Phenomenon explained: strange circular halo around the Sun

What is the phenomenon of the Sun being surrounded by a circle? This article will explain in detail to you the phenomenon of the circle around the Sun.

Table of Contents

• What is a solar halo?
• When does the Sun halo appear?

What is a solar halo?

Have you ever looked up and noticed a large ring of light around the sun or moon? Scientists call them 22-degree halos. They are so named because the radius of the circle is always approximately 22 degrees.

There’s an old weather saying: A halo means drought, a waning moon means rain. There’s some truth to this, since high cirrus clouds often appear before storms. Halos are a sign of tall, thin cirrus clouds drifting 20,000 feet (6 km) or more above our heads.

These clouds contain millions of tiny ice crystals. The halos you see are created by both refraction, or the splitting of light, and reflection, or rays of light from these ice crystals. The crystals must be oriented and positioned precisely with respect to your eye for the halos to appear.

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"Strange sun" in Nghe An at noon on May 7.

Sun halo in Hue at noon on May 9.

Video of a solar halo appearing in Hue filmed by locals.

When does the Sun halo appear?

This phenomenon occurs due to the effect of the atmosphere. When the temperature increases, the areas near the Sun often have a situation where hot air and cold air intersect. Hot air carrying water vapor rises above the cold air and rises into the sky. When the water vapor in the sky meets low temperatures, it condenses into ice crystals in the shape of hexagonal prisms. When sunlight shines on these ice crystals, they are strongly refracted, forming a circle with full colors like a rainbow surrounding the Sun.

Circular rainbow phenomenon.

This phenomenon also occurs at night with the Moon causing the "lunar halo" phenomenon.

Moon Halo.

According to experts, when a halo appears around the Sun or Moon, it predicts rain or wind in the coming days.

Are halos around the Sun or Moon more common at high latitudes?

It's a good question but not easy to answer accurately because no halo frequency statistics have been collected except for one or two mid-latitude European countries.

We need to distinguish between halos formed by low-level diamond dust in very cold weather (a) and (b) halos formed by ice crystals in high-level cirrus clouds.

• It is clear that halos (a) only appear in polar regions or countries with very cold winters (for example Canada is not a high latitude).
• Halos (b) can occur anywhere on the planet in winter or summer. Their frequency depends on the frequency of cirrostratus cloud cover and whether it has a history of containing halo-forming crystals. The latter is difficult to predict. For example, there is a large variation in halo frequency and halo types over a 200-mile [300 km] area in the UK.