I think it is also worth mentioning the role of the ionosphere - which is the (charged) part of the atmosphere that will reflect radio/EM waves, and make it possible to communicate with someone on the other side of the globe. The ionosphere has different layers, and is quite dynamic - depending on the sun and its activity.

Basically, imagine a charged shell around the earth that reflects electromagnetic waves back, and that the properties of said shell is constantly fluctuating. Solar storms (and following northern lights) are bad news for radio communication.

That's the very, very ELI5 version.