The Strange Behaviour of Water Temperature

Tony Butt

by on

Updated 8d ago

Have you ever wondered why the temperature of the sea seems to have a mind of its own?

Usually, the temperature of the place where you live depends on how much incoming solar radiation it receives, which varies according to the latitude and the season. The land responds almost directly to these variations in solar radiation, but the sea doesn’t. In fact, sometimes the water temperature appears to act of its own accord, bearing little or no relation to the latitude or the season.

There are several reasons for this. Coastal upwelling (see HERE); ocean currents, which I’ll cover in a future article, and specific heat capacity, which is what we are going to talk about here.

We've discussed at length about the differences between warm and cold water waves. But here's a little reminder of those frigid caps.

The specific heat capacity of a substance is the amount of heat energy required to raise a specific mass of that substance a certain temperature. For example, the heat energy in Joules needed to raise one kilogram one degree Celsius. Basically, it is the ability of a substance to store heat energy inside it.

Different things have different specific heat capacities. Water, for example, has a specific heat capacity of about 4,000 Joules per kg per ºC. That means, to raise the temperature of a kg of seawater by 1ºC you need to pump about 4,000 Joules of heat energy into it. Typical vegetated land, on the other hand, has a specific heat capacity of around 800 Joules per kg per ºC. In other words, you only have to put about a fifth of the energy into it to raise its temperature by the same amount.

As a result, sea and land respond differently to the Sun’s input. In theory, the same amount of solar radiation hitting adjacent areas of land and sea would make the temperature of the land increase five times as much as the sea. Put another way, with the same solar input, the sea would take five times as long to reach the same temperature, and five times as long to cool back down again.

The peak in solar radiation occurs at the summer solstice, and the peak in land temperature about a month later, but the peak in sea temperature occurs almost two months later

The relatively slow heating up and cooling down of the sea from the input and output of solar radiation is a classic case of hysteresis. Hysteresis is the delayed reaction to the input and output of energy to and from a system. It can be seen everywhere, from the changing shape of sediments on the shoreline to the power steering mechanism of your car.

Due to hysteresis, the seasonal variations in sea temperature tend to be out of phase with the seasons themselves. See the graph below of monthly averages of air and sea temperatures for Perranporth, Cornwall. The peak in solar radiation occurs at the summer solstice, and the peak in land temperature about a month later, but the peak in sea temperature occurs almost two months later.

You can also see that in early March, while the sea is still cooling down, the land has already started to warm up; and in early August, while the sea is still warming up, the land has already started to cool down.

In addition to this phase shift, the fact that the sea has a greater specific heat capacity means that, while the land has relatively large temperature swings from winter to summer, the sea temperature remains more constant throughout the year.

An important consequence of all this is the behaviour of the water temperature in areas that are primarily influenced by land, compared with those that are primarily influenced by the sea.

The most obvious example is if we compare ocean islands with inland water bodies. Ocean islands are small pieces of land surrounded by sea, and inland water bodies are small areas of water surrounded by land. One is an inside-out version of the other, if you like. The coastal water temperature in, say, Hawaii is controlled by the surrounding Pacific Ocean. In contrast, the coastal water temperature of, say, the Great Lakes is controlled by the surrounding land mass of North America.

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. The graph below shows how, in Honolulu, the coastal water temperature hardly changes throughout the entire year, whereas, in Chicago, on the shores of Lake Michigan, it varies enormously.

And just in case you were wondering, yes, there is surf in Chicago. See HERE.

Cover shot by Bryan Zinski.