A lunar halo is a captivating luminous ring encircling the Moon. This atmospheric spectacle often appears as a large, bright circle, creating an ethereal glow in the night sky.
What a Lunar Halo Is
This striking ring is typically faint and diffuse, lacking a sharp outline. Its most common form is known as a 22-degree halo, named for its consistent angular radius of approximately 22 degrees from the Moon’s center. This measurement is roughly equivalent to the width of an outstretched hand at arm’s length when held towards the Moon. While often seen as a white circle, faint colors can sometimes be discerned, with red on the inner edge and blue on the outer, though these are often too weak for the naked eye due to the Moon’s dimmer light compared to the Sun.
The sky within the 22-degree halo can appear noticeably darker than the surrounding sky, sometimes giving the impression of a “hole in the sky”. These halos are not rare events and can occur at any time of the year, though they are more commonly observed during winter months when atmospheric conditions are more favorable for their formation. Optimal viewing often occurs when the Moon is full or nearly full, providing sufficient brightness for the phenomenon to be clearly visible.
How Lunar Halos Form
Lunar halos originate from the interaction of moonlight with hexagonal ice crystals suspended in the Earth’s upper atmosphere. These crystals are typically found within high-altitude cirrus or cirrostratus clouds, which can form at elevations ranging from 20,000 to 40,000 feet. Their formation requires low temperatures at these high altitudes, allowing water vapor to freeze directly.
As moonlight passes through these hexagonal ice crystals, it undergoes refraction, meaning the light bends as it enters and exits the crystal. Each crystal acts like a miniature prism, deflecting the light at a specific angle. For the common 22-degree halo, this bending occurs at about 22 degrees, resulting in the consistent circular appearance. The hexagonal shape of these crystals is important for the distinct circular ring to form, as they collectively bend the moonlight towards the observer’s eye at this precise angle.
Lunar Halos and Weather Prediction
The appearance of a lunar halo has long been associated with impending weather changes in folklore and traditional observations. While this belief is not entirely without a scientific basis, the halo itself does not directly predict specific weather events.
The scientific connection lies in the cirrus clouds that host the ice crystals responsible for the halo. Cirrus clouds often form in advance of weather fronts, particularly warm fronts, which can precede low-pressure systems bringing precipitation. These high-altitude ice clouds can appear anywhere from one to two days before a significant change in weather occurs. However, cirrus clouds, and thus lunar halos, can also form without any associated weather changes, making the halo an unreliable standalone predictor of rain or snow. Therefore, while a lunar halo signals the presence of specific atmospheric conditions that can precede certain weather patterns, it is not a direct guarantee of inclement weather.