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.
Click
here to see the Physics Photo
of
the Week Archive.
Observers are invited to submit
digital photos to:
