Specific Heat 

In oceanography, specific heat is the number of calories needed to raise the temperature of 1 gram of seawater 1C. The specific heat of seawater decreases slightly as salinity increases. Land heats and cools much more rapidly than seawater, its specific heat being much less than that of seawater. This, in part, accounts for the land having a much greater temperature range than the sea, which results in monsoons and the familiar land and sea breezes of tropical and temperate regions.

The ratio of specific heat to seawater at a constant pressure and constant volume has a direct relationship to the speed of sound in water.

Learning Objective: Recognize the effects temperature, pressure, and salinity have on the thermal expansion of seawater, and identify one of the major roles of thermal expansion in the sea.

Liquids expand and contract when temperature changes take place; some more than others. Seawater has a higher coefficient of expansion than that of freshwater. Within the sea, the coefficient of thermal expansion is effected by salinity, temperature, and pressure. It is greater in high salinity water; greater in warm water than in cold (under similar salinity conditions); and it increases with increasing depth under constant temperature and salinity conditions. Of course, constancy is not a trademark of any of these properties; they are all quite variable. In turn, the thermal expansion that takes place in the sea varies and is difficult to assess.

A major role of thermal expansion is in the formation of ice. PURE WATER is most dense at 4C. Thermal expansion takes place when water warms above 4C, but it also expands when it cools below 4C. When expansion takes place, the volume is increased, which in turn decreases the density. When water cools below 4C, it expands slightly, and as it freezes, it expands much more. If water failed to expand during the freezing process, the density of ice would be such that it would sink to the bottom on forming. In the cold of winter, freshwater lakes would eventually become solid blocks of ice. Come summer, only the upper few feet of ice would melt, leaving the remaining ice beneath the melted water.

Learning Objective: Identify the properties that control sound velocity in the ocean and recognize how each controls the speed and direction of a sound wave.

If you remember from an earlier lesson, velocity takes into account both speed and direction. The speed of sound in seawater is governed by temperature, pressure, and salinity. An increase in temperature increases the speed of sound in water, while a decrease in temperature decreases the speed of sound. The same relation-ship applies to pressure and salinity. An increase in pressure causes an increase in sound speed, as does an increase in salinity, and vice versa. Since pressure is a function of depth in the sea, if we were to discount the effect of temperature and salinity, sound would travel faster at the ocean bottom than it does at the surface. However, we cannot discount either of these other two variables, especially temperature. It is the most important property controlling the speed of sound in water.

As far as direction is concerned, sound waves travel in straight lines only in a medium in which the speed is everywhere constant. For this to occur in seawater, the temperature, pressure, and salinity values would have to be unchanging. Changes in any or all of these variables do occur, which in turn affects the speed of sound waves. The change in speeds along the sound wave causes the wave to change direction. Sound waves are bent (refracted) in the direction of the slower sound velocities. The degree of refraction is proportional to the velocity gradient. If the velocity gradient is such that there is a rapid increase in the speed of sound with depth, a sound wave will be sharply refracted toward the surface, the direction of the slower sound velocities. On the other hand, a rapid decrease in the speed of sound with depth causes a sound wave to be sharply refracted toward the ocean bottom. Sound in the ocean, especially as it relates to anti-submarine warfare, will be covered in greater detail in Unit 2.

Learning Objective: Identify the three layers of the three-layered ocean model and differentiate between mechanical and convective mixing.

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