  Sea ice breaks up and regenerates rapidly during winter conditions in the
Arctic. Analyzing satellite data from the Kara Sea, we find that the average
ice floe size depends on weather conditions. Nevertheless, the frequency of
floes of size $A$ is a power law, $N\sim A^{-\tau}$, where $\tau=1.6\pm 0.2$,
for $A$ less than approximately 100 $km^2$. This scale-invariant behaviour
suggests a competition between fracture due to strains in the ice field and
refreezing of the fractures. A cellular model for this process gives results
consistent with observations.
