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.
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.
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.
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.
Credits:
Photos by Kerry Anderson