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.
Space weather
The ionosphere bends radio waves in the same manner that water in a swimming pool bends visible light. When the medium through which such waves travel is disturbed, the light image or radio information is distorted and can become unrecognizable. The degree of distortion (scintillation) of a radio wave by the ionosphere depends on the signal frequency. Radio signals in the VHF band (30 to 300 MHz) can be distorted beyond recognition by a disturbed ionosphere. Radio signals in the UHF band (300 MHz to 3 GHz) transit a disturbed ionosphere, but a receiver may not be able to keep locked to the carrier frequency. GPS uses signals at 1575.42 MHz (L1) and 1227.6 MHz (L2) that can be distorted by a disturbed ionosphere. Space weather events that corrupt GPS signals can significantly impact society. For example, the Wide Area Augmentation System (WAAS) operated by the US Federal Aviation Administration (FAA) is used as a navigation tool for North American commercial aviation. It is disabled by every major space weather event. Outages can range from minutes to days. Major space weather events can push the disturbed polar ionosphere 10° to 30° of latitude toward the equator and can cause large ionospheric gradients (changes in density over distance of hundreds of km) at mid and low latitude. Both of these factors can distort GPS signals.