What is that odd blue ring around the moon at night?
The bluish ring around the moon at night is caused by ice crystals high up in the atmosphere (about 7 km or 22,000 feet above the surface) which refract the moonlight in such a way that they create a halo. The effect is much like that which occurs when sunlight passes through an area of rain and creates a rainbow, or when a light passes through a prism and is split into the colours of the spectrum. A halo often precedes the onset of unsettled weather within the next 24 hours or so. This is because the ice crystals are part of cirrus clouds, which are those thin wispy high clouds that often run ahead of an approaching storm system.
