Why we need a Wetsuit for diving, even in warm water…

How clothes keep us warm

An old blind-stitched Gul wetsuit hanging next to a modern fluid-seam welded O’Neill wetsuit.

Wetsuits are not that different from ordinary, warm clothes—and they work in a very similar way. When you step outside on a chill winter’s day, you pile on layers of clothes to keep you warm. You probably know that more thin layers keep you warmer than one thick layer, because several thin layers trap warm air in between them—and it’s this air that helps to keep you warm.

How do the layers work? Heat tends to flow from hotter objects to colder ones nearby; that’s a basic rule of physics called the second law of thermodynamics (“thermo” = heat, “dynamics” = motion, so “thermodynamics” is the science of how heat moves). Let’s say you’re standing outside on a winter’s morning. If your body temperature is 37°C (98.4°F), and the air around you is just 8°C (46°F), heat flows from your body into the air and your body rapidly starts to cool. It’s worth noting that the rate at which your body loses energy is directly related to the difference between your body temperature and the temperature of your surroundings. (That’s called Newton’s law of cooling.) So the colder the water, the faster you lose energy.

Put on lots of layers and trap warm air in between them and the heat has to flow through a series of warm “airlocks”. Air is mostly empty space, so these airlocks are effectively barriers that stop heat escaping. If it’s harder for the heat to escape, it’s a whole lot easier to stay warm.

Stopping heat from escaping this way is called insulation. We insulate the walls and roofs of our homes for the same reason. Insulation means providing a barrier to stop heat escaping. We often think of ourselves trying to “stop the cold getting in.” But there’s not really any such thing as cold. Cold is just a lack of heat. What we really mean is that we’re trying to prevent the heat from getting out.

Why ocean water cools your body so quickly

Now imagine that instead of standing outside in the cold air, you’re swimming in the freezing cold ocean in the middle of winter wearing only a pair of boardshorts! Unless you live in the tropics, the ocean water where you live will get very cold in winter. In a fairly cold, coastal country like the UK, the water temperature dips to about 6–8°C (43–46°F) in February/March, when the sea is at its most bitter. Venture into water that cold without a wetsuit and you risk a life-threatening condition called hypothermia, where the inner “core” of your body gets so cold that it doesn’t warm up again. It is very dangerous to swim in water that cold. Your heart stops beating properly and you can die in a matter of minutes.

There’s an added problem because water is very different from air. Air is a thin gas, while water is a heavy, dense liquid. So, when you swim, there are far more water molecules surrounding your body. The water molecules are also much nearer to one another, so they can conduct heat more efficiently than air. This is why water carries heat energy away from your body around 25–40 times faster than air. It’s also why, on a warm summer’s day, you can get in the ocean and feel freezing even when the water and the air are the same temperature: you feel cold because the water is ferrying heat away from your body like a conveyor belt!

What’s so good about neoprene?

Put on a wetsuit and everything changes. A wetsuit is made from multiple layers and, most importantly, a thick layer of synthetic rubber called neoprene. If you’re interested in chemistry, neoprene is the generic name for an organic (carbon-based) chemical called polychloroprene, which is a polymer (a very large molecule made from endlessly repeating building blocks called monomers), typically built from the monomer 2-chloro-1,3-butadiene. Unless you’re a chemist, that will mean nothing and you won’t care! The really important thing about neoprene is that it’s a kind of foam rubber with a cellular structure that has nitrogen gas bubbles trapped inside it, which make it a particularly good heat insulator.

Layers, layers, layers!

How a wetsuit works with multiple insulating layers between your body and the sea.

Most wetsuits are made from multiple layers—and these help to trap and reflect heat much like any other insulating clothes. Some are lined with a thin layer of metal such as titanium or copper to reflect your body heat back inside. That helps to keep you even warmer than a normal wetsuit. Also, as you step into the ocean, a small amount of water seeps in between the neoprene costume and your skin—and stays there. Your body quickly warms this water up to something approaching normal body temperature. So now, between you and the sea, there’s an insulating layer of rubbery material, some warm water, and multiple layers of insulation—all working together like a kind of personal, all-over body radiator! Not all wetsuits are the same, but these layers are typical of what you might find between your warm body and the cold sea:

1. Your own skin. 2. A thin layer of trapped water warmed by your body. 3. A layer of nylon or some other comfortable fabric to stop the neoprene rubbing and chafing your body. 4. A thin layer of heat-reflecting material based on a metal oxide of titanium, copper, silver, magnesium or aluminum. 5. A thick layer of neoprene containing trapped bubbles of nitrogen. This is the most important part for keeping you warm. 6. A durable outer layer made from some water- and abrasion-resistant material.

Keep that water out!

For a wetsuit to work properly, any water that seeps in has to stay inside and stay warm. If a wetsuit fits badly, or isn’t well sealed, the warm water layer will constantly “flush” in and out and be replaced by cold water from the sea—which, if you think about it, would be almost the same as wearing no wetsuit at all. As wetsuit inventor Hugh Bradner (see below) first realized, a neoprene wetsuit keeps you warm in spite of the fact that it makes you wet, not because of it. Even so, stopping cold water from flushing in and out is vital. That’s why the seams of a wetsuit (where the separate panels of neoprene are joined together) are held together with special waterproof tape. They are also “blind-stitched”: instead of the stitch holes going all the way through, they go only part of the way through the neoprene from the inside. That means there are no stitch holes in the outside of the neoprene to let in cold water. For the same reason, wetsuits have tight-fitting cuffs and legs.