William,
There is a nice explanation of what your seeing on your refreezing drops
in this weeks Science News. It is caused by the expansion of the ice as it
freezes from the bottom up changing the curvature of the drop. The
discontinuity between the lower frozen drop and the upper liquid drop
probagates upward through the drop it freezes until all that is left is the
discontinuity as a spike on top of the frozen drop.
Gary