travelers during takeoff. The tips of rotating surfaces (such as propellers and rotors) sometimes produce visible contrails. In firearms, a vapor trail is sometimes observed when firing under rare conditions due to changes in air pressure around the bullet. A vapor trail from a bullet is observable from any direction. Vapor trail should not be confused with bullet trace, which is a much more common phenomenon (and is usually only observable directly from behind the shooter). Radiative forcing Contrails, by affecting the Earth's radiation balance, act as a radiative forcing: they trap outgoing longwave radiation emitted by the Earth and atmosphere more than they reflect incoming solar radiation. In 1992, the warming effect was estimated between 3.5 mW/m2 and 17 mW/m2. Global radiative forcing has been calculated from the reanalysis data, climate models, and radiative transfer codes; estimated at 12 mW/m2 for 2005, with an uncertainty range of 5 to 26 mW/m2, and with a low level of scientific understanding. The effect varies daily and annually: night flights contribute 60 to 80% of contrail radiative forcing while accounting for 25% of daily air traffic, while winter flights contribute half of the annual mean radiative forcing while accounting for 22% of annual air traffic. Contrail cirrus may be air traffic's largest radiative forcing component, larger than all accumulated from aviation, and could triple from a 2006 baseline to 160-180 mW/m2 by 2050 Without interventions. Condensation trails may cause regional-scale surface temperature changes for some time. NASA researched atmospheric and climatological effects of contrails, including effects on ozone, ice crystal formation, and particle composition, during the Atmospheric Effects of Aviation Project (AEAP). Bomber contrails affected climate during World War II. A hotter temperature was recorded near airbases. Diurnal temperature variation The diurnal temperature variation is the difference in the day's highs and lows at a fixed station. Contrails decrease the daytime temperature and increase the nighttime temperature, reducing their difference. When no commercial aircraft flew across the USA following the September 11 attacks, the diurnal temperature variation was widened by . Measured across 4,000 weather stations in the continental United States, this increase was the largest recorded in 30 years. Without contrails, the local diurnal temperature range was higher than immediately before. This was maybe due to unusually clear weather during the period. In the southern US, the difference was diminished by about , and by in the US midwest. Head-on contrails