Physics Photo of the Week
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
23 degrees on either side of the Sun when the Sun is shining
through thin cirrus clouds near the horizon (see
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 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
These photos were made Jan. 27, 2010 - two days ahead of last weekend's
massive snow storm.
Photo of the
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 email@example.com.
All photos and discussions are copyright by Donald
Collins or by the person credited for the photo and/or
discussion. These photos and discussions may be used for private
individual use or educational use. Any commercial use without
written permission of the photoprovider is forbidden.
see the Physics Photo
the Week Archive.
Observers are invited to submit
digital photos to: