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.
Environmental impact of aviation
Least significant on a mass basis is the release of soot and sulfate particles. Soot absorbs heat and has a warming effect; sulfate particles reflect radiation and have a small cooling effect. In addition, particles can influence the formation and properties of clouds, including both line-shaped contrails and naturally-occurring cirrus clouds. The impact of "spreading contrails and cirrus clouds that evolve from them—collectively known as contrail cirrus—have a greater radiative forcing (RF) today than all aviation CO emissions since the first powered airplane flight". Of the particles emitted by aircraft engines, the soot particles are thought to be most important for contrail formation since they are large enough to serve as condensation nuclei for water vapor. All aircraft powered by combustion will release some amount of soot; although, recent studies suggest that reducing the aromatic content of jet fuel decreases the amount of soot produced.