A Lunar Halo Means Rain
In the proud custom that wise words stick better in the mind when they rhyme, you may have heard this one stated as "ring around the moon, rain real soon" or "when a halo rings the moon or sun, rain's approaching on the run."
Both maxims have the ring of truth about them. The halos that sometimes frame the moon or the sun are produced by high, wispy clouds made of ice crystals. These sky sparklers refract the sunlight or moonlight to create a kind of luminous halo. During the day, their light-bending properties can at times also produce bright splotches, called parhelia or "sun dogs," that look like false suns [source: UIUC].
These ice crystals typically occur in translucent, sky-spanning cirrostratus clouds, which form during large-scale convergence. In a common convergence scenario, a low-level, low-pressure zone forms, pulling in air from its environs. As converging air rises, it cools and forms water vapor. If it continues to rise into the higher, colder reaches of the sky, it will solidify into ice crystals [source: UIUC].
Cirrus clouds often move in ahead of weather fronts, where temperature differentials force warm air upward, condensing moisture and forming clouds. Thus the rainy reputation [source: Pidwirny].