Broadcast range
The local terrain can also play a major role in limiting broadcast range. Mountain ranges block FM broadcasts, AM broadcasts, and TV broadcasts, and other signals in the VHF and especially UHF ranges, respectively. This terrain shielding occurs when the line of sight is blocked by something through which the radio waves cannot pass, particularly stone. At times this may be moot due to weather, such as when the tall cumulonimbus clouds of a squall line of thunderstorms reflect the signal over the top, like an extremely tall radio tower. Conversely, heavy rain may attenuate the range of even local stations. ATSC digital television is affected by wind and trees (even if not surrounding the transmitter or receiver locations), apparently related to its use of 8VSB modulation instead of COFDM.
Amateur radio frequency allocations
Band openings are sometimes caused by a weather phenomenon known as a tropospheric "inversion", where a stagnant high pressure area causes alternating stratified layers of warm and cold air generally trapping the colder air beneath. This may make for smoggy/foggy days but it also causes VHF/UHF radio transmissions to travel or duct along the boundaries of these warm/cold atmospheric layers. Radio signals have been known to travel hundreds, even thousands of kilometers due to these unique weather conditions.