Why does the quality of AM radio stations change at night?

I have noticed that some AM stations deteriorate at night, while others get stronger. Why is this?

Easy.. most stations drop their power at night because they are forced by the FCC in the US and CRTC I beleive in Canada to prevent interference to stations that are allowed full power at night (the "Clear Channel" stations (the term means 50kw stations made to travel long distances, not the company)

These other stations use the power of the ionesphere which normally absorbs those radio signals during the day actually start reflecting them back at night, hince stations hundreds of miles away come in, where as the one two towns over is history

the other fellow is right, Am is the kind of broadcasting it is, not the band itself technically (as in Europe it's called the Medium wave band) However, it is so engraved in North American Psyche, the term is interchanged here

Depends on how strong the antenna is. I worked at an am station that had a 50,000 watts (Daytime) and then15,000 watts (Nighttime). But for the most part with AM stations the watts get lower at night. 15,000 watts is still strong so you can pretty much here it. You also have to consider the weather. If there are clouds or fog the frequency may be interrupted as well.

It's got nothing to do with transmitter power, at night Medium Wave (it's wrong to call it AM) signals are reflected off the ionisphere whereas by day they pass through into space.
Therefore, at night signals can bounce around the globe covering huge distances and causing co-channel interference (that's the whistling, fading and distortion you hear). The stronger stations at night tend to be distant ones which often blot out local services!

WLW in Cincinatti has the biggest transmitter in the US (maybe the world) at 250,000 watts. But they can only crank it up once in a while for testing. The transmitter is so big and gets so hot at full power, they have a lake around the building to cool it. It's used in case of national emergency. Boy can that thing honk at night!

Many AM Radio stations in the US are required to reduce power at sunset. This means that your local station that broadcasts at 500-1,000 watts during the day will probably drop to 250 watts at night (or, if it's what we call "a daytimer," will sign off all together). Likewise a 5,000 watt station may drop to 1,000 watts.

This has been so from the early days of radio. AM radio waves travel farther at night, so the smaller stations have to make way for big "clear channel" stations Seryan mentions that broadcast at 50,000 watts (or more). If they did not, there would be interference between the stations.

But the night time is the right time. You can tune all over the dial and get some really interesting radio stations. When I worked at a powerhouse flame-thrower station in Little Rock in the '70s, we once got a "DX" (postcard reporting reception) from the Arctic Circle. That was really cool.

I have an interesting device that I bought years ago that really boosts the antenna power of your AM radio. It's a big, round plastic device called a "Select-A-Tenna." You put it next to your radio and - wow! What a difference. this would also be a good device for those kids who have written in about bad reception on Radio Disney. You can reach them at www.selectatenna.com/

I've also listed a couple of websites that give lists of big "clear channel" stations you can get at night. (NOTE: I've used the lower case "c" with clear channel, so you don't confuse it with the huge radio conglomerate of the same name, but with capital Cs.)

Back in the sixties and seventies there were some great Rock 'n' Roll DJs on big 50,000 watt stations: Wolfman Jack, Jackson Armstrong, Brother Dave, John "Records" Landecker, Jay Thomas, Beaker Street (with Clyde Clifford and later, Stuart McRae), Frank "Shoutin" Holler and many others. Those were the best days of AM radio and that little spot you see on your computer is a tear that just fell in rememberance of what was, but never again will be.
Good luck & Good listening
-a guy named duh

I can answer this one. AM signals bounce off of the Ionosphere. The clearer the night the more they bounce. The more they bounce the longer the signal and so they can be picked up easier. From here in Indiana I can pick up AM stations from New Orleans. Why they break-up you ask. Well Some times they will come into contact with things such as power lines which have a much large place on the Electomagnetic spectrum. An AM staion is only about 10KHZ (kilohertz) wide. So when they come in contact they collect static. It all depends on where they bounce. There are a few other reasons but I dont feel like going into complete detail.