Points of order:

* Tall buildings and bridges don't increase interference per se: they cast shade of the desired signal, and can funnel interference towards the receiver. It just sounds as if the interference is greater, as automatic gain control winds the gain up as the signal goes down.

* The difference between day and night is caused by the way sunlight affects the F1 and F2 layers of the ionosphere. The net result is that signals go somewhat further at night, so your friendly local radio station isnae local nae mair, and neither is the one way over the horizon on the same frequency. (A Russian transmitter always used to interfere with the medium-wave Light Programme in the UK, but only at night.)

.... Admittedly this isn't folk music, unless the tune of the USSR's national anthem counts.

Revision note to self: find why the minimum reflected wavelength gets shorter at night.