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England February 9, 2001 - Yorkshire,
Lancashire and the Midlands saw bright displays. Ray
Martin took this montage from Sutton Coldfield a few miles
north of Birmingham. Comparison with a South Pole display of
the identical solar altitude shows the same major halos and that
spectacular halos can be seen in the UK. Horizontal column crystals
formed most of the halos with very bright supralateral and
infralateral arcs, an almost blinding upper tangent
arc and a hint of a diffuse Wegener arc.
The aircraft trail was above the halo-forming ice crystal layer
and casts a shadow downwards onto it. A matching HaloSim simulation
below identifies the halos. Images ©Ray
Martin, shown with permission. |
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HaloSim
ray tracing for a solar elevation of 22.5°. Crystals: 76%
columns 1° tilt dispersion c/a=1.2, 10%
random columns c/a=2, 5% plates 2°
tilt disp c/a=0.1, 6%
plates 6° tilt disp c/a=0.1, 3%
Parry 1° tilt c/a=2. Very few Parry
crystals are needed to make the arc, Parry orientations seem
appear very improbable but, conversely, Parry rays are very efficient
at halo formation. 256
colour levels, 30 million rays. |
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