text: Disturbances in layers of the upper atmosphere allow TV and FM band signals to travel through "ducts" for several hundred miles.  Sunrise, sunset, and some weather patterns can set up temperature inversions that bend signals back to earth 100-200 miles beyond their source.  On rare occasions the ionospheric layers hundreds of miles up become dense enough to reflect TV and FM signals back to earth--500 to several thousand miles away from their source.  This last kind of DX mechanism is commonplace at lower frequencies.