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Rising global air traffic and its associated contrails have the potential for affecting climate via radiative forcing. Current estimates of contrail climate effects are based on coverage by linear contrails that do not account for spreading and, therefore, represent the minimum impact. The maximum radiative impact is estimated by assuming that long-term trends in cirrus coverage are due entirely to air traffic in areas where humidity is relatively constant. Surface observations from 1971 to 1995 show that cirrus increased significantly over the northern oceans and the United States while decreasing over other land areas except over western Europe where cirrus coverage was relatively constant. The surface observations are consistent with satellite-derived trends over most areas. Land cirrus trends are positively correlated with upper-tropospheric (300 hPa) humidity (UTH), derived from the National Centers for Environmental Prediction (NCEP) analyses, except over the United States and western Europe where air traffic is heaviest. Over oceans, the cirrus trends are negatively correlated with the NCEP relative humidity suggesting some large uncertainties in the maritime UTH. The NCEP UTH decreased dramatically over Europe while remaining relatively steady over the United States, thereby permitting an assessment of the cirrus-contrail relationship over the United States. Seasonal cirrus changes over the United States are generally consistent with the annual cycle of contrail coverage and frequency lending additional evidence to the role of contrails in the observed trend. It is concluded that the U.S. cirrus trends are most likely due to air traffic. The cirrus increase is a factor of 1.8 greater than that expected from current estimates of linear contrail coverage suggesting that a spreading factor of the same magnitude can be used to estimate the maximum effect of the contrails. From the U.S. results and using mean contrail optical depths of 0.15 and 0.25, the maximum contrail-cirrus global radiative forcing is estimated to be 0.006-0.025 W m-2 depending on the radiative forcing model. Using results from a general circulation model simulation of contrails, the cirrus trends over the United States are estimated to cause a tropospheric warming of 0.2°-0.3°C decade-1, a range that includes the observed tropospheric temperature trend of 0.27°C decade-1 between 1975 and 1994. The magnitude of the estimated surface temperature change and the seasonal variations of the estimated temperature trends are also in good agreement with the corresponding observations.
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... Contrails of planes also have optical properties similar to those of natural HLC. They not only attenuate solar radiation flux themselves, but also form cirrus [11] (e.g., Cirrus floccus homomutatus and Cirrus fibratus homomutatus [12]). The contrails existing more than 10 minutes are named by the World Meteorological Organisation as the only artificial type of ice clouds [13]. ...
... The contrails existing more than 10 minutes are named by the World Meteorological Organisation as the only artificial type of ice clouds [13]. Many years of HLC observations in northern latitudes [11] have shown an increase in the frequency of clouds formation with the growth of air traffic. Instrumental studies of the contrail characteristics are difficult, because the contact aircraftbased instruments provide little information (in addition, information on the orientation of ice particles gets lost), and from space platforms traces of aircrafts at the initial stage of formation are not distinguishable from space platforms due to their small dimensions. ...
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... average data of CC from the camera from 1 June 2017 to 31 December 2020 during Terra and Aqua overpasses are superimposed and show a similar temporal pattern to the satellite data. This result is consistent with previous measurements comparing the ISCCP reanalysis data with those from global surface synoptic weather stations and ships [47]. Over Chiba, clear and cloudy skies tend to occur during the winter (December-February) and summer (June-August) months, respectively [23]. ...
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... This value is not considered a best estimate because of the uncertainty in the optical properties of AIC and in the assumptions used to derive AIC cover. However, this value is in good agreement with the upper limit estimate for AIC RF in 1992 of+0.026 W/m -2 derived from surface and satellite cloudiness observations (36). A value of+0.03 ...
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... Early studies adjusted modeled cover and optical properties of contrail cirrus to match observations and then estimated RF with radiative transfer models (Meyer et al., 2002;Minnis et al., 1999;Stuber et al., 2006). Others estimated RF from observed multiyear trends in contrails, or cirrus cloudiness, with a factor estimating the RF from the added cirrus (Boucher, 1999;Eleftheratos et al., 2007;Minnis et al., 2004;Stordal et al., 2005;Stubenrauch & Schumann, 2005;Zerefos et al., 2003). The forcing can be derived directly from differences in irradiances from satellite observations in contrail regions relative to irradiances in neighboring contrail-free regions (Spangenberg et al., 2013;Vázquez-Navarro et al., 2015). ...
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