A sun dog also referred to as mock sun is a colorful patch of light that is caused by the refraction of sunlight by tiny six-sided crystals of ice within the atmosphere. When the sunlight passes through the crystals of ice, it is bent by 22 degrees before reaching the eyes of the observer, thus producing a sun dog. The sun dog scientifically known as parhelion is actually an atmospheric optical phenomenon (the phenomenon involving light). A group of sun dogs are known as parhelia( meaning with the sun).

The sun dog usually occurs during the time of sunrise or sunset when the sunlight is not strong and the ice crystals that form cirrus or cirrostratus clouds fill the atmosphere. Sun dog is one of the forms of weather illusions and sometimes looks like a sun or a comet owing to its luminous halo. Sometimes a band of colors varying from scarlet to pale blue are also visible in the sky.

The shapes of the sundogs depend on the size and shape of the ice-crystals that are refracted. These ice crystals that cause the meteorological phenomenon of sun dog are either flat or elongated with the shape of hexagonal prisms. The color and the shape of sun dogs depend on the varied shapes and orientation of these ice crystals. For instance poorly aligned crystals of ice result in long and colorful sun dogs. The sun dogs that occur close to the sun have redder edges while those located away from the sun are bluish in color.