This afternoon, May 21st, a halo around the Sun has appeared simultaneously in Hanoi, Quang Ninh, and several other localities.
The appearance of the Sun accompanied by a halo has drawn considerable attention from many people this afternoon. This phenomenon is easily observable under clear and cloudless conditions. Many believe that this is a rare occurrence.
The solar halo became most distinct from around 11 AM today (May 21st).
A halo is a circle of light that surrounds the Sun or the Moon. They commonly appear when there is a thin layer of ice crystals in the atmosphere. The combination of chemistry, physics, and geometry is the main cause of the solar halo. The atmosphere mixes various gases, including oxygen, nitrogen, and water vapor.
At a sufficient altitude, water vapor condenses and then freezes into ice crystals. When sunlight passes through these ice crystals, the geometry of the crystals causes the light to refract, similar to the phenomenon that occurs when light passes through a prism.
Solar or lunar halos are often a sign that rain is on the way. Because halos require ice crystals to form, these ice crystals typically appear in thin clouds at high altitudes. These clouds can form several days before a warm or cold air mass brings rain. However, not all thin clouds are accompanied by storms. Some halos simply indicate an increase in moisture in the upper atmosphere. This phenomenon can also occur with the Moon.
Most recently, a solar halo appeared near noon on May 8th, lasting for about 10 minutes, sparking curiosity among residents.
