It’s been a week of cool optical phenomena across the state. It was all capped off by about 5 or 10 minutes of this sight – a gorgeous sun pillar stretching upward from the western horizon right around sunset Saturday.
A deck of altostratus clouds overhead Saturday evening was comprised of ice crystals. Those crystals were sublimating before reaching us on the ground though they were able to produce beautiful (and unusual) sun pillars across the state.
Sun pillars form as ice crystals reflect sunlight back to an observer on the ground. This is actually a halo that manifests itself as a vertical column of light. Only specific types of ice crystals can lead to sun pillars.
Hexagonal plates or columnar ice crystals with their long axes oriented horizontally are the only types of crystals that can result in pillars.
As it turns out that’s exactly what we had in this deck of altostratus yesterday evening over Connecticut. Here’s the weather balloon launch sounding from 7 p.m. on Long Island Saturday evening. You can see areas of near saturation (high relative humidity) where the green line (dew point) and red line (temperature) are close together. This occurs
This cloud deck is generally below -10ºC and above 12,000 feet which means ice crystals were likely present in the cloud. The presence of the sun pillar confirms that those ice crystals were either columnar or hexagonal plates. Really cool stuff!
Read more: Keith Heidorn’s page on sun pillars