Estimating cloud particle size using the moon. Robin Hogan. What causes the coloured rings around the moon?. Huygens’ principle states that any part of a “wave front” can be treated as its own source of spherical waves.
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A “wave front”: Maxwell’s interpretation of this is the point of maximum electric field
Direction of incidence of electromagnetic radiation
Scattering amplitude from an obstacle (including unscattered radiation)
Scattering amplitude from the equivalent hole
Original plane wave
The first Bessel function in WCD!
Ring radius 2° -> particle radius r=9 mm
Ring radius 3.5° -> particle radius r=5 mm
Ring radius 7° -> particle radius r=2.5 mm
Droplets must be nearly all the same size so that the colours don’t wash out
Diameter of moon: 0.5°
Radius of red-ring: 3°
Droplet diameter: 4.3 mm
Ice crystals are larger so produce a smaller aureole
“The Scream” (Munch 1893) shows Krakatoa-induced red skies?