Light Pillars in Umeå

2 February 2022

On 21 December 2021 (the winter solstice), I saw an enigmatic optical illusion right outside my apartment building in Umeå. Several vertical pillars of light seemed to hover in the sky (please watch the two videos embedded in this post). Today, after reading an article from 2017 in NYT, I learnt that these pillars are virtual images formed by the reflection of terrestrial light off flat hexagonal crystals of ice floating in the sky. Such crystals usually tend to float several kilometers above the Earth's surface. However, when the weather is extremely cold, they can come quite close to the ground. As millions of flat ice crystals float at different heights, reflections off them appear to form a vertical column of light that has the same color as the terrestrial light source. As these pillars form due to simple reflection off plane mirrors, they are found approximately halfway between the light source and the observer. For more information on similar atmospheric optical phenomena, check out Les Cowley's website.