The convective critical effects of water near 4°C in a finite vertical cylinder are examined by means of local temperature measurements at the midplane of the cell. The properties of water in such a state induce a different behaviour from that observed in “Boussinesq” fluids. In particular, at the threshold the convective temperature jumps discontinuously from zero to a finite value. Convection occurs via an inverted bifurcation, the associated hysteresis loop being observed for the first time.