What causes a Halo around the Moon?

While sitting in my den watching television recently, a friend of mine from North Carolina called with the following statement. "My dad was out in the yard last night and looked up and noticed a very distinct halo around the moon, and he wanted to know what caused it?" My friend's inquiry prompted this week's science question of the week.

A halo is a ring of light that surrounds either the sun or moon and typically appears as bright white rings. However, some halos can also have color patterns. A halo is an optical phenomenon similar in concept to a rainbow but also very different.

Halos form when the sunlight or moonlight is refracted or bent by ice crystals associated with thin-high level clouds, like cirrus or cirrostratus. The most common type of halo is the 22 degree halo. In this halo, a ring of light 22 degrees from the sun or moon is projected by hexagonal (i.e. six-side) ice crystals with diameters less than 20.5 micrometers.

As light passes through the crystal, it is bent or refracted twice. The two refractions bend the light by 22 degrees from its original direction, producing a ring of light.

Often, a halo around the moon or sun is an indicator of cloudy or rainy weather as high level cirrus and cirrostratus clouds that cause halos tend to drift ahead of frontal systems (especially warm fronts) that produce rainfall. Incidentally, the seemingly innocent and wispy cirrus clouds that cause halos are poorly understood, and they represent a large uncertainty in our understanding of how clouds impact the Earth's radiation budget. Programs like NASA's CRYSTAL-FACE experiment (http://cloud1.arc.nasa.gov/crystalface/) seek to better under stand the role of cirrus and other clouds in the atmospheric system.

This week's question is provided by Dr. Marshall Shepherd. Dr. Shepherd is a research meteorologist in the Laboratory for Atmospheres at NASA Goddard Space Flight Center. He specializes in mesoscale weather systems (e.g. rainfall systems, hurricanes, and thunderstorms) and remote sensing meteorology. He also serves as the Deputy Project Scientist for the forthcoming Global Precipitation Measurement (GPM) mission.