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I watched my father put a bucket of water on a freshly laid patio overnight. In the morning, the water in the bucket was frozen but the grout under the patio was not. Is there a correlation and if so, how does it work?

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    $\begingroup$ There is significant geothermal heat coming constantly coming up through the Earth's surface, which is probably why the simple act of throwing a blanket over a small tree can help in saving it from cold temperatures and frost. Putting a bucket of water on the grout probably helps in (1) providing some thermal insulation from the surrounding freezing air just like a blanket does and (2) helping to decrease the rate of temperature decrease due to all the thermal mass of the water as well as temporarily halting further temperature decrease once the water temperature hits freezing. $\endgroup$ – Samuel Weir Feb 3 '17 at 23:10
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This is a technique used in gardening in spring with delicate items such as tomato plants when the tender plant is surrounded by containers of water. The general principle takes advantage of the latent heat of fusion of the water. A calorie is the amount of thermal energy required to heat 1 gram of water by 1 degree (C or K), or conversely to cool it 1 degree 1 calorie needs to be removed from it. It takes about 80 calories to melt ice without changing its temperature. Or freeze it. So, while that higher amount of energy is spent freezing the water it cannot be spent cooling or freezing other things. With plants, the water in them is less pure than that in the containers, so they will not freeze as soon as the water in the bucket and this buffer may provide them with several degrees of frost protection overnight. The theory would apply similarly to the patio tiles, their freezing point for the water in solution should be a bit lower than the bucket of water, so the water should freeze first. In general, it should provide some protection until the bucket is fully frozen which may be long enough to get through the night.

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