December 2001

By John Francis

Even tropical water as warm as a swimming pool is cool compared to your body's core temperature of 98.6F. You have to burn extra energy to replace lost body heat, and by the end of a diving day or the middle of a week of diving, you'll feel run down.

Becoming more chilled at the end of the dive than you were at the beginning also increases your risk of DCS. That's because warm tissues transfer gas more efficiently than cold tissues—they have more blood flow. So you absorb nitrogen efficiently at the beginning of the dive when you're warm and offgas less efficiently at the end when you're cold.

Here's how to end the dive as warm—or nearly as warm—as you began:

• Wear a hood. It should be the first piece of thermal protection you consider, not the last, because up to 40 percent of your heat loss is through your head. Many divers prefer bathing-cap shaped "beanies" because they don't restrict your head movement. Wear a thick one, or wear two—there's no flexibility penalty because your skull doesn't change shape.

• Stop the leaks. The best wetsuit or dry suit is worthless if it lets in too much cold water. A fairly tight fit is best, though you don't want to feel choked or unable to expand your chest fully. Repair broken zippers and split seams. Pay special attention to the neck opening—look for a wetsuit with an adjustable, well-sealed collar that fits snugly.

• Cover up. Heat loss is huge where cold water flows over skin, so a thin full-length suit is warmer than a much thicker shorty.

• Break the wind. Between dives, a wet exposure suit becomes a swamp cooler. Many divers actually lose more heat between dives than they do in the water. Towel off the suit and wear a parka or a windbreaker, or take off the suit and dry off. Also, dry your hair and wear a hat.

• Stay shallow. Wetsuits are more effective in shallow water because they compress in proportion to the depth. Compression flattens the bubbles, which are the suit's only insulation. Also, breathing air under less pressure chills you less.

• Add layers. A vest or step-in jacket, perhaps with a hood attached, is an economical way to add warmth to a full-length wetsuit. But make sure it fits snugly so water doesn't flow between the layers.

• Forget miracle fabrics. Polypro and the like may work fine in air, but under water, all the insulating air is driven out. For wetsuits, the real deal is neoprene. A fleece suit under your wetsuit will probably just make wrinkles that promote water flow.

• Surface if you shiver. Uncontrollable shivering is the warning sign of hypothermia. That means it's time to get out of the water and warm up.