photometeor

A photometeor is any optical effect you can see in the sky or atmosphere produced by light interacting with water droplets, ice crystals, dust, or air molecules themselves. Unlike hydrometeors (rain, snow, hail), photometeors are purely optical events: the light itself changes direction or splits, creating halos, glories, coronas, rainbows, and similar phenomena. The atmosphere acts as an optical medium, bending and scattering light in ways that depend on the size and shape of the particles present and the geometry of the sun or light source relative to the observer.

The main physical mechanisms are reflection (light bouncing off a surface), refraction (light bending as it enters or leaves a medium of different density), diffraction (light spreading around obstacles or through narrow gaps), and interference (light waves reinforcing or canceling each other). A 22-degree halo around the sun forms through refraction in hexagonal ice crystals in cirrus clouds; a rainbow occurs when light refracts into water drops, reflects internally, and refracts back out. A corona (the colored rings close to the sun, not the stellar corona) results from diffraction in cloud droplets; a glory on clouds below you uses backscattering and interference. Mirages are refraction effects in layers of heated air near the ground.

Common photometeors and observation

Observers and optical instruments encounter photometeors constantly in field work, surveying, and astronomy. Optical designers and atmospheric scientists study them to understand particle size distribution and air quality. In remote sensing, photometeor effects introduce noise into measurements; in camera and lens testing, they can mimic optical artifacts. The 22-degree halo is perhaps the most frequent photometeor in temperate zones, visible on roughly one day in three when cirrus clouds are present. Upper-atmosphere ice crystals of specific shapes and orientations produce sharp, repeatable optical patterns, whereas liquid water droplets produce softer, broader effects due to their spherical shape and size distribution.

The term 'photometeor' is less common in everyday use than specific names like rainbow or halo, but it is standard in atmospheric optics, physics textbooks, and optical metrology. It encompasses phenomena ranging from obvious (rainbows) to subtle (the cloud iridescence near the edge of a cumulus cloud, caused by diffraction in small water droplets). Ice crystal halos include not only the 22-degree halo but also the 46-degree halo, sun pillars, light pillars, and parhelia (sun dogs), each corresponding to a different crystal orientation and refraction angle.

Understanding photometeors matters in precision optical work because they can contaminate measurements, degrade image quality, or indicate atmospheric conditions that affect instrument performance. In aerial photography and satellite imaging, ice crystals in the upper atmosphere can produce artifacts in the data. Conversely, deliberate observation of photometeors provides real-time information about the type, size, and concentration of atmospheric particles without needing a probe or sampler.