Circumhorizon Arc Above Ithaca, New York

July 15, 2010

: Michael Roman
Summary Author
: Michael Roman

A few weeks back, the high solar altitude of the summer season of the northern hemisphere conspired with some gauzy cirrus clouds and icy fall streaks to produce a colorful optical display known as a circumhorizon arc (CHA). This display came and went with the clouds, producing an intermittent show that lasted several hours. These fetching arcs, like all halo phenomena, are formed when rays of sunlight enter ice crystals that compose cirrus clouds and are redirected towards the observer's eye. The observed colors are due to each component of "white" light being bounced by the similarly aligned ice crystals at slightly different angles. This is referred to as dispersion due to refraction.

Due to the specific geometric path of sunlight required to produce this particular kind of arc, they're are only seen when the Sun is more than 58 degrees above the horizon. Consequently, at my latitude in Ithaca, New York, CHAs are somewhat rare and restricted to midday hours of early summer and late spring. Always protect your eyes when looking in the direction of the Sun. Photo taken on June 15, 2010.

Photo details: Camera Maker: Canon; Camera Model: Canon EOS 40D; Lens: EF17-40mm f/4L USM; Focal Length: 24.0mm; Aperture: f/5.6; Exposure Time: 0.0020 s (1/500); ISO equiv: 100; Exposure Bias: none; Metering Mode: Matrix; Exposure: Manual; Exposure Mode: Manual; White Balance: Manual; Flash Fired: No (Manual); Color Space: Adobe RGB (1998) ISO 100; polarizing filter.