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.
TWA Flight 1
Unbeknownst to Ferguson, the course he flew was about 8 degrees south of his plan. After passing the Harrisburg, Pennsylvania radio beam, TWA Flight 1 began to drift south. Heavy cloud cover prevented Ferguson from seeing any landmarks, and he was relying wholly upon instruments. Shortly after 10 a.m., Ferguson began his descent to Pittsburgh, believing it to be much closer than it actually was. At 10:09, he asked for weather conditions and was told the skies were overcast, with thick clouds above . He confirmed the report and said he was about east of the airport, flying in clouds at . Ferguson said the tower's radio signal was "very weak" and asked, "Is it OK to come on in?" It was the last communication from the plane.