The shape of the snowflake goes on to determine how stable the snow cover is. Snow that is stable and solidly packed does not slide easily and usually doesn’t cause avalanches. Crystals make the most stable snow because the six points of each flake interlocks with other crystals. When both granules and pellets fall from the sky, they roll over each other, making snow loose and unstable. This is the kind of snow most likely to cause an avalanche because it consists of different kinds of flakes or different layers of snow. A layer of loose snow can slide over a layer or more solid snow.
Another, very dangerous snow condition is a layer of snow known as a depth hoar, which is also known as sugar snow. Large, round and cup-shaped snow crystals act like ball bearings, causing any snow on top to slide right off. It would be like trying to walk across a floor covered with marbles. The slightest disturbance, from a person or an animal, can trigger an avalanche.
Forces of Nature: ThinkQuest 2000 (Team #C003603)
http://library.thinkquest.org/C003603/english/avalanches/thecauses.shtml
No two snowflakes are the same, right? Snow falls in many different shapes - as crystals, pellets, or granules. The flakes are always six-pointed stars in enchantingly different patterns. Granules and pellets can fall as needles, pyramids, plates, or bullets. However, these flakes change after falling. Because of changing air temperature and pressure from additional snow, the points of each crystal get smaller. The center then grows larger until a rounded grain of snow has formed. The six-pointed crystals are pushed together as they lose their points, in a process known as settling. A foot of fresh snow can settle down to four inches in twenty-four hours. These changes occur slowly at low temperatures, and all metamorphosis stops at -40 degrees Fahrenheit.