NAUI Scuba Diver 182 The two layers of water can vary in temperature as much as 10°C (18°F). They remain distinctly separated and do not mix. If your diving suit is appropriate for only the surface water temperature, you might find that this drastic temperature change at depth would force an early end to your dive. Thermoclines are commonly found in many lakes, but they also occur in the ocean. Freshwater lakes also experience changes in water temperature throughout the year. During the fall, as the surface water cools, the wind mixes it with the deeper water and eliminates the thermocline. During the winter, the surface temperature of a freshwater lake can drop to near freezing and a reverse thermocline develops, with the colder water as the top layer, and warmer water as the lower layer. During the spring, as the surface water warms, the wind mixes it with the deeper water and eliminates the reverse thermocline. You must obtain information about the water temperature at the depths to which you plan to dive before entering the water. This information is usually available from local instructors, divemasters, or dive stores. You must select suitable thermal protection for both the water temperature and your individual physiological needs. Verify What You Have Learned Review the following questions about water conditions: 7. A thermocline is a ____________________________ ____________________________. 8. __________, _____________, ___________, and ______________ can affect visibility. 9. Diving in water with no visibility can cause ____________________________. MOVING WATER AND ITS EFFECTS Water is heavy. If you think about the weight of a sealed can of water and how it would feel to have one dropped on your foot, you can imagine the enormous potential energy contained in a large amount of moving water. It is useless to fight against strong water movement, such as a strong current, because you cannot effectively swim against it. It is possible to use the movement of the water to assist you in diving, and this is something you will learn during your NAUI Scuba Diver certification course. You must understand what sets water into motion, how the water moves, and how to function effectively in moving water. This understanding will help you plan ahead to cope with the movement of water and use it to your advantage. Waves and Surf As wind blows across the water, it transfers its energy to the water. The water starts to push into peaks and valleys to become waves. In the open ocean, these waves are called swells. With increasing wind, or as the wind blows for a longer time, larger swells form. Unless the swells are quite large and the wind blows hard, waves usually do not break in the open ocean. The energy of the waves – but not the water itself – can travel across thousands of miles of ocean. As waves enter shallow water and are affected by the bottom, they break and form surf. The larger the waves, the larger the surf. The surf zone is the area where waves are breaking as the water gets shallower closer to shore (figure 8-13). In coastal areas where beach diving is popular, divers must go through the surf zone to enter the water. Chapter 3 provides an introduction to surf entries. Your instructor will cover these entries in detail if they are common in your diving area. Surge is the underwater back-and-forth movement of water you will experience when diving in areas close to a shore with wave action. When you are in shallow water close to shore and there are large waves, you will feel the effects of surge. As the waves move over you toward the shore, the surge will tend to push your body first away from and then toward the shore. Diving Environment
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