In meteorology, a sun dog or parhelion is a bright spot on one or both sides of the Sun. Sun dogs or parhelia are around around 22° directly to the left and right of the Sun, sometimes accompanied by a 22° halo around the Sun. Sometimes sun dogs appear as bright white light or a mock sun, but often they are colored from red (closer to the Sun) to orange to blue and finally white (further from the Sun).
How Sun Dogs Form
Hexagonal plate-shaped ice crystal refract and scatter light. As the plates fall toward the ground, they orient horizontally and collectively act as a prism. The horizontal aspect of the crystals refracts the light in a horizontal line with respect to the Sun. Larger crystals wobble more as they fall and produce taller sun dogs. Other ice crystal shapes similarly refract light and affect a sun dog’s shape and size. These ice crystals occur in cirrus and cirrostratus clouds and in diamond dust drifting at lower altitudes.
Why Are They Called Sun Dogs?
The word parhelion comes from the Greek word parēlion, which means “beside the sun”. The reason parhelia are called sun dogs remains shrouded in mystery. In Norse mythology, constellations of two wolves hunt the Sun and the Moon, one preceding and the other following the heavenly body. In English, the word dog refers either to the animal or is a verb that means “to hunt or track”. So, either sun dogs accompany the Sun like dogs follow their master or else sun dogs track the Sun.
In Cornwall, United Kingdom, sun dogs are called weather dogs. Weather dogs (and moon dogs) are a portent of bad weather.
Do They Predict the Weather?
Sun dogs and moon dogs that appear due to ice in cirrus clouds are a decent weather forecasting tool. Clouds in the upper atmosphere, like cirrus clouds, move faster than clouds at lower altitudes. So, you see cirrus clouds before you see lower clouds that produce precipitation. Sun dogs often precede an incoming front and stormy weather.
How Often Do Sun Dogs Occur? Are They Common?
Sun dogs are not rare. They occur anywhere and during any season. While people usually notice them in winter when the sky is low on the horizon, they are visible whenever there are ice crystals in the air or cirrus clouds in the sky.
What Is a Moon Dog?
A moon dog is a lunar equivalent to a sun dog. Other names for a moon dog are a mock moon or paraselene (plural paraselenae). A moon dog is a bright spot on either or both sides of the Moon that is caused by refraction of moonlight by hexagonal plate ice crystals. Like sun dogs, moon dogs appear around 22° directly to the side of the Moon. Moon dogs have all the color of sun dogs, but often appear as simple spots of light because moonlight is rarely bright enough to activate the cone cells in the human eye that perceive color. Like sun dogs, moon dogs commonly occur before cloudy weather.
References
- Minnaert, Marcel (1993). Light and Color in the Outdoors. Springer. ISBN 978-0-387-97935-9.
- Ping-Yü, Ho; Needham, Joseph (1959). “Ancient Chinese Observations of Solar Haloes and Parhelia”. Weather. Wiley. 14 (4): 124–134. doi:10.1002/j.1477-8696.1959.tb02450.x
- Ulanowski, Zbigniew (2005). “Ice analog halos”. Applied Optics. The Optical Society. 44 (27): 5754-5758. doi:10.1364/ao.44.005754