Rare Halos, Sweden
Imaged by Lotta Gustavsson December 30, 2009 at Huskvarna at the southern end of Lake Vattern.  ©Lotta Gustavsson, shown with permission.

About - Submit Optics Picture of the Day Galleries Previous Next Today Subscribe to Features on RSS Feed

A notable diamond dust display dominated by the halos from horizontal column and Parry oriented crystals.

At upper left is an intense upper tangent arc with a much less common sunvex Parry arc tucked above it. This arc only appears at low sun elevations. The sun was 8.7° here.

The coloured arc at right is a supralateral from column crystals. Why not a 46° halo? Because the colours are pure, the inner 22° halo is weak (few randomly oriented crystals that would otherwise make a 46° halo) and the tangent arc is strong (plenty of horizontal columns).

But the fainter arcs are the gems of this display. The lower image enhanced by changes in levels and unsharp masking shows them more clearly but less aesthetically – avoid ‘enhancement’ unless for technical purposes.

A rare sight is the Wegener arc generated by a convolved ray path in horizontal column crystals.

Even rarer sights are two arcs from Parry oriented crystals. (1) A helic arc made by sunlight reflected from the crystal side faces curves upwards from the sun. (2) On the supralateral arc is a brightening with curved offshoots. This is a Parry supralateral arc from similar rays to those making the ordinary supralateral arc but this time passing through the Parry crystals.

Not so many years ago these arcs had been observed only in Antarctica. Now, the high quality crystals that grow downwind of ski-slope snow machines produce them but they are still a rare occurrence. The source of the crystals at Huskvarna is not known with certainty. However, the temperature was -13 Celsius and the wind only 0-1 m/s from W-NW. There was probably a weak northern wind over Lake Vattern (no ice) reaching the southern end where Huskvarna is located there were some clouds forming and at higher altitudes where the images were taken (5-6 km from the lake up a hill) there might have been small halo forming ice crystals. The nearest ski centres were 10 km away or at greater distances and mostly not upwind.

My thanks to Weine Josefsson of SMHI for weather data and helpful discussions.