Question

Answering which question would best allow you to dig deeper into the content at the heart of the passage?

Why does a fresh water body, when cooled, start freezing from the surface downward?

Why does the behavior of fresh water cooling by convection change at 4°C?

Can the temperature of the air above a fresh water body be subzero at the end of summer?

Can ice form on the surface of fresh water if the lower layers are not frozen?

Answer 1

How Does Arctic Sea Ice
Form and Decay?
by Peter Wadhams (excerpt)

Consider a fresh water body being cooled from above, for instance a lake at the end of summer experiencing subzero air temperatures. As the water cools the density increases so the surface water sinks, to be replaced by warmer water from below, which is in its turn cooled. This creates a pattern of convection through which the whole water body gradually cools. When the temperature reaches 4°C, the lake reaches its maximum density. Further cooling results in the colder water becoming less dense and staying at the surface. This thin cold layer can then be rapidly cooled down to the freezing point [0°C], and ice can form on the surface even though the temperature of the underlying water may still be close to 4°C. Thus a lake can experience ice formation while considerable heat still remains in the deeper parts.

Answer 2

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