When you look up at the night sky and see a circle surrounding the moon, you are witnessing an atmospheric optical phenomenon known as a lunar halo, or a moon ring. This common event has captivated observers for centuries. The appearance of this perfect circle results from light interacting with specific atmospheric conditions in the upper layers of Earth’s atmosphere.
The Science Behind Lunar Halos
The formation of the lunar halo depends on the presence of high-altitude clouds composed of ice crystals. These clouds, called cirrus or cirrostratus, float in the upper troposphere, typically at elevations between 20,000 and 40,000 feet (6,000 to 12,000 meters). At this height, water vapor freezes into tiny ice crystals instead of liquid droplets.
The light from the moon, which is reflected sunlight, hits these ice crystals as it travels toward the ground. Each crystal acts like a miniature prism, bending the light through a process known as refraction. Since the ice crystals are suspended randomly, the collective refraction of light from millions of these tiny hexagonal structures creates the appearance of a luminous ring centered on the moon.
Why Is the Ring Always 22 Degrees?
The moon ring always appears to have a radius of approximately 22 degrees. This specific measurement is a direct consequence of the hexagonal structure of the ice crystals in the clouds. Light enters one face of the hexagonal prism and exits another, passing through a 60-degree angle of the crystal.
Physics dictates that when light travels through a 60-degree ice prism, the angle at which it is deflected is a minimum of about 22 degrees. This is known as the angle of minimum deviation for ice. No light is refracted toward the observer at an angle less than 22 degrees, which is why the sky inside the halo appears darker. This optical property ensures the resulting halo always maintains its characteristic 22-degree radius.
Predicting Weather Using the Moon Ring
The appearance of a moon ring has long been associated with the saying that “ring around the moon means rain soon.” This saying has a legitimate basis in meteorology, linking the halo’s atmospheric cause to future weather conditions. The high-altitude cirrus clouds responsible for the halo are often the first sign of an approaching warm front.
Warm fronts are slow-moving weather systems that bring moisture and eventually lead to precipitation. The cirrus clouds precede the lower, thicker clouds that ultimately produce rain or snow. Therefore, seeing a lunar halo indicates that a large, moisture-bearing weather system is moving into the area, suggesting precipitation could arrive within the next 12 to 24 hours.