Measurements have been made on the infrared and visible reflection properties of cirrus (ice) clouds formed at Old Faithful Geyser, Yellowstone Park. The optical data have been correlated with the meteorological parameters and the solar scattering angle. Intense specular reflections called “undersuns” were observed from fogs of hexagonal plate crystals.
RICHARD I. MCDONALD and RALPH W. DELTENRE, "Cirrus Infrared Reflection Measurements," J. Opt. Soc. Am. 53, 860-868 (1963)