> but then won't the heat radiate and conduct outwards in a "sphere" from the heating element including up and down, and not make any layers? Suspicion of mine

> but then won't the heat radiate and conduct outwards in a "sphere" from the heating element including up and down, and not make any layers? Suspicion of mine is part of the answer is that the viscosity of water drops with temperature. Rising hot water in a warm layer hits the cold layer above it and spreads out then turns down. Also water is a middle of the road non metallic solid when it comes to heat conduction. But it has high heat capacity and is a liquid. Complicating things stratification at least in a well designed water heater happens rarely. So it's easy enough to set up conditions where it doesn't happen. But of course it's a known problem. And not a totally solved one either. Suspect that simplified models probably don't display the behavior.