Moondogs Seeing Double by Meteorologist Kerry Anderson

It is not often I see a moondog, the halo of light around the moon, but this past weekend I saw the phenomena twice. And not only did it happen twice but Thursday's event produced an unusual double, intersecting bow.

Double intersecting moondog
Single moondog 3/8,2020

Before satellite pictures and numerical models these phenomena were used to forecast the weather. Ancient peoples noted that storms often followed the appearance of a moon or sun dog. And their observations were usually right. In order for a moondog to appear we need to have cirrus clouds in the sky. Cirrus clouds can form along warm frontal boundaries, which are the front runners of storm systems.

Moon/sundogs are created by the same optical principles that produce rainbows. Light from the sun is reflected and refracted by drops in the sky. However in this case, the droplets are frozen. These ice crystals are found in cirrus clouds.

The ice crystals have an octagonal prism shape. As the sunlight (or the reflected sunlight from the moon) hits the crystal, some of the light is reflected back from the surface of the crystal while some of the light enters the prism, slows down causing the angle of the beam to bend. This light is then is reflected back off another surface of the prism. The projected light is split so we see all the different wavelengths of color producing the rainbow effect.

In order for a moondog to appear we need a mostly clear sky and lots of reflected sunlight from the moon. This will occur when the moon is full or almost full. We had these ideal conditions twice this weekend.

And the moondogs proved to be accurate forecasters of an incoming storm. A large area of Low pressure has developed off the California coast and will be sending surges of moisture over the next few days as well as southerly flow for our beaches.

The surface Low is sitting below an upper level Low that has cutoff from the flow. These systems are notoriously difficult to forecast as they no longer have steering winds. They tend to wobble about. So be prepared for tweaks to the forecast as we watch this system and the rain we expect it to send our way.


Photos by Kerry Anderson