Home
  OpticsPOD
  What's New 
  Rays & Shadows
  Water Droplets
  Rainbows
  Ice Halos
    Contents
    Crystals
    Frequent Halos
      22° circular
      Sundogs
      Tangent Arcs
      Circumscribed
      Pillars
        Lower Pillars
        About Pillars
          Plate Pillars
            Solar Altitude
          Column Pillars
          Lowitz Pillars
        Moon Pillars
        Venus Pillars
        Light Pillars
        More Images
      Circumzenithal
      Circumhorizon
      Parhelic Circle
    Infrequent Halos
    Multiple Displays
    Other Worlds
    Observing Halos
    HaloSim
  High Atmosphere
  Links & Resources
  Search - Index





 
123456789012345678



  Plate Pillars - Solar Altitude 
  Sun pillars change as the sun descends. At first the lower pillar is strongest but as the sun descends the upper pillar grows taller and brighter. The subhorizon components are visible in nearby diamond dust, from mountainsides or from aircraft. Simulations by HaloSim; plate crystals of various tilts**, 10 million rays traced per pillar.  

Upper pillars are short and faint when the sun is a few degrees high. The sun needs to reflect off the underside of plate crystals to make an upper pillar and when its rays are slanting downwards a few degrees, only strongly tilted crystals have their lower faces lit. In contrast, sunlight easily shines on upper faces and so the lower pillar is strong. Most of the lower pillar is of course beneath the horizon. The subsun - at the same distance below the horizon as the sun is above it - is especially bright.

As the sun sinks, the upper pillar lengthens and brightens. At sunset both pillars are equal.

After the sun has set it still illuminates high clouds in the west. These almost horizontal or even slightly upward going (from atmospheric refraction) rays illuminate the lower crystal faces. They reflect downwards to form a strong upper pillar. Watch a pillar strengthen after sunset (the reduced glare and skylight also helps make the pillar more apparent) and creep slowly northward following the sun hidden below the horizon.
 

** 


Large crystal tilts produce tall pillars but the distribution of tilts also affects pillar appearance. Crystals in the simulation had a uniform distribution of tilts up to ±3° from horizontal then tailing off so that a few crystals had tilts of ±6°. Other tilt distributions produce pillars of different appearance. Try it in HaloSim.