Radio propagation has more to do with the state of the F2 layer of the ionosphere. And yes, that does change with the season and time of day. Also depends on what frequency you listen to - some are more likely to be reflected than others.
AM is much more likely to be affected than FM, since it bounces off the atmosphere more. That's why, under certain conditions, you can pick up AM stations hundreds or thousands of miles away. Even moreso for shortwave and other frequencies. This is why you can pick up shortwave all over the world when it's broadcast from just a single place.
Broadcast FM also bounces, but not nearly as much. It depends a lot more on line-of-sight propagation. Pretty much as long as your antenna can "see" the broadcast tower, you're going to get good reception. Things like steel buildings can mess with this, however. A skyscraper can reflect and scatter the radio, so you'll pick up both the intended broadcast and some scatter reflected back off the building.
If you want to improve reception, get an external antenna and put it on your roof. Radio Shack sells a few decent FM models for $30 or so. You might be surprised at how clear and good analog broadcasts can sound with a good antenna. The antenna is every bit as important as the radio itself.