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.
Capture effect
Because AM assumes short term changes in the amplitude to be information, any electrical impulse will be picked up and demodulated along with the desired carrier. Hence lightning causes crashing noises when picked up by an AM radio near a storm. In contrast, FM suppresses short term changes in amplitude and is therefore much less prone to noise during storms and during reception of electrical noise impulses.