What Is Cold Water Swimming? Part 1 – What Governs Ocean Temperature?

I went for a swim one evening recently, my first time in the sea in about three weeks. The air was warmer, the day nicer, the sun setting later than it had in mid- February. But the water felt bitter. My skin thermometer told me it was about seven degrees (and feeling colder due to physiological factors). Seven degrees is cold, by almost any definition.

Cold water swimming is obvious when it’s the traditional northern European sport of cutting a hole in the ice for a swim. But what about if there’s no ice or polar bears?

For someone that has written a lot about cold water swimming, I’ve never really defined cold water and cold water swimming apart from the humourous open water temperature scale. A more common question variant is to ask what temperature of water is tool cold to swim in. But it’s come to seem like somewhat of an oversight to not directly address the definition of cold water swimming.

To look at this, I’d first like to consider some major factors that govern ocean temperature.

Christmas Day swims are moderately traditional and also increasingly popular in Ireland and the UK (and elsewhere), but for Ireland at least, I always feel that the real test for irregular cold water dippers would much better be a St. Patrick’s Day swim. So it seems appropriate to write this when the water here is at its coldest while on land people are experiencing spring. But since it’s St. Patrick’s Day (alternatively Paddy’s Day but never, ever, ever Patty’s Day) and you are obviously not out swimming or swam earlier, what you need to do is go to kitchen and make yourself a proper Irish Coffee using my recipe and then spend a couple of hours catching up on Loneswimmer.com.

On this year when Ireland celebrate 100 years as an independent state, Beannachtaí na Féile Pádraig oraibh go léir! (The blessings of St. Patrick’s Day on you all). Here’s the annual St. Patrick’s Day message from our much-admired President).

Experienced open water swimmers therefore usually maintain one personal calendar for land, another for the sea.

In the Gaelic (Irish) calendar, Winter, which was also called the Dark Half of the year, begins on the first of November with Lá Samhna, following the previous day of Samhain, (which you all know better as Halloween). The Christian calendar later co-opted the first day of November as the Day of All Hallows. So Halloween was the “eve of Winter”, (later the Eve of All Hallows, the day when the dead were remembered). November is usually the wettest month in Ireland, (though that’s often difficult to ascertain, given the rain in the other … eleven months). December has the shortest day of the year, so it was logical that it was almost halfway through the middle month of winter. January is the coldest month, with the lowest light levels (therefore the only month that the Irish National Art Gallery puts its Turner watercolour collection on display). By Paddy’s Day, we are half way though the the Gaelic season of spring which starts at the beginning of February.

The calendar year confuses, in that for people not acquainted with the ocean, they assume marine temperatures and seasons match land temperatures and seasons. Our experience of land seasons however doesn’t always align to what we can expect or experience in the sea.

Land and ocean heat and cool at different rates. These rates are mostly (not exclusively, but enough for this discussion) to do with size and volume.

Land surface is easily and quickly heated by the Sun. But Land also transfers heat within itself slowly, so heat from the Sun down’t penetrate deeply, even at a very hot latitude (the distance north or south of the equator). Then at night heat is lost quickly by infrared radiation. Even those of who who don’t live near a desert have heard they become cold at night. This is because the heat is radiated away quickly and there is no or little cloud cover to slow that irradiation. Similarly anyone living in a temperate latitude will know that nights without cloud cover are always colder.

Land is not a homogeneous unit. Heat doesn’t transfer across or through it. Visit a moderately deep cave or even a cellar and the temperature remains remarkably uniform throughout the year without any electrical heating or cooling.

Image of the Global Thermohaline Circulation currents

The Global Thermohaline Circulation

The global ocean has significant features that don’t apply to the land.

  • The oceans are really a single unit.  Different parts are connected by a circulatory process called the Global Thermohaline System, sometimes called the “global ocean conveyor belt”, which are massive persistent ocean currents such as the Labrador current, the Gulf Stream, North Atlantic Drift or the Aghulas Current from across geographical areas.
  • Water stores or loses heat easily.
  • The ocean has immense volume. 90% of the ocean’s volume is below the variable temperature surface levels so it has huge “heat capacity” It takes more heat or lower air temperature to change the ocean temperature, (the same as a large pot of water heats or cools slower than a small pot).

The ocean being three-fifths of the earth surface is exposed over large areas to direct heating by solar radiation. There is so much water versus the surface area of the ocean, that even though the surface is good at transferring heat away from the surface during night or winter, there is more water underneath “holding”more heat. A single hot or cold day or even few days is not enough to greatly affect ocean temperatures. Nor does the ocean temperature drop overnight. Even when such cold day is noticeable, it’s a very local effect. For example a prolonged warm calm spell may lead to rise in local surface water but one windy day even in warm weather will mix water from lower temperatures and the various strata will once again settle to a stable temperature.

In addition the combination of volume with the thermohaline circulation system moves extraordinary large volumes of water from warmer to cooler regions regions. The exposure to different rates of prolonged solar heating drives the conveyor, as according to the Laws of Thermodynamics heat always dissipates by flowing to colder areas. As most Irish children learn, it’s the Gulf Stream carrying warm water from the Caribbean that keeps Ireland from freezing in the winter or having winters as cold as those of the same latitudes on the North American continent. A facet of the global conveyor belt is that as water cools, it becomes more dense and heavy. Dense cold water that sinks and less dense warmer water that rises is the part of the motive factor for the Global Conveyor Belt. Warm water flowing up into the north Atlantic from the Caribbean gradually cools due to less sunshine, and so becomes heavier and drops lower into the deeps.  This cycling of deep cold layers with warm upper layers  (which is also the base of the oceanic food chain, as deep water that moves to the surface caries the most nutrients) is seen in other places. The Atlantic coast of South Africa even when the land temperature is hot, can have cold water at the coast as deep Southern Ocean currents rise to the surface.

Lakes, regardless how large, as not as complex or have as much volume, but still undergo the same processes. When comparing rivers or lakes with the ocean, swimmers from those locations will know that lakes warm quickly in early summer and drop quickly in autumn. The Atlantic around the south coast of Ireland rarely gets below six degrees in the winter whereas smaller or higher altitude lakes in Ireland can easily freeze. Rivers can have warmer water entering and movement also slows freezing but frozen river edges are a common sight here in winter. Ireland isn’t special in this regard, but the moderate climate (in comparison with hotter and colder climes, and latitudes which are both) makes observation of the marginal conditions more obvious.

These factors, along with latitude, help define the local ocean season. When you apply heat to water, or put a hot drink in a fridge, they neither heat nor cool instantly. So it is with the ocean. The land responds more quickly to the weather and season whereas the ocean responds slowly. In Ireland the months with most sunlight in are May, June and July. But the warmest months are June, July and August.

Seasonal lag, as it’s called, differs depending on location and altitude. San Francisco’s summer seasonal lag is longer for example with September being the warmest month but its winter seasonal lag is much shorter with its coldest time close to the winter solstice. (Similarly, the warmest part of the day is after noon, which is when the surface actually receives the most solar radiation).

I’m not intending writing a meteorological discourse on climate here. There are complex factors that determine climate and weather, (where climate is the overall pattern and weather is the day-to-day stuff). Other significant factors include:

  • Latitude and longitude (for factors such as solar insolation, i.e. radiation received from from the Sun, location with respect to a Hadley cell boundary or prevailing jet stream patterns).

    hadley cell on a uniform body of water

    Hadley Cell on a uniform body of water

  • Depth of the contiguous ocean floor and significant local currents.
  • Location with respect to a continental mass.
  • Location with respect a major circulatory ocean current, or trade current or trade wind.

All these details don’t make your ability in cold water any different. What they do affect is  whether you are more likely to be a cold water swimmer because of your location and local conditions.

  • Cold water doesn’t always occur at the same time as the coldest temperature on land.
  • Cold water can occur in warm places, due to bathymetry (the shape and depth of the ocean floor) and large-scale oceanic currents.

If you live in Sydney or Capetown or the Phillipines, your perception of cold is most likely very different to my perception of cold, and my perception is very different to someone in Canada or Russia.

What the above explanation demonstrates is that cold water isn’t exclusively the privilege or curse of the very northern or southern latitudes.

If you want to swim in open water without a wetsuit, and the local water is cold, then you become a cold water swimmer.


In part two, we’ll drop from the global to the personal, and consider just what we can consider cold water swimming.

Related Articles

St Patrick’s Day swimming nutrition. How To make a great Irish Coffee!

Weather and Climate, Part 1

Weather and Climate, Part 2

Weather and Climate, Part 3

How To: “What does 965hPA mean?”


5 thoughts on “What Is Cold Water Swimming? Part 1 – What Governs Ocean Temperature?

  1. Pingback: What Is Cold Water Swimming? Part 2 – The Open Water Swimmer’s Rule of Thumb – When Is The Water Too Cold? | LoneSwimmer

  2. Hi Donal,
    I’m in Enniscrone this Easter weekend and was hoping to have a dip and a swim. I’ve never swam outside at this time of year before but I love sea swimming in the summer. I invested in a 5mm wetsuit (Boo says you) so that I can get out more often, I see that temps are aprox 7 or 8c at the moment. I found your blog and am inspired to get in, thanks but maybe it’s safer I keep the wetsuit on for a few months!


  3. As you said, cold water defined by geographic location. In my country water under 25° consider cold by about 95% of the population although about 15% of the population are immigrate from Russia. (My wife was born in Cyberia but she would not dare entering the water when the sea is under 25°)
    The minimum water temperature at the east Mediterranean Sea is 17°. The temperature going up or down less the 1° a week. (From September to April)
    Very few enter the sea in the winter, mostly for less than ten minutes. Less the 100 swimmer swim in the sea all the year. (Between 1 KM and 5). Less than 10 young swimmer under 25 swim in the winter in the open water. (They are called crazy.)


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