Pastoral Paraselene ~
A colourful moondog, a paraselene, lights the Shropshire, England sky. Around the cloudy moon a corona hints its presence, its diffraction no rival to the hearty refraction of the moondog. Radiation fog layers a foreground barley field. An image by Andrew Steele ( andrewsteele.co.uk ).
©Andrew Steele, shown with permission.
|Paraselenae like sundogs are formed by hexagonal plate ice crystals in high cloud.
Colour separation occurs as rays enter a crystal side face. Blue refracts more strongly than red. Colours disperse further when rays leave through another side face angled 60° to the first.
The route through the crystal is only straightforward when the sun is close to the horizon. To get light of a higher sun through the thin ice plate the rays must reflect up and down between the large upper and lower crystal facets.