Lunar Halo and Cloud Layers
March 07, 2005
This photo showing a partially obscured 22-degree lunar halo was taken from Green Bay, Wisconsin and dramatically demonstrates the cloud altitude necessary for prismatic ice crystals to form. The thin cirrus cloud layer, responsible for this faint halo, likely resides at an altitudes of at least 20,000 feet (about 6,000 meters) and contains microscopic, hexagonally-shaped ice crystals that act as prisms in the presence of light from the nearly full moon. If the crystals are more or less randomly oriented, a 22-degree halo may be observed, as was the case this night. The cloud cover streaming in to eclipse the halo is a ragged stratus layer at an altitude of approximately 6,500 feet (about 2,000 meters), and it's composed primarily of water droplets, which tend to absorb the light rather than refract it. These low-level clouds appear to have a reddish tinge because they're close enough to the ground to reflect the light emanating from street lights -- a prime source of light pollution. Although this photo was taken in winter (January 23, 2005), halos can occur any season of year, independent of the Earth's surface temperature.
Camera details: Canon EOS Digital Rebel, ISO 1600, f-5.0, exposure time 2 seconds.
This week we're featuring photos that illustrate meteorological optics -- light reflection and refraction in ice crystals and raindrops.Related Links: