A Moon halo, or lunar halo, happens because of the refraction of moonlight by ice crystals in the upper atmosphere.
Understanding Lunar Halos
Lunar halos are optical phenomena that appear as large, bright rings surrounding the Moon. They are a captivating sight, often sparking curiosity about their formation.
The Science Behind Moon Halos
The primary cause of a lunar halo is the presence of tiny ice crystals suspended high in the Earth's atmosphere. Specifically, these crystals are typically found in cirrus or cirrostratus clouds.
Ice Crystals and Refraction
- Refraction Defined: Refraction is the bending of light as it passes from one medium to another. In this case, it's the bending of moonlight as it enters and exits the ice crystals.
- Hexagonal Shape: The ice crystals are hexagonal in shape. This shape is critical to the halo's formation.
- 22-Degree Halo: As moonlight passes through these hexagonal ice crystals, it bends at an angle of approximately 22 degrees. This bending creates a ring of light with a radius of roughly 22 degrees around the Moon, hence the common term "22-degree halo."
Conditions for Halo Formation
Several conditions must align for a lunar halo to appear:
- Presence of Ice Crystals: The upper atmosphere must contain a sufficient amount of ice crystals.
- Cloud Type: Cirrus or cirrostratus clouds are ideal because they are composed of these ice crystals.
- Moonlight: A bright Moon is necessary for the halo to be visible.
- Atmospheric Conditions: The atmosphere must be relatively clear between the viewer and the ice crystal clouds.
Common Types of Halos
While the 22-degree halo is the most common type, other types of halos can occur, each with different characteristics:
- 22° Halo: The most frequently observed halo.
- 46° Halo: Rarer and larger than the 22° halo.
- Circumhorizontal Arc: A brightly colored arc appearing below the horizon.
Practical Implications
Observing lunar halos can have practical implications:
- Weather Prediction: The presence of cirrus clouds, which cause halos, often indicates an approaching weather system.
- Photography: Lunar halos provide unique photographic opportunities for capturing atmospheric phenomena.
