night.
Now the reason why skywave only happens at night is because when the sun hits the atmosphere, there's a lower layer of atmosphere (the E-layer) that is energized by the solar radiation and becomes opaque to MW frequencies. So they never reach the higher layers of the ionosphere to get reflected. At night, there's no sun, the E-layer isn't energized, and it becomes transparent and we in Rhode Island can hear stations like WWVA 1170AM all the way from Wheeling, West Virginia (about 500 miles away, just west of Pittsburgh).
All AM radio stations cause skywave reflections. That's why many stations (including RIPR's old home, 1290AM) have different transmission patterns for day and night; they must reduce power in certain directions to avoid interfering with other stations. Sometimes other stations that are hundreds or thousands of miles away!
Only the biggest have the right to transmit at full power at night, and thus are really listenable over wide areas. Stations like WBZ 1030, WTIC 1080 and two or three dozen others in the entire USA. Those are FCC-classified as Class A "clear channel" stations, which is where the company of the same name took its name from even though they own lots of stations that aren't actually "clear channeled" licenses. Go figure.
All the other Classes of AM stations (Class B regional, Class C local and Class D daytime-only) are required to reduce power at night in one way or another. Even so, they all tend to blend into one giant mess of interference/static...at least, when listened to outside of their licensed NIF or "nighttime interference free" area.
For grins, also check out: http://en.wikipedia.org/wiki/Skywave and http://en.wikipedia.org/wiki/Clear-channel_station
Also for grins, some folks may note that you can hear 1290AM really well along a lot of Narragansett Bay, even really far south along Aquidneck Island. That's not skywave but instead is "groundwave" propagation, where the electrical energy of AM is conducted through the earth. Or in this case, through the saltwater of the ocean, which is a fantastically good conductor. It's why 1290AM sometimes has a rock-solid signal along the coastlines but can disappear just a quarter-mile inland!