Sun Pillar – Beaming With Light
Clouds made of ice crystals can interact with sunlight to form a whole range of bright arcs, spots, and lines known collectively as halo phenomena. This family of optical effects includes the sun pillar. It is a vertical beam of light that extends above the Sun, when it’s known as an upper sun pillar, or down from it, known as a lower sun pillar. Paolo Bardelli (Member 45,963) spotted this upper sun pillar formed by Cirrus and Cirrostratus clouds over Sumirago, Varese, Italy at around sunrise. The effect is associated with sunrises and sunsets, for it generally depends on the Sun being low in the sky. In fact, sun pillars often look best when, like Paolo’s, the Sun’s just over the horizon. Even though it is hidden from vision, it makes its presence felt by glinting off the tiny ice crystals in the high clouds.
That’s because a sun pillar is made up of the countless momentary sparkles of sunlight reflecting towards the observer by the faces of the cloud’s ice crystals. For the pillar to appear tall, the crystals need to be rocking back and forth as they fall gently through the atmosphere. The effect is similar to the beam of light known as a glitter path that can appear along the surface of the sea as light from a low Sun reflects off the waves. Whether the light is bouncing off rippling waves on the water below or falling ice crystals in the atmosphere above, the result is a steady beam of light made from all the momentary and fleeting reflections.
The larger and more plentiful the crystals, the brighter the sun pillar will be, with the tallest forming when these ice crystals wobble a lot as they fall. This is because the tilting reflects even more sunlight back at the observer. Paulo’s sun pillar heralded the dawn of a new day, and we hope that, as January’s Cloud of the Month, it will also herald a beaming start to your new year.
Sun pillar in Cirrus and Cirrostratus spotted over Sumirago, Varese, Italy by Paolo Bardelli (Member 45,963). View this image in the photo gallery.
