2.2.11 Snow microphysics
All snow, be it wet or dry, originates as ice crystals in the atmosphere. These crystals form different shapes (habits) depending on the temperature and moisture content of the atmosphere. The primary habits are dendrites, plates, columns, and needles. In general, each is produced at the following atmospheric temperatures.
- Dendrites and plates: -22°C to -10°C (-8°F to 14°F)
- Needles: -10°C to -3°C (14°F to 26°F)
- Columns: -10°C to -3°C (14°F to 26°F) and colder than -22°C (-8°F)
Dendrites are the fastest-growing crystals and tend to aggregate into larger snowflakes that result in low-density snowpack at cold temperatures. In contrast, smaller crystals, such as columns, needles, and plates, tend to accumulate into a higher-density snow layer.
Other factors that also increase snowpack density include:
- Riming, which occurs when any type of ice crystal passes through a super-cooled cloud (one whose liquid water droplets are below 0°C or 32°F). Rimed crystals are partially or completely coated in tiny frozen water droplets and are associated with higher-density snow accumulation.
- The presence of broken crystals. Crystals can fracture when they hit each other as they descend through the atmosphere or when they strike the ground. Higher winds lead to more fractured crystals, which increases snowpack density.
For more information on atmospheric microphysics, see the COMET module “Topics in Precipitation Type Forecasting” at http://www.meted.ucar.edu/norlat/snow/preciptype/.