Published by the American Geophysical Union as part of the Special Publications Series. Atmospheric halos are visible phenomena of much beauty and fascination. From our earliest known records, dating from the Sumerian-Babylonian culture of 4-5,000 years ago, to modern researchers, halos have kept us looking skyward. Caused by light refracted through ice crystals floating in the atmosphere, they can appear at nearly any time and place. What do the ice crystals look like, especially those that make the exotic "odd radius" halos? What is the value of the elusive angle x, so critical to determining the shapes of these crystals? What halo displays do we understand well and what displays do we not understand at all? This book responds to such questions, and more, with a multifaceted view of halo science. Experts and novices alike will find this book the definitive resource on the subject. Features include: The "how to" in halo observation and photography Carefully analyzed photos of rare halo displays and pyramidal ice crystals The early history of halo science Elegant new explanations of how halos form Authoritative discussion of odd radius halos We invite you to gaze up and discover the world of halos.