"Matt Beckwith" wrote in message
...
The clarity of local AM radio stations is less at night.

At night, nearly every AM in the US gets skywave intereference, due to the
nature of AM night propagation. Distant signals interfere with what are
unobstructed day signals, even if the station has the same power.

The only exceptions are clear channel stations, which get a coverage
increase due to skywave and being on realtively protected channels.

Is there
some sort of law that radio stations have to cut their output when it
gets dark?

Stations, to operate at night (nearly 35% don't) have to protect other
stations on the same frequency, sometimes as much as 1000 miles or more
away. Because of this, at night they may cut power or directionalize where
htey send it to avoid interference, or both.

I've noticed that it's the amount of light outside, not
the time of day. That is, the lack of clarity occurs whenever it gets
dark, not at a certain time each day.

The ionesphere makes AM signals skip form just before sunset to just after
sunrise. This is why many US stations are daytime only, since they could not
run at night without causing interference to others.

I always presumed the stations
were cutting power to save money, but then why would it be based on
light vs dark, rather than on time of day?

It's due to the laws of physics. Newer stations protect older ones. And many
can not even run at night.