What is a ';clear channel'; AM radio station and how many are there currently in the U.S.?

I remember listening to KEX in Portland, OR, which used to advertise they were ';clear channel';, still on 1190 kHz.What is a ';clear channel'; AM radio station and how many are there currently in the U.S.?
When talking about standard broadcast ';AM'; band, stations operating at night have their signals propagated very long distances. Since the same frequencies are used by local stations all over the country, this can create so much interference that you couldn't hear any clearly. Therefore regulations were made to allocate certain channels to be used exclusively by just ONE station at night. That way the local interference is eliminated and the designated ';clear channel'; station will be heard at great distances.





640, 650, 660, 670, 700, 720, 750, 760, 770, 780, 820, 830, 840, 870, 880, 890, 1020, 1030, 1040, 1100, 1120, 1160, 1180, 1200, and 1210 kHz.





There is one designated 50KW station for each of these channels. A local station using one of these frequencies turns its transmitter off during night-time hours.What is a ';clear channel'; AM radio station and how many are there currently in the U.S.?
based out of san antonio they have 52 frequencies and 1200 stations with over 700 of them on am, many on both broadcasts.