Avalanche Canada’s Mountain Weather Forecast tracks four different types of precipitation: snow, rain, freezing rain, and ice pellets. Each one has a different impact on snowpack structure.
- Snowfall builds the snowpack. Recently fallen snow is normally easily transported by wind. Heavy snowfall of more than 2 cm per hour or 30 cm within the last 48 hours, increases avalanche danger. Consistent light snow (5-10 cm per day) makes avalanches less likely.
- Rain adds moisture and heat to the snowpack and causes melt. Rain crusts form if surface snow refreezes after rainfall. Any amount of rain on snow can make avalanches more likely. If the rain freezes into a thick crust (3 cm or more), it makes avalanches less likely.
- Freezing rain adds less heat to the snowpack than rain and causes minimal snowpack melt. It can rapidly form surface crusts.
- Ice pellets form when snow melts into rain droplets and then they fall into arctic air trapped in valley bottoms causing it to change into frozen rain droplets. They are much larger than snow grains and may resist bonding to surrounding snow grains.
Precipitation of any kind adds load to the snowpack and stresses the weak layers within it. Critical loading of weak layers present in the snowpack can occur during rain or heavy snowfall.
The video illustrates how precipitation particles fall to the ground as snow, sleet (ice pellets), freezing rain, or rain depending on the vertical temperature structure in the atmosphere. If the air temperature remains below zero throughout the particle’s descent, it falls as snow. If the air temperature rises above and remains above zero, rain occurs. When a mid layer of warm air overrides cooler, valley bottom air, ice pellets or freezing rain form, the deciding factor being the depth of the cold air in the valley bottoms. Video by The Comet Program.
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