Growth Mode of Ice Crystals in Natural Clouds

Abstract

The growth mode of ice crystals in natural clouds has been studied by examination of replicas of more than 10,000 crystals sampled in about 120 clouds over the temperature range ?2 to ?32C. The relationship between the basic ice crystal form deduced from the shape of recently formed small ice crystals and the temperature at which they have grown is found to agree well with the results of laboratory experiments. Within each category of basic crystal form, there is some characteristic and systematic variation in the ratio of crystal axes which depends strongly upon the growth temperature. The growth of an ice crystal along either the a or c axes almost ceases after it reaches a certain length which depends upon the growth temperature. For columnar ice crystals, the ratio of the length of the c axis to the a axis at a given column length has a maximum value at about ?5C. For plane ice crystals, the ratio of diameter to thickness for a given diameter has a maximum value at about ?15C. Some consideration is given to the growth rate of ice crystals based upon the observed ratio of the crystal axes and the bulk density. It is shown that the two pronounced peaks in the growth rate obtained by Hallett, a major peak at about ?15C and a minor one at about ?5C, can be explained in terms of the characteristic axial ratio of the crystals.