22-degree halo
= 22-degree halo
{title2=$\Delta_{\min}\simeq22^\circ$}
The common <optical halo> has a radius of approximately $22^\circ$ around the Sun or Moon. Refraction through a $60^\circ$ ice-crystal <prism> supplies a minimum-deviation edge. Nearby incident directions are concentrated near that edge, forming a <caustic>.<International Cloud Atlas by World Meteorological Organization>[Section 3.2.3.1.1, 22° halo]{ref}