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Where When How March/April 2017 : Page 99

temperature or below. Using alcohol fur-ther aids hypothermia by dilating your blood vessels, which pumps more heat to your skin, which means the cooler water can steal it more quickly. To a diver, one of water’s most impor-tant attributes is weight. Water is heavy; about 8 pounds per gallon, or 64 pounds per cubic foot. (Salt water is heavier than fresh water.) On the surface, al-though we don’t notice it, we’re always under pressure from the air above us. (Air has weight too, a little over one once per cubic foot.) Aptly called “one atmosphere,” this pressure refers to the 14.7 pounds that bears down on every square inch of Earth. In other words, if you measured the pressure exerted by a one-square-inch column of air that rises from the surface to the edge of space (that’s the thermosphere, about 52 miles high), that pressure would equal 14.7 psi (pounds per square inch). To double that pres-sure, all a diver needs to do is descend 33 feet. Every additional 33 feet adds an-other atmosphere of pressure, so if you are at 66 feet, you have three atmos-pheres of pressure to contend with (one from the thermosphere to the surface, another for the first 33 feet, and the third for the second 33 feet). We don’t notice this one atmosphere of pressure on land because we’re de-signed for it. We do notice it when fly-ing. As the airplane climbs, there is less “atmosphere” above, which means less “weight,” or pressure (the atmospheric pressure is halved at 18,000 feet). As the plane descends, the pressure in-creases. It works the same way under-water. Because the majority of our body is comprised of incompressible water, whether we’re flying or diving, we feel these pressure changes only in our natu-ral air spaces, our ears and sinuses. These air-filled cavities are most com-fortable at ambient pressures, or when the pressure inside equals the pressure outside. Diving as it should be Diving • Snorkelling • Instruction Tel: 649.946.5040 Visit the Turks & Caicos Islands at MARCH/APRIL 2017 • • • • • 99

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