April 06, 2005
It was a chilly day in late January (2005) when I saw these spectacular contrails at sunset near Sienna, Italy. The exhaust of an airplane engine can generate a condensation trail (contrail) by saturating the surrounding air with moisture. Additionally, the wings of a plane may encourage contrail development by causing a drop in the ambient temperature. This temperature decrease causes small ice-crystals to form. The otherwise clear sky on this winter day was suffused with contrails, which were enhanced by the low Sun. Over the last several decades, contrails have significantly increased the amount of high altitude clouds.
Contrails, like cirrus clouds, may act to slightly cool the Earth during the day since they reduce solar irradiance at the surface. For instance, a reduction of solar irradiance at the surface by 3 to 5% (depending on the wavelength) was measured at Lausanne, Switzerland, when the Sun passed trough a contrail. However, cirrus and contrails are thought to have a warming effect on the global climate since they tend to trap more outgoing longwave radiation (emitted by the much warmer Earth surface) than they block incoming solar radiation.Related Links: