Low-temperature, scanning electron microscopy of artificial snow

TitleLow-temperature, scanning electron microscopy of artificial snow
Publication TypeJournal Article
Year of Publication2005
AuthorsWergin W., Erbe E., Rango A., Foster J., Pooley C.
Date PublishedMarch 1, 2005
ARIS Log Number178338
Keywordscrystals, electron, LTSEM, microscopy, scanning, snow
AbstractLow temperature scanning electron microscopy (LTSEM) has been used to observe and characterize fresh and metamorphosed naturally formed snow crystals1. These crystals form in the atmosphere by a process known as vapor deposition, in which molecules of water vapor (gas) bind to form a crystal (solid). No transitional liquid phase occurs. Atmospheric temperature during crystal formation largely influences the structure of the crystals, which may develop into dendrites, plates, columns needles, or irregular crystals, whereas the time of formation influences their sizes. Once formed, the descending snow crystals may encounter super-cooled cloud droplets. The droplets impact and immediately freeze to the surface of the crystal, which is then referred to as being rimed. Continuation of this process, which is known as accretion, results in a mass of frozen droplets, referred to as graupel. Artificial snow appears as microscopic pellets. The types of snow guns and use of ice nucleating agents, influence the sizes of the pellets and the efficiency of production. Artificial snow, which results from the freezing of a liquid, most closely resembles the natural process of accretion that results in riming and graupel. Because sufficient time for vapor deposition seldom occurs, artificial snow does not exhibit the crystalline shapes found in natural snow.