A world-famous tourist destination offered a rare weather phenomenon as a bonus. This photo released by NASA on the 9th shows the sky over Chamonix-Mont-Blanc in France at night and again after sunrise in late Dec. 2025. It is a halo created by the moon and sun in the sky. The site is a famous mountain village and a world-renowned destination for skiing and hiking at the foot of Mont Blanc, the highest peak in the French Alps.
◇ ring of light made by ice crystals
A ring of light around the moon is called a lunar halo, and around the sun a solar halo. Both are produced by cirrostratus clouds at altitudes of 5–13 kilometers. Cirrostratus are upper-level clouds that spread like a thin white veil, also called hairlike layer clouds or bridal veil clouds.
Cirrostratus consist mainly of ice crystals. A halo is an optical phenomenon caused when ice crystals in the cloud reflect or refract light. Most halos form at an apparent radius of 22 degrees around the sun or moon, and the rarer 46-degree halo can also appear.
In the photos, both the moon and the sun are surrounded by the commonly observed 22-degree halo. In the left image, an oval ring surrounding the 22-degree lunar halo, the circumscribed halo, is visible, and at the very bottom, a horizontally rainbow-tinted band called a circumhorizontal arc appears.
In both images, a paraselenic circle and a parhelic circle appear as horizontal white lines at the same altitude as the moon and sun. The paraselenic and parhelic circles are optical phenomena in which horizontal white lines appear at the same altitude as the moon and sun. Bright small spots at the points where the 22-degree lunar and solar halos intersect their respective circles are called Moon Dogs and Sun Dogs. In the right photo, it is amusing that a Sun Dog is in the sky and a real dog is on the ground.
◇ same shape, different mechanism and colors: corona
In Korea, halo phenomena are also observed from time to time. There has long been a saying, "When a solar or lunar halo appears, rain will come." In fact, cirrostratus that produce halos can be followed by nimbostratus bringing wind and rain.
A corona is a similar phenomenon. As light passes through a thin cloud made of tiny water droplets or ice crystals, a circular rainbow-like ring appears around the sun. While both coronae and halos form ring patterns, their mechanisms differ.
A halo arises from the refraction or reflection of light. Reflection is when light hits an object's surface and returns, and refraction is when light enters a different medium, changes speed, and bends its path. A straw appearing bent in water is due to refraction.
A corona, unlike a halo, arises from diffraction of light. Diffraction is when light spreads out after passing through a narrow slit or the edge of an obstacle. The rainbow sheen on a CD surface results from diffraction. Halos and coronae also differ in color. A halo is red on the inside and blue on the outside, while a corona is blue on the inside and red on the outside.
References
Astronomy Picture of the Day, https://apod.nasa.gov/apod/ap260109.html