Physics Photo of the Week

February 8, 2008

Parhelia or Sun Dogs

Parhelia (Sundogs) are often viewed when the Sun is low and there are thin cirrus clouds in the sky.  They are a fairly common occurance.

The picture above shows both Sundogs - at 22 deg left and right of the Sun.  The Sun is deliberately blocked by the spruce tree in the center.  The photograph below shows a closeup of the parhelion on the left.

These colored displays are caused by the refraction of ice crystals in the upper troposphere found in the thin cirrus clouds.  The basic shape of the ice crystals here consists of thin hexagonal plates that act like prisms.  The size of these crystals is smaller than 0.3 mm.  In order for the crystals drifting randomly in the air to form these coherent "images", there must be some form of ordering.  Small, falling, flat objects that are affected by air friction, tend to fall such that the large dimension is horizontal.  Think of falling leaves or confetti.  The air friction causes any orientation but horizontal to be unstable. 

Each thin horizontal hexagonal prism refracts the light as shown in the drawing at right.   Horizontal (or nearly horizontal) light from the Sun enters a vertical face of the hexagonal prism of ice.  The refraction causes the Sun's ray to be bent both upon entering and leaving the prism.  The horizontal plates or prisms are oriented randomly in the horizontal plane, and the light becomes deflected at many angles.  However, the deflection is a minimum (22 degrees) if the light enters and leaves the prism symmetrically.  That means that if one looks 22 degrees to the left or the right of the Sun's position, you will see this refraction.  Different colors refract at slightly different angles, therefore the parhelia show distinct colors (red, green, and blue in a smooth transition).  The differene between this and a glass prism is that the dispersion is caused by ice instead of glass.  Look for parhelia or sun dogs next time you see thin cirrus clouds in the sky.



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 dcollins@warren-wilson.edu.


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