The combination of sunlight and water, either in its frozen or liquid form, can lead to some pretty stunning optical effects.
The other day toward evening, I noticed a rainbow of colors visible across a streak of cirrus clouds, in an otherwise blue sky. This phenomenon is called a sundog, and occurs when sunlight is refracted (bent) by ice crystals.
Sundogs, also called “mock suns,” can be seen as a bright spot of whitish light on either side of the sun. The angle of the sun, and the shape and orientation of the ice crystals, can lead to many other types of beautiful effects. The most spectacular of those occurred two years ago in June across the Inland Northwest sky. Called a circumhorizon arc, it appeared as a fiery rainbow (see image at http://www.spokesmanreview. com/blogs/video/thumbs/s undog.jpg).
A rare phenomenon, the circumhorizon arc requires a high sun angle and ice crystals oriented nearly perpendicular to the sunlight. Because of the high sun angle requirement, there is only a small window of time around the summer solstice in which the ingredients can even come together.
A ring around the sun or moon, is called a halo, and is also caused by the bending of light by ice crystals. These ice crystals are usually found in cirrus clouds. Since cirrus clouds are often produced by the lifting air ahead of a low pressure system, a visible halo has been noted as a harbinger of foul weather. The old saying goes something like “Halo ’round the sun or moon, rain will be a-fallin’ soon.”
Speaking of rain, water in its liquid form, along with the right orientation of sunlight, is responsible for producing the rainbows we see. To see a rainbow, we must be facing the water droplets, with the sun at our backs.
The water droplets don’t have to come from a rain shower, but can originate from any source, such as your backyard sprinkler. As sunlight enters the water droplets, it is bent and reflected back to our eye. The degree of bending determines the color that we see, and as sunlight hits the myriad of drops at different angles, we are treated to a spectacle of color.
Rainbows, like halos, have also been used in the past, to forecast the weather. As the saying goes, “Rainbow in morning, sailors take warning. Rainbow at night, a sailor’s delight.”
Red sky is also used in place of the word rainbow. The reason this saying makes sense, is that to see a rainbow in the morning you would have to be facing water droplets in the west, with the sunlight at your back in the east. Since most storm systems move from west to east, this would indicate an approaching storm.
Once into the fall season, which begins on the morning of Sept. 22, we should have an opportunity to see more rainbows. The weather pattern will begin to shift from the dry stable pattern of our summer season, to a more unsettled pattern. Average rainfall for September is .76 inches for Spokane and 1.58 inches for Coeur d’Alene. In October the averages increase to 1.06 inches for Spokane and 1.93 inches for Coeur d’Alene.