Scientists have long known that airplane condensation trails act to both cool and heat the atmosphere. Formed by jet engines' hot exhaust, contrails act as thin cloud barriers that not only reflect sunlight but also prevent the earth's heat from escaping into space. During the day, the effect of blocked incoming radiation tends to outweigh that of trapped heat, thereby cooling the atmosphere. Indeed, after the events of 9/11 grounded all commercial U.S. flights for three days, daytime temperatures across the country rose slightly, whereas nighttime temperatures dropped. This evidence supported the hypothesis that contrails reduce the temperature range by cooling the atmosphere during the day and heating it at night.