Solar halos appeared in the sky over various locations in Hong Kong today (April. 10), prompting many citizens to capture and share photos on social media. As people looked up, the sun appeared surrounded by a halo or bubble.
According to the Hong Kong Observatory, a "halo" is an optical phenomenon that occurs when sunlight or moonlight passes through cirrus clouds composed of ice crystals. The light is refracted, forming a circular or arc-shaped halo around the sun or moon. The visibility of the halo depends on the concentration of ice crystals in the clouds. The halo is more prominent and more accessible for observation if the clouds contain a higher concentration of ice crystals. Conversely, if the concentration is lower, the halo may be difficult to observe or not form.
Halos can be classified into two types: "22-degree halo" (or slight halo) and "46-degree halo" (or large halo). The 22-degree halo appears as a faint red inner circle with occasional purple or white colors in the outer circle, with the sky inside the halo noticeably darker than the sky outside. The 46-degree halo, which is rarer, has a larger radius and is generally darker compared to the slight halo.
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