White parhelia

120� from the sun, these colourless patches on the parhelic circle are easily missed especially against a milky white or cloudy sky. Look for them if there are bright sundogs or traces of the parhelic circle. Whenever there is an atmospheric optical effect it is often rewarding to check the sky in the opposite direction.

Plate crystals drifting with their large hexagonal faces nearly horizontal form them.

The ray path at right is one of several that contribute to the parhelion. Sunlight enters the top face, reflects twice internally from adjacent near vertical side faces and then leaves through the large lower face.

The ray�s angular deviation is a constant 120� regardless of the crystal�s rotational position.

The angles of entry and exit are equal and so there is no net dispersion of different wavelengths � the resulting parhelion is without colour.

It is best made by relatively thick plates or non-regular ones that have a triangular aspect.