The weather at the time was fine with base of cirrostratus at a height of 10000 metres. The most common halo is a luminous ring of 22 degrees radius surrounding the sun or moon, the space within the ring appearing less bright than that just outside.
Halo is produced by refraction through ice crystals.
Usually, the sky is covered by cirrostratus or thick cirrus. In Hong Kong, halo is not uncommon. On the average, we can observe halo on about 3 days in a year. The following table shows the number of days with solar halo observed at the Observatory.