Microphysical Studies of Noctilucent Clouds

Abstract

Abstract : We conclude from our experiments that the ice particles present in noctilucent clouds may often exhibit cubic habit. It appears even possible that high tropospheric ice crystals may occasionally exhibit such habit. That would be consistent with rare, but reliable observations of the Scheiner's halo. In the case of noctilucent clouds, however, it is doubtful that the crystal habit is after all of much practical importance, because due to low growth rates and limited duration of supersaturation at the mesopause, the crystals must be very small. Ice crystals of dimensions much smaller than the wavelength of transmitted electromagnetic waves interact with the waves as Rayleigh scatterers, which allows for fairly simple theoretical treatments of the interactions. No refraction or surface reflection would be occurring. It appears that even after a few days of growth noctilucent cloud particles are still only a fraction of a micron in diameter. That is less than or equivalent to the wavelength of visible light.