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Physics Photo of the Week

February 24, 2006

Vivid Contrails

On the morning of February 1, 2006 at 8:00 am, the jet vapor condensation trails - "contrails" - were especially prominent in the sunrise photo here.  There are always about the same number of planes in the sky, at least at the same time of day every day.  Why are the condensation trails much more visible on some days than on others?

The answer lies in the conditions of the air at the 30,000 foot altitude that most commercial jets fly.  The normal temperature at 30,000 ft altitude is about -50 °C.  This is the top of the troposphere - the lower part of the atmosphere that experiences the weather in the form of clouds, warm fronts, and cold fronts.  February 1 occurred after several days of cold, clear weather.  By this time, a warm front was approaching.  A warm front consists of a layer of warm air flowing over the top of  a mass of cold air.

The drawing below shows the cross-section of the weather pattern called a warm front.

Warm, humid air that is less dense than the cold air from the past few days has begun to move into the area at the high altitudes.  When the jets fly in the region of cold air, the humidity is very low and the vapor from the jet engine exhaust soon evaporates and becomes invisible.  Thus on cold, clear days, we usually don't see the contrails, although the airplanes are still flying.  In the region to the right of the leading edge of the warm front, the airplanes fly in the upper layer of more humid air with the cirrus clouds.  The high humidity in this region prevents the condensed water vapor emitted by the engines' exhaust from evaporating.  The condensation trails thus remain visible, condensed either as supercooled fog droplets or as ice crystals, and join the cirrus clouds.

As time passes, the warm front proceeds further north (left in the drawing above).  The first thickening involves the whispy cirrus clouds turning into a complete cirrus blanket.  The cirrus blanket provided a sun halo later in the day in the photo at right taken about 1:00 pm.  (For more information on the sun halo or "ice halo" see the PPOW for March 25, 2005).  As the warm front progresses further, the clouds become progressively thicker and eventually lead to rain.  Indeed, we did receive a little rain the following day (Groundhog Day) according to the WWC Farm Weather Page.

All photos and drawing by Donald F. Collins.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an 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

Click here to see the Physics Photo of the Week Archive.

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