An Analog Light Scattering Experiment of Hexagonal Icelike Particles. Part II: Experimental and Theoretical Results

Abstract

The scattering properties of hexagonal icelike crystals as measured in the analog manner by the experimental apparatus described in Part I are presented. The crystals are made out of sodium fluoride (NaF), which has an index of refraction similar to that of water ice. The experimentally determined light intensities scattered from fixed and integrated random orientations of a NaF hexagonal crystal, oriented to produce a two-dimensional scattering profile, compares favorably to the expectations derived from geometric ray tracing methods. Also, the three-dimensional scattering properties of a simulated NaF Parry column, a NaF crystal aggregate, and a NaF plate with a rough surface are compared to results computed from the geometric ray tracing approach. From these comparisons the authors conclude that within the experimental measurement uncertainties and to the degree in which the NaF crystal models approach the geometric and optical ideal, the geometric ray tracing approach is an excellent method to determine the single-scattering properties of hexagonal ice crystals of various shapes in the geometric optics domain.

Laboratory Measurements of Light Scattering by Single Levitated Ice Crystals

“…However, the results show, for x ϭ 100 and ␤ ϭ 60, a broad halo peak and hints of a reflection peak with ripple structure, features that also appear in our data. Barkey et al (1999), in an analog experiment, report halo and reflection peak positions similar to those we observe (note that their definition of ␤ differs from ours by 30Њ).…”

“…Experimental efforts to measure scattering from single well-characterized nonspherical particles have been quite limited. Analog experiments scattering He-Ne laser light from macroscopic hexagonal crystals of sodium fluoride (Barkey et al 1999) did provide careful control of the crystal orientation and do show reasonable agreement between the measured scattering and geometric ray-tracing calculations. However, the size parameter for these experiments was orders of magnitude larger than that of atmospheric ice crystals.…”

Artificially generated halos: rotating sample crystals around various axes

“…Whenever the effective index is close to the one of ice in air (1.31) correct halo scattering angles can be observed. Besides single crystal experiments that visualize halos, NaF single hexagonal crystals have been used to measure accurately the scattering functions [20].…”

“…In addition, several attempts to generate artificial halos for quantitative analysis or qualitative visualization in the laboratory or lecture hall have been reported. These are based on at least three different concepts and have succeeded in demonstrating a number of well-known halos [11][12][13][14][15][16][17][18][19][20][21][22]. The most common experiment-similar to the demonstration of rainbows using single droplets in the laboratoryuses single 60°-prisms or hexagons attached to a drill.…”

Ice analog halos

“…However, the materials used either did not have the correct, hexagonal shape, had wrong values of the refractive index 4 and/or resulted from refraction through single, macroscopic objects (prisms) which due to their size cannot represent atmospheric ice crystals for the purpose of studying light-scattering properties. 2,[5][6][7] An outstanding question concerning atmospheric halos is why they accompany cirrus clouds so rarely, at least at lower latitudes. 8 The presence of water droplets, liquid or frozen, is one possible explanation.…”