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Winter divers "Chill NOT!"

The transition from fall reminds us that the weather – and the water – is beginning to get cooler. So for the next few months, I’m going to discuss the "ins and outs" of thermal protection from just your swimsuit at one end of the spectrum to drysuits at the other.

You’re Getting Warmer (maybe) …

We can stand around in 70F (21C) degree air temperatures all day long and be quite comfortable. However, a half hour submerged in the same temperature water will leave most people shivering with the onset of hypothermia in which the body can not generate enough heat for the amount of heat being lost. Amazing fact: most people will eventually become hypothermic in water less than 94F (34C).

Why? Two reasons. First, water is much denser than air; the molecules are closer together. So water can transmit heat much faster - up to 20 times faster- than air. The second reason we lose heat in the water is that everything in nature strives for balance. Since our body temperature is 98.7F (37C), we are usually swimming in water of a cooler temperature. The water and our body temperatures try to become equal. In other words, you’re trying to heat the whole ocean with your body. That’s a battle you’re going to lose!

The heat that our body is able to generate comes from two main sources: metabolism and movement. Our metabolism creates a little internal furnace by transforming the food we eat and digest into chemical energy for our cells and heat to keep those cells operating at the optimum temperature. Sitting on the sofa doing absolutely nothing will generate a certain amount of heat, known as our basal rate, our baseline, if you will. When our body is moving, it burns additional calories to keep our cells fed and generates additional heat. The more you move, the more heat you generate (up to a point.)

In the water, we can only move so much or we become fatigued and burn through our air faster. And we can’t generally eat and drink underwater so we can’t add fuel to our fire. The only thing left for us to do is to preserve the heat that we do generate. This is where wet suits and other types of "thermal protection" can help.

Choose wisely (and warmly)

Choices for thermal protection range from nothing to a 6 or 7 mm wet suit to a drysuit. It all depends on on how much  body heat the diver needs to preserve. The colder the water, the more protection needed. Of course, your personal susceptibility to feeling cold and the temperatures you’re used to also comes into play. Here in the dead of winter in the Florida Keys, we may see water temperatures as low as the mid-70’s F(25C), which may have us wearing a 5 or 6 mm wetsuit with hood! At the same time, visitors from the northern United States or Europe are happily diving in 3mm wetsuit or even a shorty. It’s all in what you’re used to. And remember, you can always make yourself cooler in a wetsuit by letting in water, but it’s difficult to make yourself warmer, so err on the side of warmth when making your selection of thermal protection.

Need some guidelines? Typical temperatures and the corresponding protection required look something like this:
           

82F(28C) and above	no protection needed, lycra, swimsuit, etc
77F(25C) – 82F(28C)	shorty wetsuit or wetsuit jacket
68F(20C) – 85F(29C)	2-3 mm full wetsuit
45F(7C) – 70F(21C)	5-7 mm full wetsuit, hood, gloves, booties
30F(-1C) – 70F(21C)	Drysuit with appropriate thermal undergarments, hood, gloves, booties

As you can see, there are no definite cutoff points for switching from one level of thermal protection to another. These are general guidelines and allow for individual preferences.

Dive smart. Dive warm.

Hypothermia is serious business and can sneak up on divers, even in "warm" water. Remember your thermal protection.