ICE FORMATION IN FRESH WATER
For water to freeze and change into ice, it must be cooled to its freezing
point, which implies heat loss. That heat loss occurs when the ambient
temperature of the air is lower than the temperature of the water.
The freezing point of fresh water is 0°C; however, its maximum density is
reached at 4°C. From an ecological perspective, the latter point is extremely
important, since the deeper water which is located under the ice – except in
some small lakes or ponds - does not freeze, which means that the creatures
living there can survive winter under the ice. Let us take the example of a lake
whose temperature decreases to 4°C in the fall. Since density (the weight of
the water per unit of volume) increases with the decrease in temperature, the
entire lake – from the surface to its bottom – will reach 4°C after a
certain length of time. When it cools, the thin layer close to the surface of
the lake becomes denser, therefore heavier, and sinks toward the bottom of the
lake, and is replaced on top by “warmer” water which rises to the surface.
If the temperature continues to drop and dips below 4°C, the layer close to the
surface becomes less dense when approaching the freezing point; it then
increases in volume instead of shrinking, and becomes “lighter” than the
water below it at 4°C. Therefore, that cooler layer will float on the surface
and will continue to cool until ice is formed. Ice (a solid) is lighter than
water (a liquid) due to its larger volume, and that is why it floats. The water
located under the ice below remains at 4°C, except for the layer just below the
ice’s surface. That layer will approach the freezing point of 0°C. As it
reaches that temperature, it turns into ice as well, making the layer of ice on
the surface even thicker. The colder it gets, the thicker and the more solid the
layer of ice becomes.
If water behaved like other physical bodies, it would increase in density as it
cools. If that were true, rivers, streams and lakes would be frozen from the
surface right down to the bottom. In the summer, only the top layer would melt,
while the deeper water could remain frozen throughout the year. In such
conditions, it is obvious that aquatic life in our rivers and lakes would be
impossible.
Salt water – the exception to the rule
Since the freezing point is lower in salt water than in fresh water and that the
density of salt water continues to rise with the decrease in its temperature,
ice is formed less easily in salt water than in fresh water. Due to its
properties, salt water must be cooled to a greater extent before ice can begin
to form; contrary to fresh water, an entire column of water must be cooled
before ice can be formed on the top.
So, what happens with salt water – that is, water with 25 parts per thousand (ppt)
or grams per liter – is entirely different. Its maximum density is reached at
a temperature below the freezing point. This means that during the cooling
process, the entire column of water (at least the water with the same degree of
salinity, because the deeper layers - those beyond 50 m. - are often more saline
and therefore denser than the upper layers) must be cooled sufficiently before
ice can begin to form on the surface.
In the Gulf of St. Lawrence, salinity can reach 35 ppt; the freezing point in
that case is close to – 2°C and the temperature of the maximum density is
even lower. However, in the tidal estuary where salinity is 20 ppt (therefore
less), water freezes at –1.08°C, whereas the temperature of its maximum
density is –0.5°C. The temperatures of the maximum density and the freezing
point intersect at a salinity of 24.7 ppt. So we can see that this
characteristic of water will influence the time when ice is formed in the
various reaches of the river, and dictate what thickness it will attain.
Note: I would love to give credit to whomever wrote this article, but the
author is unknown to me.