Photometric Measurements of the Radial Halo Profile

Despite the growing scientific awareness of ice-crystal halos, there is surprising lack of quantitative observational data about them. Even the most basic (and often-referenced) angular measurements sometimes consist of casual determinations of the radii of circular-halos with no mention whatever of the intensity distribution. More-complex halos are reported with either freehand drawings or photographs. Seldom if ever are the photographs measured to ascertain the trajectories of noncircular halos. Intensity measurements are unknown, the closest being a smoothed, uncalibrated photographic density scan across a 220 halo by Bruche and Bruche. Quantitative color measurements do not exist for any halo, and only recently have polarization studies been made by Lynch2 and Konnen.3 Most of the growing wealth of information about ice crystal halos is theoretical because naturally occurring halos are short lived, episodic, and seldom occur under the conditions necessary for detailed observation. Still, the most basic properties of halos (intensity versus angular position on the sky) are not quantitatively known. In this paper we present quantitative measurements of the intensity distribution of the 220 halo.