Freshwater reaches its maximum density at approximately 3.98 degrees Celsius — not at its freezing point. This anomaly, a consequence of hydrogen bonding geometry, means that cooling a lake from above does not simply produce a cold surface layer sitting on warmer water. Instead, the cooled surface water is denser than the water below and sinks, driving convection that mixes the entire water column. The lake overturns. But only until the surface cools past the maximum-density temperature — at which point, further cooling makes the surface water lighter, not heavier. Convection dies. The surface stabilizes. Ice forms.
Olsthoorn's computational simulations identify three distinct regimes in this process. Free convection dominates while the surface remains above maximum density — cold parcels sink, warm parcels rise, and the lake mixes vigorously. Penetrative convection begins as the surface crosses the density maximum — the downward-moving parcels encounter water that is still warming toward maximum density, and the mixing depth oscillates. Decaying convection follows — the driving temperature difference vanishes, turbulence fades, and the water column locks into a stable stratification with the coldest water on top.
The transitions between regimes are predictable from the surface heat flux and the initial temperature profile. The lake does not freeze suddenly. It passes through a sequence of dynamical states whose ordering is determined by a single material property — the nonmonotonic relationship between temperature and density.
The through-claim reaches into any system where a driving force reverses sign as a variable crosses a critical threshold. When the direction of buoyancy flips, the system does not merely pause — it reorganizes. The convective engine that mixed the lake is the same physics that later prevents mixing. The transition from stirring to stillness is not an external imposition but a consequence of the medium remembering its own equation of state.