All flights were grounded after the September 11, 2001 attacks on the twin towers. A team of NASA scientists noticed that it got much colder that night than usual. They came to the conclusion that cirrus clouds generated by aircraft contrails were trapping heat at night. This study changed world and sent the airline industry into a tail spin trying to figure out how to deal with their contrail conundrum. It is possible that aircraft contrail induced cirrus clouds are trapping more heat than their CO2 emissions meaning the airline industry could incur hefty carbon tax charges if they don't create "less warming and more cooling clouds."
Before 9/11/2001, one could make the argument that contrails creating clouds was just pollution. After this monumental study, scientists and geoengineers have been trying to figure out how to alter jet fuel to create clouds that ONLY cool the planet and have no intention on stopping the creation of artificial clouds or removing these clouds all together.
The grounding of all commercial aircraft within U.S. airspace for the 3-day period following the 11 September 2001 terrorist attacks provides a unique opportunity to study the potential role of jet aircraft contrails in climate. Contrails are most similar to natural cirrus clouds due to their high altitude and strong ability to efficiently reduce outgoing infrared radiation. However, they typically have a higher albedo than cirrus; thus, they are better at reducing the surface receipt of incoming solar radiation. These contrail characteristics potentially suppress the diurnal temperature range (DTR) when contrail coverage is both widespread and relatively long lasting over a specific region. During the 11–14 September 2001 grounding period natural clouds and contrails were noticeably absent on high-resolution satellite imagery across the regions that typically receive abundant contrail coverage. A previous analysis of temperature data for the grounding period reported an anomalous increase in the U.S.-averaged, 3-day DTR value. Here, the spatial variation of the DTR anomalies as well as the separate contributions from the maximum and minimum temperature departures are analyzed. These analyses are undertaken to better evaluate the role of jet contrail absence and synoptic weather patterns during the grounding period on the DTR anomalies.
It is shown that the largest DTR increases occurred in regions where contrail coverage is typically most prevalent during the fall season (from satellite-based contrail observations for the 1977–79 and 2000–01 periods). These DTR increases occurred even in those areas reporting positive departures of tropospheric humidity, which may reduce DTR, during the grounding period. Also, there was an asymmetric departure from the normal maximum and minimum temperatures suggesting that daytime temperatures responded more to contrail absence than did nighttime temperatures, which responded more to synoptic conditions. The application of a statistical model that “retro-predicts” contrail-favored areas (CFAs) on the basis of upper-tropospheric meteorological conditions existing during the grounding period, supports the role of contrail absence in the surface temperature anomalies; especially for the western United States. Along with previous studies comparing surface climate data at stations beneath major flight paths with those farther away, the regionalization of the DTR anomalies during the September 2001 “control” period implies that contrails have been helping to decrease DTR in areas where they are most abundant, at least during the early fall season. 
Contrails during day cause cooling because of reflecting of sunlight back into space. During night, they trap infrared heat causing heating. So it is a balance between the two time intervals. We would like to have more CICs (contrail-induced cirrus clouds) during day and none during night. 
If the time and place of seeding is selected with care, the climate effect of cirrus thinning can be enhanced. For that, only the long-wave warming effect of cirrus clouds should be targeted, and their solar effect should be avoided. This can be achieved if seeding is limited to high-latitude winters or to nighttime seeding. 
Travis said he has mixed feelings about contrails. On the one hand they have been a prime focus of his research interest for the last 10 years. On the other hand they almost spoiled a recent vacation in the Pacific Northwest.
"There were so many contrails in the sky we weren't having any nice days," he said. "The contrails were making the days kind of gloomy and overcast. There aren't as many sunny days as we had when we were kids." 
High in the sky where the cirrus ice crystal clouds form, jet contrails draw their crisscross patterns. Now researchers have found that these elevated ice cloud trails can influence temperatures on the ground and affect local climate, according to a team of Penn State geographers.
"Research done regarding September 2001, during the three days (following 9-11) when no commercial jets were in the sky, suggested that contrails had an effect," said Andrew M. Carleton, professor of geography. "But that was only three days. We needed to look longer, while jets were in the air, to determine the real impact of contrails on temperature and in terms of climate." 
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