A field experiment was developed to measure the critical energy release rate for fracture propagation in a weak snowpack layer. A snow block was isolated on a slope and tested in-situ by cutting along the weak layer. Critical cut lengths of about 25 cm were required to start fracture propagation along the weak layer. The critical energy release rate was determined numerically from the critical cut length with a finite element simulation. The mean critical energy release rate for the tested weak layers was about 70 mJ m-2. Numerical simulations showed that slope normal bending of the slab, in addition to the slope parallel shear deformation, contributed considerably to the energy release rate in our experimental setup.