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The later it gets at night, the more AM stations I'm able to pick up, and from farther away. Is the night are more clear? Are there fewer other signals at night to cause interference? This has always baffled me.
Read a little bit about radio propogation. Here's one site that discusses it: Radio Propogation Click "Next" when you see the page above for more information.
That is way too much information for me to sit still and read, but it is very informative. I am only guessing that the signals are better when there are less people on.