Lower Tangent Arc Observed Over San Rafael, California

May 10, 2012

LTAIMAG0799 (2)

: Eric Muller; Eric's Exploratorium Web site
Summary Author: Eric Muller; Jim Foster

The photo above shows a wonderful example of a lower tangent arc (LTA). It was captured while watching a little league baseball game in San Rafael, California at about 5:00 p.m. on March 19, 2012.The Sun was about 30 degrees above the horizon. This colorful arc is tangent to the 22-degree halo (barely visible on photo) and is positioned directly below the Sun. The Sun must be rather low in the sky, but at least 22 degrees above the horizon, in order to best observe this arc. LTA's result when columnar ice crystals, composing cirrus clouds, are similarly aligned -- their axes are all horizontal. Note that though these crystals are oriented horizontally, they're not all aligned in the same direction. Sunlight enters one of the crystals' side faces and is refracted through an alternate side face. In the foreground are the Coast Range Mountains (The Marin Hills). Be sure to protect your eyes when looking toward the Sun. Photo taken with an Android cell phone in automatic setting.