"Thank God we're surrounded by water," went the words of a song that was very popular some centuries ago when I was young. We are indeed an island nation, with water all around us. And yet, no matter how cold may be the weather, this water remains most stubbornly mobile. The Atlantic never freezes over on our shores. Whence, one might wonder, this strange aversion to solidity?
The main difference between sea-water and the contents of the village pond is that the former contains salt. The amount of salt in the world's oceans varies considerably from place to place; the average concentration is about 35 parts per thousand, and it has important implications for the way in which the water behaves at low temperatures.
The presence of the salt has two consequences. Firstly, it lowers the temperature at which freezing takes place by about half a degree Celsius for each 10 parts per thousand of salt in the water. The temperature of the sea, therefore, must fall to about 1.5 Celsius before ice begins to form.
But there is another even more complex effect in operation. You may remember that fresh water has its maximum density, and is therefore heaviest at 4 Celsius. So when the temperature of our village pond falls close to zero, the cold water near the surface is relatively light and stays at the top. And with just a little more cooling by the air above, a layer of ice begins to form.
Salt, however, depresses the temperature at which maximum density occurs. As sea-water is cooled it continues to become denser and heavier until the temperature is well below zero. This means that as the surface layers of the ocean cool towards their freezing point of 1.5 Celsius, the colder and heavier water at the top continually sinks to the bottom, and is replaced by warmer and lighter water from below. Freezing cannot take place until the temperature of the whole body of water, from surface to sea-bed, is uniformly reduced to near 1.5 Celsius. For this reason ice forms much more quickly on shallow seas than on deep ocean waters.
Another factor that inhibits the freezing of the sea is the disturbance of the surface of the water by waves, tides and currents. And in our own special case, the proximity of the Gulf Stream means that our coastal waters are relatively warm to start with; before freezing could take place a great deal more heat would need to be removed from the water than in most seas at these latitudes.