When the fracture of a lower snow layer causes an upper layer to fall or collapse, making a whumpfing sound. This an obvious sign of instability. See Collapse.
Whumph 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 whumphing today,” or “the snowpack whumphed 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 whumphing occurs on very wet snowpack as well.