So it's been a long time since I learned all the theory on this stuff, so I don't remember the exact specifics.
HF Radio (think ham radio) is on a frequency that has the potential to bounce all the way around the globe. Early/mid 2000's when we where flying missions over Iraq in the Air Force we would use it to connect with state side people that could patch us in on a phone call. Generally people used it to talk to people back home, as this wasn't a secure thing. It essentially works because the frequency is low enough that the signal can bounce off the atmosphere and the ground. Weather conditions have an impact on that, so you might be bouncing the signal past things you'd like to hit based on those conditions. When we would do it stateside you might be able to get a guy in Washington State while flying over South Carolina, but not be able to get the guy in Tennessee.
Rambling, but essentially radio waves bounce, and if memory serves they bounce better at night due to atmospheric conditions. Sometimes farther away will see a better signal
Found a video that explains it pretty similar to how we learned it
View attachment 532107
This is what I did in the military and this is a good explanation of how the signals work. You also get much further range at night usually due to the atmosphere.