Staying warm while freediving is an art, especially in colder waters. During a normal 2-hour dive session you can easily lose more than 700 kcal just on heat loss, and if you lose heat too fast, your dive session will be cut short. Here I’ll share some surprising information about the performance of your wetsuit, and what you can do to stay warm in the water.
High heat loss despite a wetsuit
Conductive heat loss is something we can calculate. I am not going to try to bore you too much with the details, so here is the condensed story.
How much heat you lose through conductive heat transfer depends on four things. 1) The temperature difference of your skin and the water around you, 2) the thermal conductivity of your wetsuit, 3) the thickness of your wetsuit, and 4) the surface area of your body.
Wetsuits contain up to 94% nitrogen bubbles. The nitrogen is what prevents heat loss. The neoprene itself is actually quite conductive. Nitrogen compresses just like air, so at depth the suit becomes thinner and the overall heat conductivity increases.
Conductive heat transfer does not include water sloshing through your wetsuit. It is simply the amount of heat you lose if you were to be still, in still water.
Here are some interesting results based on conductive heat transfer alone:
- A diver in a 3 mm suit and 25ºC (77ºF) water loses more heat than a diver in an 8 mm suit in 8ºC (46ºF) water
- Heat loss at 20m is 4 – 5 times as high as at the surface
- Heat loss at 40m is 7 – 8 times as high as at the surface
- You will commonly lose at least 500 kcal of energy during a 2 hour dive session
- At 40m depth, you can lose as much heat as a medium sized space heater generates (3000W), depending on the thickness of your wetsuit
Planning your dives so you stay warm
When I initially calculated the difference in heat loss between cold and warm water divers, I thought I made a mistake. How on earth can a diver in 25ºC water with a 3 mm lose as much heat as a diver in 8ºC water with an 8 mm suit? I know from experience that I can spend all day in warm water, and only a couple of hours in cold water.
The answer is that conductive heat transfer is not the full story. If the sun is directly above you (midday on the equator), it will supply the earth surface with >1000 W/m2 of heat. Those 25ºC waters we talked about earlier get much more direct sunlight. In a 3mm suit you only lose ~300 W at the surface, so the sun is instrumental in keeping you warm.
The sunlight hits regions farther from the equator such as Canada and New Zealand at an angle (see also: solar irradiance). Because of that, the sun supplies much less heat to the freedivers that call these waters home. In these regions, if you want to stay warm you need to plan your dives when the sun is high. That is not difficult in the summer, but in spring and fall you should try to dive in the afternoon, preferably on days without cloud cover. In the winter… may Poseidon be with you.
Wind and evaporation
Part of your body is above the surface during your breathe up. If the apparent air temperature (apparent air temperature takes into account wind chill) is lower than the water temperature, this will cause you to lose heat faster. The increase in heat loss can be as much as 10%, if you dive during a 0ºC day.
More important is the effect of evaporation. When you are above the surface, the water on your suit will evaporate. During evaporation, liquid water extracts heat from its surroundings. Unfortunately its surroundings are you, or rather, your suit. Evaporating water costs a lot of energy, and how cold you become during a dive might be directly proportional to the rate of evaporation. Wind will increase the rate of evaporation, and so does heat. If you have a smooth skin suit, there will be less water to evaporate than if you have a lined suit.
Diving style and getting in and out of your wetsuit
Diving style is another big part of staying warm. Our muscles are very inefficient, and about 80% of the energy we use is lost as heat. A more active diving style will keep you warm for longer. You will of course sacrifice some performance.
Lastly, what you do before you get in the water will also affect how warm you will be. Make sure to store your wetsuit in a warm(ish) place before a dive session, and try to stay warm while you change. I know one diver that changes in a bathing robe, and transports his wetsuit in a cooler with jugs of hot water. The jugs of hot water do excellent service as a post-dive shower.
- Try to dive when the sun is high if you are far from the equator
- Try to dive in low winds
- Check the weather forecast and wear a (thicker) suit if there is cloud cover
- Adjust your diving style to suit the water temperature. Long line diving sessions, deep targets and hangs are for the summertime
- Stay horizontal at the surface, to allow your entire body to be heated by the sun (if there is some)
- Try to stay as warm as you can before you get into the water
- Diving with a smooth skin suit is warmer than diving with a lined suit of the same thickness