When the fracture of a weak snow layer causes an upper layer to collapse, making a whumpfing sound. This an obvious sign of instability.
Whumpf has actually been adopted as a technical avalanche term to describe the sound of a collapsing snowpack when you cross the snow. For instance, “we got a lot of whumpfing today,” or “the snowpack whumpfed like rolling thunder just before it released and caught us.” This is the sound of nature screaming in your ear that the snowpack is very unstable. Most snowpacks collapse onto a “persistent” weak layer such as faceted snow, depth hoar or surface hoar, although occasionally whumpfing occurs on very wet snowpack as well.
Also see “Collapse”.