Consequences of water temperature for surfing

 


Finally, we will look at how the temperature of the water affects our experience while surfing, focusing on the effect it has on our perception of the waves themselves.

We have all probably agreed at some time or other that summer swells just don’t seem to have the same punch as winter ones – winter swells are thicker and gnarlier. While wiping out in warm water is almost a pleasant experience, wiping out in cold water is anything but, as wave after icy-green wave slams down on you like a ton of bricks.

There are several possible reasons for this. Perhaps summer swells come from weaker storms, and are therefore somehow weaker; or perhaps the temperature of the water in the generating area is warmer, which in some way makes the waves have less power. But maybe the water temperature itself simply makes winter swells ‘heavier’ than summer ones because the water is that much denser. Apart from between zero and 4°C, water becomes denser as you decrease its temperature, so a cold-water wave of the same size must weigh slightly more than a warm-water one, which means it must pack a bigger punch, right?

To find out the answer to that question, we need to know how much more energy a cold wave contains than a warm one. The kinetic energy of a wave is proportional to its density times its volume times the square of its velocity. If we compare two hypothetical waves of the same volume and velocity, then the difference in energy just becomes proportional to the difference in density. So all we need to know is the densities of seawater at two different temperatures.

In Atlantic City on the east coast of North America, the water varies from 2°C in winter to 22°C in summer. The density of seawater is typically about 1,028 kg per cubic metre at 2°C, and 1,024 kg per cubic metre at 22°C – a difference of about 0.4 per cent. So a wave in Atlantic City weighs about 0.4 per cent more in winter than in summer. If the wave is travelling at the same speed, it will contain about 0.4 per cent more energy. That is not very much, and it is difficult to imagine noticing the difference when a wave breaks on your head. The volume of water falling on your head when you get caught by the lip of, say, a one-metre wave, is probably about two cubic metres, which weighs about two tonnes. With a couple of tonnes of water coming down on you, an extra 4 kg is not going to make much difference.

But what really makes cold waves ‘heavier’ than warm ones is the effect the cold water has on the human body. Cold water not only feels uncomfortable, but can quickly cause the body to stop functioning. The fact that water conducts heat away 32 times more rapidly than air, means that submersion in cold water can cool the core temperature very quickly. In cold water, even with a good wetsuit, any involuntary time spent under the water will not be a very pleasant experience. A cold-water hold-down will seem to last longer than a warm water one. When you do eventually surface, you feel a lot more drained of energy.

There are also a host of psychological factors that make cold waves seem heavier than warm ones. Warm waves are often an inviting light blue colour, conjuring up images of sunny days and fun on the beach. Cold waves are often an ominous dark green colour, conjuring up images of huge clean-up sets, strong rips and sharks. All this gets the adrenaline pumping before you even enter the water, let alone catch a wave or get a set on the head.

So, yes, waves are definitely ‘heavier’ in winter than summer, at least as perceived by us. It is the effect of the cold water on our bodies that without doubt overshadows the real increase in density and energy of a winter wave.

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