[Petrochemicals (OP)]

It's a bit nippy at present, ice is forming at night on the water. I cannot believe however that ice only forms once the entire 300l of water in my water butt has achieved 0C, I do not believe it would cool that quickly, even though at present it is quite dry accelerating evaporation. But the ice must be 0C maximum, so how does ice form?

[SeanB]

NO, bulk of the water is below the surface, and at 4C, because that is where the density of water is at it's highest. Then the upper layers cool down further, and as they cool down they float on the deeper cold layer, so only the top layer will eventually reach 0C, and being at the top, and lowest density, it will continue to lose heat by radiation, till eventually it has lost enough energy for the ice crystals to start to form, and grow to cover the top.

[alancalverd]

It's interesting to speculate as to whether life could have evolved, or what the climate would be like, if ice was denser than water.

[Petrochemicals (OP)]

NO, bulk of the water is below the surface, and at 4C, because that is where the density of water is at it's highest. Then the upper layers cool down further, and as they cool down they float on the deeper cold layer, so only the top layer will eventually reach 0C, and being at the top, and lowest density, it will continue to lose heat by radiation, till eventually it has lost enough energy for the ice crystals to start to form, and grow to cover the top.

On the base of the container water will be 4C, the rest of the container is someother temperature between 0 and 100. How does ice actually come  to being if the water crystals circulate amongst water of a higher temperature?

[Bored chemist]

On the base of the container water will be 4C, the rest of the container is someother temperature between 0 and 100.

No.
The rest is at some temperature between 0 and 4 C
Try to keep up.

How does ice actually come  to being if the water crystals circulate amongst water of a higher temperature?

Ice floats.
Why on earth do you imagine it circulates?

[alancalverd]

And before ice floats, water at temperatures below 4°C rises, so the top layer is colder than lower down. The anomalous convection of water is fascinating to watch.