Physics Photo of the Week

February 5, 2010

Contrail Parhelion

Notice the vivid colors in the center of this picture.  This is called a parhelion.  It literally means "next to the Sun".   It is caused by prismatic refraction by millions of tiny ice crystals.  The Sun is hidden behind the building to the right of the picture.  Notice also that the colors of the parhelion are concentrated in the jet contrails.  Contrails are the trails formed by the condensation of water vapor in the exhaust of jet aircraft.  The ambient temperature is so low at the high elevations of commercial aircraft that the water vapor freezes or sublimes directly into tiny ice crystals.  The ice crystals are flat hexagonal plates.  The plates are all oriented horizontally - the only stable orientation for falling flat bodies. 

Parhelia are also commonly referred to as "sun dogs".  They often resemble additional Suns 23 degrees on either side of the Sun when the Sun is shining through thin cirrus clouds near the horizon (see the PPOW for Feb. 8, 2008).  On the day of the above photo, the "mate" to this parhelion was indeed present, but it resulted from smooth cirrus clouds rather than a contrail.  It is shown at  the left.

The presence of ice crystals in the atmosphere is a good prediction of precipitous weather to come.  The ice crystals are condensed water vapor.  The increasing amount of water vapor in the upper atmosphere signals the beginning of a warm front consisting of moist air.  In the next day or two, the moisture increases to such an extent that rain or snow is almost a sure prediction.  More jet contrails are visible with the increased water vapor in the atmosphere.  When the air is too dry, the contrails quickly evaporate.  But when the air is more saturated, the contrails persist for long times and eventually dissipate as increased cirrus clouds.

These photos were made Jan. 27, 2010 - two days ahead of last weekend's massive snow storm.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to 

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