Halos and Arcs Above Buenos Aires
April 26, 2014
Photographer: Andrea Anfossi
Summary Author: Andrea Anfossi; Jim Foster
The colorful arcs shown above were captured from my backyard in Buenos Aires. The brightest arc is the circumzenithal arc (CZA). It forms when sunlight enters the upper horizontal basal face of plate-shaped ice crystals (found in cirrus clouds) and exits through one of the six vertical side faces. Look for it high in the sky when the Sun is low.
The fainter arc, tangent to the CZA and bending sunward (down), is part of a 46 degree halo -- formed by minimum deviation of sunlight through randomly oriented 90 degree ice prisms. It's infrequently observed and quite a bit fainter than the much more commonly seen 22 degree halo (shown at bottom) because only a small percentage of the light emerges from the 90 degree prisms that form it. Moreover, dispersion of whatever light does emerge spreads over a wider stretch of the sky than is the case with the 22 degree halo. When the 22 degree halo and or CZA are visible look for the 46 degree halo -- it's about twice as far from the Sun as the 22 degree halo. Photo taken on February 10, 2014.
Photo details: Camera Model: NIKON D3100; Lens: 18.0-55.0 mm f/3.5-5.6; Focal Length: 18mm (35mm equivalent: 27mm); Aperture: f/22.0; Exposure Time: 0.013 s (1/80); ISO equiv: 100; Software: Adobe Photoshop Lightroom 5.3 (Windows).