Although soft slabs are defined to be slabs less than 30 percent
water weight (300 kg/m3), there is a rather fuzzy boundary between
hard slabs and soft slabs. We generally think of soft slabs
as composed of new snow while hard slabs are usually composed
of old, denser layers of snow or very wind-hardened new snow.
Also, soft slab avalanche debris tends to be soft and powdery
while hard slab debris often contains chunks hard snow.
The most important distinction for people in avalanche terrain
is that soft slabs tend to break at or near your feet while
hard slabs tend to break above you, making them much more difficult
to escape being caught.