4 Ingredients
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Just like snow storms, avalanches are somewhat predictable. Hi, I'm Dave Thurlow for The Weather Notebook. Forecasters like Knox Williams from Durango, Colorado look for ingredients in a mountainside snowpack that most often indicate that an avalanche may soon happen:

Knox: "We say that there are four ingredients. First is a slope. About 95% of all avalanches fall on slopes of 30 to 45 degrees. The second ingredient is a slab, anywhere from maybe six inches to tens of feet."

Dave: "And covering an area of approximately what?"

Knox: "Oh boy, it can be hundreds or thousands of feet across and in extreme instability sometimes the fracture can go more than a mile."

Dave: "Okay. So we have two ingredients now. We have the slope and the slab."

Knox: "Right, and the third ingredient is a weak layer underneath the slab. There has to be something for that slab to break loose from.

Dave: "You're talking about a layer of snow, I assume."

Knox: "Yes, it's older snow that has no strength. Or a snow layer that loses its strength over time."

Dave: "And the fourth ingredient?"

Knox: "Is a trigger. And the trigger can either be just the accumulated weight as the slab gets heavier because of falling or windblown snow. Or it can be an explosive. Or it can be a person."

It's obviously better for an explosive to trigger an avalanche on purpose than to have a skier or a hiker trigger one by mistake. You can hear an avalanche triggered by a howitzer tomorrow right here on The Weather Notebook. Our senior consulting producer is Jay Allison. Funding for the show is provided by the National Science Foundation and by Subaru.