How big should hexagonal ice crystals be to produce halos?

Area of Science:

• Atmospheric optics
• Cloud physics

Background:

• Halos are optical phenomena typically associated with ice crystals.
• Frequent absence of halos in cirrus and contrails suggests a deviation from expected optical behavior.
• This absence is often attributed to small ice crystal sizes falling outside the geometrical optics domain.

Purpose of the Study:

• To investigate the hypothesis that small ice crystal sizes cause the lack of observed halos.
• To determine the size parameters at which halos become observable in ice crystals.
• To explore the implications for remote-sensing applications.

Main Methods:

• Utilized ray tracing phase functions for hexagonal and circular ice cylinders.
• Employed T-matrix computations for electromagnetic scattering by circular cylinders.
• Analyzed scattering properties for size parameters up to 180 in the visible spectrum.

Main Results:

• Demonstrated a strong similarity between ray tracing phase functions of finite hexagonal and circular ice cylinders.
• Calculations indicate that well-defined halos should be observable for ice crystal size parameters of 100 and larger.
• Particle size is a critical factor in halo formation and observability.

Conclusions:

• The hypothesis that small ice crystal sizes lead to the lack of observed halos is supported.
• Observable halos are expected for larger ice crystals (size parameters ≥ 100).
• Results have significant implications for interpreting remote-sensing data of ice clouds and contrails.