Question

In winter water tanks starts freezing from the surface and not from the bottom

Answer 1

yes this is true because in winter the cold air and some small particle of ice came from the clouds to the earth that will be settle on the surface of anything and it become dry as droplets and that droplet it look. like freezing

Answer 2

In cold countries when the tanks are laid open, this phenomenon occurs. the water at the surface starts to cool, the surface water cools faster then the lower water due to being in direct contact with the cold air. when the water cools below 4C it expands, hence the heat in the lower water stays inside the tank and it does not freezes.
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