Contrails

It's Tuesday so we were down at the beach this morning, of course.  I happened to notice this bright white contrail coming from the northeast.  It is unusual to see contrails around here because you really need a clear blue sky, with very little haze.  So I grabbed my binoculars from the car, but could not identify the aircraft, other than it had four engines and I couldn't see any colors or markings on the fuselage.  So I grabbed my iPhone and opened an app that shows airplanes flying nearby, and then you can click on the icon of the plane, and it tells you the airline, and flight number and the departure airport and the destination, which is really cool!  But this airplane did not show up, so I am betting it was a military aircraft.  And then it began a gradual turn to the west.  Contrails, which is short for  "condensation trails" are line-shaped clouds produced by aircraft engine exhaust.  They only occur at altitudes of 25,000 to 40,000 feet.  They are composed primarily of water, in the form of ice crystals.  The combination of water vapor in the exhaust and the low air temperatures at high altitudes allows the formation of ice crystals.  The air temperature at 25,000 feet is about -60 degrees Fahrenheit at this time of year!  Brrrrrrr...