halo (optical phenomenon)

halo (optical phenomenon)
Name	Halo (optical phenomenon)
Phenomenon	Atmospheric optical phenomenon
Causes	Ice crystal refraction, reflection, scattering
Wavelength	Visible light

halo (optical phenomenon) is an atmospheric optical phenomenon produced by the interaction of sunlight or moonlight with ice crystals suspended in the Earth's atmosphere. It appears as circular rings, arcs, spots, or pillars of light around the Sun or Moon and has been observed and recorded across many cultures and historical epochs. Scientific study of the phenomenon links it to crystallography, atmospheric physics, and optical geometry.

Description and Classification

Halos are classified according to angular radius, geometry, and brightness into categories such as 22° halos, 46° halos, parhelia, and circumzenithal arcs, terms used in meteorological catalogs maintained by institutions like the Royal Meteorological Society, the American Meteorological Society, and the World Meteorological Organization. Observers include field scientists from Royal Observatory, Greenwich, researchers at Scripps Institution of Oceanography, and photographers associated with the National Geographic Society. Classification schemes draw on optical taxonomy developed by figures linked to historical observatories such as Greenwich Observatory and modern groups like the International Commission on Illumination.

Physical Mechanisms

The optical processes responsible for halos include refraction, dispersion, internal reflection, and scattering by hexagonal ice crystals found in cirrus and cirrostratus clouds. The geometric optics framework used to model the phenomenon was advanced by scientists connected to institutions like University of Cambridge, Massachusetts Institute of Technology, and Max Planck Institute for Meteorology. Ray-tracing analyses reference canonical work associated with scholars from Imperial College London and computational methods refined at California Institute of Technology. Polarimetric and spectroscopic measurements performed at facilities such as Mount Wilson Observatory and Mauna Kea Observatories link the microphysics of ice to macro-scale halo morphology.

Types and Visual Characteristics

Common halo manifestations include the 22° halo, supralateral arcs, infralateral arcs, circumhorizontal arcs, sun dogs (parhelia), sun pillars, and light pillars; each has distinctive angular positions, color dispersion, and dependence on crystal orientation. Observational records from projects at Smithsonian Institution collections and aerial campaigns by National Aeronautics and Space Administration teams document spectral gradients resembling dispersion effects first analyzed in the context of optics at University of Oxford. Photographic archives curated by Royal Photographic Society and natural history exhibits at American Museum of Natural History preserve rare formations like the 9° halo and Lowitz arcs, which have been subjects in specialized monographs from researchers affiliated with Princeton University and University of Chicago.

Atmospheric Conditions and Formation

Formation requires ice crystal populations in upper-tropospheric clouds, typically cirrus or cirrostratus, where temperature and humidity profiles measured by radiosonde campaigns from National Weather Service, Met Office (United Kingdom), and Météo-France favor hexagonal plates and columns. Synoptic patterns monitored by agencies such as European Centre for Medium-Range Weather Forecasts and Japan Meteorological Agency correlate halo prevalence with frontal systems and jet stream dynamics studied at NOAA National Centers for Environmental Prediction. Microphysical properties are inferred from lidar and satellite remote sensing missions like CloudSat, CALIPSO, and imaging from platforms operated by European Space Agency.

Observation and Measurement

Quantitative observation uses all-sky cameras, photometers, polarimeters, and spectrometers in networks maintained by research centers such as University of Helsinki, University of Toronto, and Stockholm University. Citizen science initiatives organized by groups including the Royal Meteorological Society and the Citizen Science Association contribute image data, while algorithmic detection methods are developed in collaborations involving Google Research and computational teams at ETH Zurich. Historical measurements by expeditionary science teams from institutions like Royal Society and contemporary field campaigns run by National Science Foundation provide datasets for radiative transfer models.

Cultural Significance and History

Halos have been recorded in chronicles, art, and literature, appearing in accounts associated with events like the Battle of Towton, reports by explorers linked to the British East India Company, and paintings held by museums such as the Louvre and the National Gallery, London. Astronomers and natural philosophers across epochs—from correspondents of the Royal Society to observers at the Vatican Observatory—documented halos alongside auroral and cometary phenomena. Literary references appear in works connected to authors like William Shakespeare, while visual representations influenced artistic movements represented in collections of the Metropolitan Museum of Art and the Uffizi Gallery. Folklore tied to halos features in regional traditions from Scandinavia recorded by institutions like the Nordic Museum and in accounts preserved by the British Library.

Category:Atmospheric optical phenomena