Peculiar Ground Halo Spokes

Peculiar Ground Halo Spokes

Jari Luomanen (atmospheric phenomena) noticed on one afternoon (Feb 27, '12) unusually bright and glittering snow. Well aware that ground lying ice crystals can form ground halos he searched the fields and found this strange phenomenon.

Bright but diffuse halo spokes radiate across the snow from the low Finland winter sun at about 5.30 and 6.30 as clock angles go. There are wider spokes at 3-4.30 and 7.30-9. Individual crystals glint brightly.

All images ©Jari Luomanen, shown with permission

The ice shards, clearly dendritic.

The two populations are evident, some plates strongly tilted upwards, other lying almost flat. The plates are about 1mm long. By eye the snow/crystal crust looked quite normal, the unusual crystals were only revealed by the macro shots.

From Finland to Alaska.

An image captured by Walt Tape (Atmospheric Halos) showing remarkably similar 5.30 and 6.30 spokes. However, there is no sign of Jari's 4 and 8 o'clock ones. The Alaskan sun was 11° high compared to the 6° in the Finland images. Simulations using the same crystals at 6° and 11° sun show that the outer halos would be less visible.

Is this coincidental? Or do dendrite shards under some meteorological conditions stack naturally with tilts of 20 and 65 degrees?

We need more observations. Observe ice and snow closely this winter and look for halo spokes.

Ice crystals were obviously responsible but of what type and how oriented?

Jari took some macro lens images that revealed (example below) very thin shard-like dendritic plates. Assuming external reflection from their larger faces Jari ran ray tracing simulations using HaloPoint 2.0   The simulation at left (green points) employed two crystal populations.

The 5 and 7 o’clock spokes come from plates with a mean tilt of 20° from horizontal (tilt standard deviation only 5°).    The broader 4 and 8 o’clock spokes are generated by plates with a mean tilt of 65° from horizontal and an equally small standard deviation. Both populations take all azimuthal rotational orientations.

Jari spent some considerable time in optimisation and later we played with other orientations in a video session.   Tilted stubby hexagonal crystals offering two populations of faces tilted 90° (suggested to avoid two apparently arbitrary populations) simulate rather less well.

Two populations really are needed and the spokes are remarkably sensitive to the mean tilt and the tilt distribution width.

Jari has a weather station 5km away and here are the conditions from the previous midnight through to the following one. The halo photo was taken at ~1600 and the crystal images at 1930 by flash illumination.

The previous night favoured large crystal growth with the dew point closely following the air temperature. The day was also sufficiently cold that the crystals did not significantly sublime.

"Of course, the microclimate just on top of the snow sheet is bound to have its quirks but this log from my station gives us hints about the reasons behind this extreme xtal growth."