Observations of snow crystals using low-temperature scanning electron microscopy

Abstract

Low-temperature scanning electron microscopy (SEM) was used to observe precipitation particles commonly known as 'snowflakes.' The snowflakes were collected in Beltsville, Maryland, at temperatures ranging from -6° to +1°C, mounted on SEM stubs, frozen in liquid nitrogen (LN2), and then transferred to a cryosystem mounted on a field-emission SEM. Neither sputter coating with platinum nor irradiation by the electron beam affected their delicate fine structure. SEM observations revealed that snowflakes consisted of aggregations of snow crystals that occurred as hexagonal plates, prismatic columns, needles, and dendrites. In some cases, the snow crystals contained minute surface structures that consisted of rime, microdroplets, short prismatic columns, and amorphous films. Snow crystals from wet snow, which were collected at temperatures of 0° and +1°C, exhibited varying degrees of metamorphism or melting. The discrete crystalline faces and their sharp intersecting angles were gradually replaced by sinuous surfaces that tended to exhibit more spherical shapes. This study indicates that low-temperature SEM is a valuable technique for studying the formation and metamorphosis of snow crystals. The results suggest that combining low-temperature SEM and x- ray analysis could also provide qualitative elemental information on the nucleation particles of snow crystals us well as on the composition of acid snow.