Oslo - Nov 17, 2010
A rare lunar and light halo display captured by Steinar Midtskogen (site).
   ©Steinar Midtskogen

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"In the transition from clear sky to ice fog there was a brief ice halo display around 22:45 MET.   There were several [fluted] light pillars and an arc below the Moon.  The arc was initially very bright, but had dimmed by the time I had my camera ready.  Jupiter is also visible in the picture.  The temperature was -7.5C. Exposure 4s at f/4 ISO200."

Both the moon halos and light halos have rare features.

Moon halos - Directly beneath the moon is a bright chromatic halo. This is a lower tangent arc or more correctly the lower section of a circumscribed arc since the lunar altitude was 35°. The arc shows that the ice fog was rich with horizontal column crystals.

Below the circumscribed arc and crossed by a vertical light pillar is a hint of a second halo that echoes the shape of the upper one. This is a rarely seen lower (moon)vex Parry arc from column crystals additionally constrained in Parry orientations. More of Parry arcs in the next OPOD.

Light halos - ice crystals are glinting the Oslo lights towards the camera to produce the appearance - an illusion - of upward going pillars of light.

The pillars are unusual in that they have fluted, trumpet shaped, tops.   The clue to their formation is the preponderance of horizontal column crystals as evidenced by the bright lunar halo.

Light passing through side faces of horizontal columns is refracted through a minimum angle of 22° to form the lower tip of the familiar upper tangent arc from the sun or moon. The corresponding spatial halo made by a nearby artificial light is a little different.

At left is an Oslo light. The dotted red curve joining the light and the eye marks the path where horizontal columns deflect light through 22°. They all glint light to form a weak apparent pillar. Some plate crystals might also be contributing. At any one point, column crystals above the parabola form a fluted halo shape but it is not visible because those from other sections are vertically displaced and the result is a faint blur.

The solid red curve section near the eye is almost linear. The fluted halos from sections along this section overlap and strengthen each other to form the flutes in the sky.