It had been hypothesized that in regions such as the United States with heavy air traffic, contrails affected the weather, reducing solar heating during the day and radiation of heat during the night by increasing the albedo. The suspension of air travel for three days in the United States after September 11, 2001 provided an opportunity to test this hypothesis. Measurements did show that without contrails the local diurnal temperature range (difference of day and night temperatures) was about 1 degree Celsius higher than immediately before; however, it has also been suggested that this was due to unusually clear weather during the period.
So, for the best answer, we again ask the rocket scientists:
NASA scientists have found that cirrus clouds, formed by contrails from aircraft engine exhaust, are capable of increasing average surface temperatures enough to account for a warming trend in the United States that occurred between 1975 and 1994. According to Patrick Minnis, a senior research scientist at NASA’s Langley Research Center in Hampton, Va., there has been a one percent per decade increase in cirrus cloud cover over the United States, likely due to air traffic. Cirrus clouds exert a warming influence on the surface by allowing most of the Sun’s rays to pass through but then trapping some of the resulting heat emitted by the surface and lower atmosphere. Using a general circulation model, Minnis estimates that cirrus clouds from contrails increased the temperatures of the lower atmosphere by anywhere from 0.36 to 0.54°F per decade. Minnis’s results show good agreement with weather service data, which reveal that the temperature of the surface and lower atmosphere rose by almost 0.5°F per decade between 1975 and 1994.
This enhanced infrared image from the Moderate Resolution Imaging Spectroradiometer (MODIS), aboard NASA’s Terra satellite, shows widespread contrails over the southeastern United States during the morning of January 29, 2004. Such satellite data are critical for studying the effects of contrails. The crisscrossing white lines are contrails that form from planes flying in different directions at different altitudes. Each contrail spreads and moves with the wind. Contrails often form over large areas during winter and spring.
Wednesday, January 03, 2007
BRRR! from Cleveland, OH!
I got up this am intending to run on the treadmill on my front porch (portable heater in place), but was guilted into running outside by a lone jogger passing by my house in the dark. It was cold, but what many people call "brisk," and getting to experience the world lighten with the dawn was well worth it.
As I approach folks standing by the curb, awaiting a bus to work, I wonder whether the cloud by their heads are from cigarette smoke (not pleasant, since I'm gasping for air) or just their breath. Which got me thinking about contrails. I read recently about the environmental implications of these sky stripes and wanted to do a little research...
Wow, there are even websites devoted to them: http://www.contrails.nl/
But first the facts from Wikipedia: Contrails are condensation trails (sometimes vapour trails): artificial cirrus clouds made by the exhaust of aircraft engines or wingtip vortices which precipitate a stream of tiny ice crystals in moist, frigid upper air. Contrary to appearances, they are not air pollution as such, though might be considered visual pollution.
Interestingly, this same posting refers to a September 11, 2001 climate impact study proposing that the presence of jet traffic contributes to global warming: