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Snow to ice animation
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This animation shows how snow on the ground is rounded and compacted, sealing off pores between the grains, trapping atmospheric gases. Snow that survives one melting season is called firn.
Falling snow crystals are usually hexagonal plates, needles, stars, or other shapes. On the ground they become unstable and sharp points are rounded and condensed into hollows. The crystals fit more closely together and over time the density of the snow increases.
At a density of about 830 to 840 km m3, the air spaces between grains are sealed off, and the material becomes impermeable to fluids. The time it takes for pores to be closed off is critical for climate-history information from ice cores.
Snow that has survived one melting season is called firn (or névé); its density usually is greater than 500 kg m3 in temperate regions but can be as low as 300 kg m3 in polar regions.
