Monthly Archives: February 2009
It has been good having all the snow. So exciting and makes a real impact in both good and bad ways. Snow is a type of precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. The process of this precipitation is called snowfall. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by external pressure. Snow crystals form when tiny supercooled cloud droplets (approx 10 μm in diameter) freeze. These droplets are able to remain liquid at temperatures lower than 0 °C because, in order to freeze, a few molecules in the liquid droplet need to get together by chance to form an arrangement close to that in an ice lattice; then the droplet freezes around this ‘nucleus’. Experiments show that this ‘homogeneous’ nucleation of cloud droplets only occurs at temperatures lower than -35 °C. In warmer clouds an aerosol particle or ‘ice nucleus’ must be present in (or in contact with) the droplet to act as a nucleus. Our understanding of what particles make efficient ice nuclei is poor – what we do know is they are very rare compared to that cloud condensation nuclei which liquid droplets form on. Clays, desert dust and biological particles may be effective, although to what extent is unclear. Artificial nuclei include silver iodide and dry ice, and these form the basis of cloud seeding. I love snow!