Critical hours
Encyclopedia
Critical Hours is that time from sunrise
to two hours after sunrise, and from two hours before sunset
until sunset, local time. During that time, certain American radio stations may be operating with reduced power as a result of Section 73.187 of the Federal Communications Commission
's Rules.
Canadian Restricted Hours are similar to Critical Hours, except that the restriction results from the January 17, 1984, U.S.-Canadian AM Agreement. Canadian Restricted Hours are called "Critical Hours" in the U.S.-Canadian Agreement, but in the AM Engineering data base, the FCC calls them "Canadian Restricted Hours" to distinguish them from the domestically defined critical hours. Canadian Restricted Hours is that time from sunrise to one and one-half hours after sunrise, and from one and one-half hours before sunset until sunset, local time. U.S. stations operate with restricted hours because of Canadian stations, and vice-versa.
Those radio stations that are required to lower their power during the Critical Hours are required to do so because this is when the propagation of radio waves changes from groundwave to skywave
(at sunset) or vice versa (at sunrise). This can cause radio stations to be picked up much farther away, possibly causing interference with other stations with nearby frequencies.
Sunrise
Sunrise is the instant at which the upper edge of the Sun appears above the horizon in the east. Sunrise should not be confused with dawn, which is the point at which the sky begins to lighten, some time before the sun itself appears, ending twilight...
to two hours after sunrise, and from two hours before sunset
Sunset
Sunset or sundown is the daily disappearance of the Sun below the horizon in the west as a result of Earth's rotation.The time of sunset is defined in astronomy as the moment the trailing edge of the Sun's disk disappears below the horizon in the west...
until sunset, local time. During that time, certain American radio stations may be operating with reduced power as a result of Section 73.187 of the Federal Communications Commission
Federal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
's Rules.
Canadian Restricted Hours are similar to Critical Hours, except that the restriction results from the January 17, 1984, U.S.-Canadian AM Agreement. Canadian Restricted Hours are called "Critical Hours" in the U.S.-Canadian Agreement, but in the AM Engineering data base, the FCC calls them "Canadian Restricted Hours" to distinguish them from the domestically defined critical hours. Canadian Restricted Hours is that time from sunrise to one and one-half hours after sunrise, and from one and one-half hours before sunset until sunset, local time. U.S. stations operate with restricted hours because of Canadian stations, and vice-versa.
Those radio stations that are required to lower their power during the Critical Hours are required to do so because this is when the propagation of radio waves changes from groundwave to skywave
Skywave
Skywave is the propagation of electromagnetic waves bent back to the Earth's surface by the ionosphere. As a result of skywave propagation, a broadcast signal from a distant AM broadcasting station at night, or from a shortwave radio station can sometimes be heard as clearly as local...
(at sunset) or vice versa (at sunrise). This can cause radio stations to be picked up much farther away, possibly causing interference with other stations with nearby frequencies.