I looked out the window this morning to see this amazing set of contrails. Contrails are the narrow ribbons of cloud that form behind high-flying aircraft. They’re a frequent sight if you live under a commercial airlines flyway. Airplane engine exhaust, along with water vapor in the air, condenses into clouds of small water droplets and ice crystals in the bitter cold air at high altitudes. The same happens on cold days with your car exhaust when every vehicle blows “smoke” out of its tailpipe like a chainsmoker.
Depending upon weather conditions in upper troposphere — where commercial and other aircraft fly — contrails come in three varieties: short-lived, persistent non-spreading and persistent spreading. Brief contrails form in relatively dry air where ice crystals or water droplets quickly evaporate and disappear from sight. Long-lasting contrails form under more humid conditions and sometimes spread into broad ribbons of clouds that cover large parts of the sky.
They also form when air becomes rarified (less dense) as it flows over an aircraft’s wings and fuselage, forcing water vapor to condense into visible droplets or ice. Contrails were first noticed during early high altitude flights undertaken in the mid-1920s. They’re composed almost entirely of water vapor but also contain small amounts of soot and dissolved gases left over after the fuel burns.
Routine cross-country commercial aircraft flights originating from Denver, Chicago, Detroit and other large cities arc across northern Minnesota’s southern sky, making contrails a common sight nearly every clear day of the year. Sometimes two or more are visible but seeing four in parallel was unusual. They persisted and spread for many minutes, long after the aircraft that spawned them had departed. My hunch is that conditions were ideal for their preservation which allowed multiple trails left by planes arriving at slightly different times to stick around and create the striking pattern.
Thousands of planes are in the air at any moment. You can track them in real time to find out which flight is making a particular contrail by going the flightradar24 website. It’s kind of fun to watch the site the plane icons move along their paths as the site updates. Click on any one of the icons for flight details.
Later in the day, a halo formed around the sun and clouds thickened. The thicker clouds provided a backdrop to see shadows cast by the human-made clouds not only obvious but strikingly three-dimensional. Contrail shadows like the one in the photo sometimes look counterintuitive. It looks like the contrail is casting its shadow upward onto the cloud. Of course, that’s impossible because the sun is the light source and shadows are cast downward.
Instead, we’re looking through the clouds past the shadow which is cast by the contrail located higher up still. If you remember that the contrail is always higher than the cloud deck you can wrangle your brain to make sense of the scene. (But if you can’t, here’s a helpful graphic). For more on the topic check out NASA’s Contrail Education Project.
Contrails demonstrate how something created artificially can be refashioned into an eye-catching hybrid phenomenon with a little help from nature.