Contrails

Not all clouds are purely natural.  We probably know this, but we rarely think of it.  And I'm not talking about cloud seeding and other deliberate attempts to manipulate clouds.  Airplanes leave behind a trail of exhaust gases, including water vapor.  The warmth of the airplanes' engines leaves the exhaust warmer than the surrounding air, and when it cools, it condenses into thin, narrow, linear clouds.  Over time, these clouds diffuse, as can be seen in the photo above. 

Though they seem to be fairly small, contrails have become an important part of the climate system, worthy of study by meteorologists and climate scientists.  In fact, a study was published a few years ago that chronicled the differences resulting from the few days following the September 11 attacks, when all North American air traffic was grounded and the atmosphere was contrail-free. 

Here in Freeville, we're largely out of the way of the major air routes, but planes do occasionally fly overhead, and contrails move into the area from upwind areas.  After all, I was able to take these two photos within a 15-minute span while I was outside last weekend.