On the 10th of January of 2022, I arrived back home in Finland after spending Christmas with my family. The arrival back home was very welcoming: there was a big layer of snow, it was cold and I even saw something I had never seen before: light pillars!
Light pillars are optical phenomena that generally occur above artificial light, such as street lights or lights from buildings. It looks like beams of light aiming upwards, as one can see in the photo’s below. It’s an optical illusion that may appear when there are ice crystals in the air.
How do light pillars form?
Light pillars occur when hexagonal ice crystals are present in the lower atmosphere. These crystals reflect the light from light sources, which causes an optical effect that shows as light beams.
For these crystals to form, very cold and very calm weather is required, in combination with high humidity. There should be barely any wind and temperatures should be below -10 C. For this reason, light pillars occur most frequently in cold areas in wintertime, under the presence of a high pressure system.
Perfect cold conditions in Helsinki
Before landing, one could already suspect that it was quite cold in Helsinki. A plume emitted from some kind of industrial building made a long horizontal stretch over the town, without rising. This indicated a strongly stably stratified boundary layer with the coldest air on the bottom. This is generally the conditions that cause the lowest temperatures in wintertime. When I landed and checked the temperatures, this indeed seemed to be true. The temperature at the airport was almost -16C, while the city center measured -13C. On the ground, there was no noticeable wind at all. These conditions favored the ‘formation’ of the light pillars.
Day after: rime
The high humidity and very low temperature caused a thick layer of rime during the following night and morning. The very thick rime covering trees and other objects created a beautiful winter scenery that made me decide to go on a nice morning walk. Enjoy the pictures of the rime, made during my walk, below.
