"Matt Beckwith" wrote in message
...
The clarity of local AM radio stations is less at night. Is there
some sort of law that radio stations have to cut their output when it
gets dark? 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. 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?


Here is something the FCC prepared about AM radio at night:

Why AM Radio Stations Must Reduce Power, Change Operations, or Cease
Operations at Night
Most AM radio stations are required by the FCC's rules to reduce their power
or cease operating at night in order to avoid interference to other AM
stations. Our rules governing the daytime and nighttime operation of AM
radio stations are a consequence of the laws of physics. Because of the way
in which the relatively long wavelengths (see Footnote 1) of AM radio
signals interact with the ionized layers of the ionosphere several miles
above the earth's surface, the propagation of AM radio waves changes
drastically from daytime to nighttime. This change in AM radio propagation
occurs at sunset due to radical shifts in the ionosphere layers, which
persist throughout the night. During daytime hours when ionospheric
reflection does not occur to any great degree, AM signals travel principally
by conduction over the surface of the earth. This is known as "groundwave"
propagation. Useful daytime AM service is generally limited to a radius of
no more than about 100 miles (162 km), even for the most powerful stations.
However, during nighttime hours the AM signals can travel over hundreds of
miles by reflection from the ionosphere, a phenomenon called "skywave"
propagation. (Shortwave stations, which operate using AM modulation on
several bands between between 2.3 MHz and 26.1 MHz, also use this phenomenon
to broadcast still greater distances, up to thousands of miles). Because of
this change in signal propagation from daytime to nighttime, if every AM
station kept its daytime operating power at night, massive interference
would result. (See also Hours of Daytime-Only AM Broadcast Stations, First
Report and Order, BC Docket 82-538, 95 FCC 2d 1032 (1983) for a similar
description.)

The FCC was established by the Communications Act of 1934, and was given the
responsibility of making a "fair, efficient and equitible distribution of
radio service" to the various communities of the United States. In order to
meet this responsibility, and in recognition of the physical laws which
govern AM radio propagation, the FCC established three basic classes of AM
stations (Clear Channel, Regional, and Local), each of which operates on
specified frequencies, or channels There have been some changes since then,
but the main features of the plan have remained in place. Here is a quick
summary of the three types of AM stations:


a.. Clear channels. On these channels, only one station, called the
"dominant" station, was permitted to operate during nighttime hours in order
to provide service to a wide area. Footnote 2. A typical clear channel
station, transmitting with power of 50 kilowatts into an omnidirectional
antenna, provides nighttime skywave service to an area approximately 750
miles in radius. Some examples of clear channel stations are WCBS in New
York, WGN in Chicago, WHO in Des Moines and KFI in Los Angeles.

b.. Regional channels. On these channels several stations are permitted to
operate during nighttime hours, protecting each others' nighttime service
areas by use of directional antennas. As the name implies, stations on
regional channels provide nighttime service to moderately-sized or
"regional" areas.

c.. Local channels. Essentially, all stations on local channels are
permitted to operate during nighttime hours with the same facilities as
employed during the daytime. The nighttime service of such stations is
limited to very small "local" areas by massive interference from the
numerous other stations on the channel.
For the most part, the basic structure of the FCC's original frequency plan
has remained unchanged over the past several decades. Additional stations
gradually have been permitted to operate on the clear channels. In order to
preserve the wide-area nighttime service provided by the dominant clear
channel stations, when nighttime operation is permitted, many of these these
stations are required to use directional antennas in order to protect the
dominant clear channel station from interference to its nighttime skywave
service area. Most of these stations are also required to reduce power at
night, to avoid causing interference to the dominant stations and to each
other. Other stations, which cannot afford the necessary protection at night
to other AM stations, are presently licensed to operate during the daytime
hours only. (New daytime-only stations are no longer being authorized, since
December 1, 1987.)

To find out what daytime and nighttime operations are authorized for your
favorite AM broadcast station, you may use our AM Query. The Hours of
Operation for this Record field for the station will indicate what hours the
station may operate with the listed station parameters, either as Unlimited
hours operation, Daytime, or Nighttime. If the record shows the "Hours of
Operation" as "Daytime" or "Nighttime", then you can find the approximate
monthly times that the station will change from day to night operation (or
cease operation for the night in the case of a daytime-only station) by
entering the coordinates (latitude and longitude) into the the Sunrise /
Sunset Times Calculations program. Note that a station may have multiple
records, for different daytime and nighttime operations.