Why The Temperature of the Water is Very Important



The temperature of the water is a very important factor for surfing. In some parts of the world you must have adequate protection from the cold water, or you will miss the best surf of the year. The temperature may even keep some people out of the water altogether – sometimes even the best waves are not worth the pain of ice-cream headaches, numb hands and feet, and the hassle of having to enter the water Michelin-Man like, with boots, gloves, hood and several millimetres of neoprene.

In this chapter we will look at what controls the temperature of our surfing waters. In some places, water temperature changes radically from season to season; in others it hardly varies throughout the year. Changes in water temperature are not completely controlled by latitude or season. So we will examine some of the factors that completely override any temperature differences due to latitude and season, which go some way towards explaining why water temperature seems to have a mind of its own.

Radical and not-so-radical temperature variations

Around the world there are places where, at first glance, you would think the water would be so cold that it would be madness even to contemplate surfing. But in some of these places the water is actually quite warm. There are other places where the air temperature gets blazing hot during summer, but the water remains freezing cold. Sometimes, water temperature doesn’t seem to relate in any way to latitude, or to whether it is summer or winter. In some places, water temperature remains remarkably constant throughout the year; in others, it goes from one extreme to the other.

If you go towards the poles, logically you would expect the water to get colder in winter because the solar radiation hitting the surface of the Earth is much less than it is in summer. But sometimes this does not happen. In Europe, some of the most popular surfing areas have quite unexpected water temperatures. For example, in France in early September you can happily surf without a wetsuit, whereas in Portugal, which is nearer the Equator, the water is much colder. In Portugal, you sometimes need a steamer and boots in early September.

There are physical mechanisms apart from the simple local heating of the water by the Sun’s radiation, which have a major say in controlling the temperature of the water. Among these are (a) the relatively slow heating up and cooling down of seawater compared with land, (b) coastal upwelling, which regulates coastal waters by constantly replacing them with cold water from underneath and (c) ocean currents, which also regulate coastal waters by constantly replacing them with water from some other part of the ocean. The entire ocean-atmosphere system of the planet is all connected together, so these factors are interlinked. I will explain a little about them below, one by one.

Land-sea influence and specific heat capacity

If you compare mid-ocean islands with inland water bodies, you will find a great difference in temperature variability throughout the year. On most ocean islands, water temperature changes very little from summer to winter, even on islands such as the Azores that are located at latitudes where land temperatures can vary greatly. But on inland water bodies such as the Great Lakes (in north-eastern North America), water temperature changes dramatically from one season to the next. Here it can be over 20°C in summer but pitch to zero degrees in winter.

Ocean islands are small pieces of land surrounded by a vast sea, and inland water bodies are small areas of water surrounded by vast areas of land. One is an inside-out version of the other, if you like. The temperature of the coastal waters of small islands like the Azores is controlled by the surrounding ocean, without any influence from the relatively tiny amount of land that makes up the island. In contrast, the temperature of the coastal waters of inland water bodies such as the Great Lakes is controlled by the huge land mass surrounding the relatively small amount of water. To understand why they behave so differently, we need to go back to the concept of specific heat capacity (SHC), as mentioned in Chapter 2.

 

The specific heat capacity (SHC) of a substance is the amount of energy in joules it takes to raise the temperature of one kilogram of that substance by one degree Celsius.

The SHC of a substance is the ability of that substance to store heat energy inside it. Water is much better at storing heat energy than land – in fact the SHC of seawater is about five times as much as the SHC of typical substances that make up coastal land. So, to raise the temperature of an equal mass of land and sea by equal amounts requires about five times as much solar radiation to be pumped into the sea as into the land.

As a result, the seasonal variations in solar radiation have a much greater effect on land temperatures than on sea temperatures. Seasonal temperature swings of the oceans as a whole are much smaller than those of the continents. Therefore, on small oceanic islands, the seasonal temperature variations of the coastal waters follow the relatively small temperature swings of the surrounding ocean; but on inland water bodies, the seasonal temperature variations follow the much larger swings of the surrounding land.

Hysteresis

The particularly high SHC of seawater also means that it takes a long time to heat up and cool down. To raise the temperature of the sea by one degree requires a lot of solar radiation being pumped into it, which takes a long time. Likewise, for the temperature to fall by one degree, the sea has to lose all that energy, which also takes a long time. The delayed reaction to the input and output of energy to and from a system is known as hysteresis. It can be seen everywhere in science and engineering, from the changing shape of sediments on the shoreline to the power-steering mechanism of your car. The delayed heating up and cooling down of the sea due to the input and output of solar radiation is a classic case of hysteresis.

This delay also means that seasonal variations in sea temperature tend to be out of phase with the seasons themselves. Maximum and minimum sea temperatures do not occur mid-summer and mid-winter – they occur up to one or two months later. In early spring, for example, just when the weather is starting to warm up, the sea might still be cooling down. Then, in early autumn, just when the air is starting to become a little chilly, the sea might still be warming up

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