For AM broadcast stations, the term critical hours refers to the time periods of sunrise to two hours after sunrise, and two hours before sunset to sunset. During these periods, the ionosphere has commenced its transition from daytime to nighttime conditions (or vice versa), resulting in greater coverage than would be expected from a daytime-only analysis. But because the transmitting station operates with its daytime power between sunrise and sunset, the extended skywave signal can be strong enough to interfere with other stations. This daytime skywave phenomenon was the focus of a protracted rulemaking proceeding that commenced in 1947 and culminated in 1959 (see Docket 8333 ) with the adoption of Section 73.187, Limitation on daytime radiation  (also called the "critical hours" rule), and Figures 9, 10, and 11 in Section 73.190. The rule provides for operation at a lower power during critical hours time periods to limit interference from new or changed Class B or D stations (where the changes were made after 1959) operating on frequencies specified in Section 73.25 , to Class A AM stations on the same frequency.
Figures 9, 10, and 11 in
Section 73.190 
have been digitized and converted to metric units:
|Critical Hours Graphs||500 kHz
|English units||PostScript  or PDF ||PostScript  or PDF ||PostScript  or PDF |
|Metric units||PostScript  or PDF ||PostScript  or PDF ||PostScript  or PDF |
The digitized curves data has been distilled into the search function above, which is primarily intended
for use by AM radio broadcast engineers. Select the frequency, then enter the
distance and azimuth from the AM station being considered to a point on the
0.1 mV/m contour of a Class A station on the same frequency. A
field strength value will be returned. The critical hours AM station may not exceed
the computed field strength toward that point during the critical hours time periods.
Several computations should be made for various points on the Class A station's
0.1 mV/m contour to ensure compliance with the rule.
Questions on the Critical Hours function may be directed to Dale Bickel, firstname.lastname@example.org .