Coronas and Iridescence in Mountain Wave Clouds Over Northeastern Colorado

Abstract

The small, uniformly sized particles in mountain wave clouds give rise to spectacularly colorful optical displays that can be explained with diffraction theory.

Icy wave-cloud lunar corona and cirrus iridescence

“…Figure 1 is a photograph of this iridescence display taken at 1600 Mountain Standard Time (MST ¼ UTC − 7), showing a typical variety of pastel reds, greens, and blues. The significant amount of green present in this display may suggest that this particular cloud does not have quite the low optical depth and narrow particle-size distribution that simulations show are required to produce blue as the dominant short-wavelength color [3,8]. However, the colors observed in iridescence displays also can change significantly with the angle of illumina-tion.…”

“…The firstorder blue ring is lost in the overexposed center region containing the waxing Moon, which was full on 9 February. Coronas with multiple visible diffraction orders are fairly common in wave clouds because of the tendency of these clouds toward monodisperse particle-size distributions with relatively small mean particle sizes [7,8].…”

“…Fraunhofer diffraction theory allows estimation of the size of the cloud particles from the angular extent of the corona rings [1,7,8]. For circular objects of diameter D, the diffraction pattern contains concentric colored rings with maxima located such that where r is the radial coordinate in the observation plane, z is the distance from the diffracting object to the observation plane, θ r is the angular radius of the diffraction ring, and λ is the optical wavelength [9].…”

“…A related study was performed for mountain wave clouds [7,8], using photographs and meteorological analysis to show that coronas were generated in mountain wave clouds in northeastern Colorado under situations for which the clouds must have contained ice. That study did not benefit from either cloud particle samples or lidar data, but instead used previous in situ samples that showed that small quasi-spherical ice particles with diameters near 10-20 μm can exist in mountain wave clouds, presumably being unable to grow into more common sizes and shapes because of the high vertical velocities found in these unique clouds.…”

Noncircular glories and their relationship to cloud droplet size

“…Such simulations work well when the glory consists of circular colored rings centered on the antisolar point at scattering angle θ ¼ 180°(i.e., where the shadow of the observer or aircraft might appear on clouds). Some of the most vivid glories seem to be caused by lenticular clouds (which also produce vivid coronas [3,4]), but such glories are almost always seen as fragments, typically at the edge of clouds, rather than complete circles. In these cases, there can be considerable uncertainty about the relative position of the antisolar point, and consequently it is very difficult to estimate the size of the water droplets in such clouds.…”