Geographic Phenomenon: Contrails

You are probably familiar with contrails — the thin, vaporous streaks of what looks like engine exhaust left in an airplane’s wake.  What you might not realize, however, is that these vapor trails are not in fact engine exhaust, but artificially-induced clouds resulting from temperature and pressure disturbances airplanes create as they travel through the atmosphere.

“Jet Trails, 2007” from artist Chris Jordan

Contrails generally arise in one of two ways, depending on atmospheric conditions.

  1. First, and most frequently, hot water vapor released as a by-product of jet fuel combustion comes into contact with cold air at high altitudes causing the water vapor to condense into visible, cloud-like contrails.
  2. Less frequently, pressure differences created by the wingtips of aircrafts traveling through extremely humid, water-saturated air can be great enough to induce water condensation and contrail formation.  These pressure differences at the wingtips are called wingtip vortices, and their contrails look like this.

Again, depending on atmospheric conditions, contrails can either last a long time or dissipate quickly.  In humid air, which allows contrails to hang around well after their creation, the initial water condensation can sometimes induce further water condensation, and even full cloud formation.

Contrail patterns over Florida, Georgia, Alabama, and South Carolina (Photo credit: Courtesy NASA Langley Research Center)

Scientists believe that increased air travel, resulting in increased contrail formation, is having a significant effect on global climate patterns.  The high cirrus clouds that persistent contrails often resemble are known to trap the sun’s radiative energy, which has a net warming effect on atmospheric temperature.  Whether this contributes to global warming is, like all things climate change related, up for debate.  Interestingly, nearly all of the data linking contrails to global climate were collected in the three days following the September 11th attacks during which all commercial air travel was suspended over the United States, an unprecedented event in aviation history.

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Monday News and Links

  • Those of us living in the Midwest got our first taste of tornado season this past weekend as 100+ tornadoes touched down late Saturday and early Sunday across Oklahoma, Kansas, Iowa, and Nebraska.  The New York Times reports that casualties were minimal thanks to advanced warning and increased severity of warnings in the wake of last year’s unusually deadly season.  “This is a life-threatening situation. You could be killed if not underground or in a tornado shelter,” read one Wichita, KS warning sent out this weekend.  The National Weather Service is experimenting with new, “blunter” language in an attempt to more directly convey the severity of their warnings.  It’s unclear whether the new warnings saved any lives this past weekend, or whether people are paying more attention after what happened last year, but it’s an interesting move by the NWS whose warning system has sometimes been criticized for “crying wolf.”
  • This link has gone somewhat viral over the past two weeks but, if you haven’t seen it, here is a stunning live map of US wind patterns.
  • More weather phenomena: 15 Incredible Cloud Formations
  • A year after the massive tsunami that devastated Japan last spring, the first of the debris expected to wash up on North American shores has done so.

Amazing Geography Photos

GSC Board Member Steve Jansen passed along a collection of incredible geography images from the photo site twisted sifter.  Here’s one…

The Cloud Covered Island of Litla Dimun

Click through to view the rest…

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Geographic Phenomenon: Sun Dogs

Sun Dogs, known scientifically as parahelia, are optical phenomena characterized by bright spots directly to the right and to the left of the sun.  Although the name is supposedly a mistranslation of an Inuit term referring to “the sun’s dogs,” sun dogs can be seen anywhere in the world, at any time of the year, though are most often visible when the sun is at or near the horizon.  This has to do with the underlying cause of sun dogs: hexagonal ice crystals in cirrus clouds and ground-level “Diamond Dust” which forms at exceptionally low temperatures.  As these ice crystals drift downwards they do so parallel to the ground, creating the perfect prism through which to refract a sun at or near the horizon.

Due to the specific optical properties of ice crystals, sun dogs are always located 22° away from the sun, and may be accompanied by a 22° halo, which occurs when enough ice crystals “wobble” from their parallel-to-the-ground orientation.

Sun dogs have a long history in art, literature, and religion — as both a source of trepidation and inspiration — with references dating back to Aristotle, Cicero, and Seneca before the common era.  Shakespeare mentioned them in Henry VI.  Someone in 1535 Sweden dramatically depicted them very dramatically.  And they’re still a source of conspiracy theories in modern times.