This is known as Ionosperic Skywave Propogation Phenomenon. I managed a small class C “day-timer” AM station in the early 90s and we had to reduce power based on a very specific schedule of dates and times. For example, start the day at 6:00 AM at low power (50 watts) and increase to full power (1000 watts) at 6:15 AM. This was to “protect” the class A station which had the rights to broadcast in our area at night. I believe the class A was KGO in San Francisco. My station was 810 KKGD in western Colorado (no idea if it is still broadcasting).