Читать книгу Life in the Open Ocean - Joseph J. Torres - Страница 14

The physical properties of water do much to shape the Earth’s climate, while the density of water literally influences the shape of marine species. Water exhibits many unusual characteristics. It is the only compound that can be found in all three phases (solid, liquid, and gas) at the temperatures encountered at the Earth’s surface. Water has a high specific heat, which means that it must absorb a great deal of heat energy for its temperature to rise by 1 °C (1.0 cal g⁻¹ at 10 °C). Because it can absorb a lot of heat, it can also deliver a lot of heat to regions far removed from where the heat was absorbed, and ocean currents such as the Gulf Stream do just that. The climate of Great Britain, for example, would be far colder than it is were it not for the warm maritime influence of the Gulf Stream.

Figure 1.1 The Earth's surface area. (a) Earth’s surface area as a function of elevation and depth. (b) The Hypsographic Curve; cumulative percentage of earth's surface area as a function of elevation and depth.

Source: Gage and Tyler (1991), figure 2.4 (p. 12). Reproduced with the permission of Cambridge University Press.

When water changes its physical state, a great deal of energy is gained or lost. When ice is formed, 80 cal g⁻¹ is liberated as the latent heat of fusion. This property is particularly important at the poles during the freezing and melting of sea ice. Similarly, 540 cal g⁻¹ must be applied to cause liquid water to evaporate (latent heat of evaporation) and an equal amount of energy is released upon condensation. Water’s freezing (0 °C) and boiling (100 °C) points are far higher than those of closely related compounds: H₂S, e.g. boils at about −59 °C.

Water has a high thermal conductivity (0.587 W m⁻¹°K⁻¹ at 10 °C), so heat diffuses readily through it. When well mixed, like the wind‐mixed surface layer of the ocean, large volumes of water can be quite homogeneous in temperature. Water has a high surface tension, which makes it fairly “sticky.” Small volumes will form drops, capillary action will cause water to readily invade a small tube, and small water‐dwelling animals must contend with a fairly viscous environment.

Water’s most unusual characteristic by far is that it is less dense in its solid form than in its liquid phase. It is unique in the physical world in that regard, and it is because of that distinctive characteristic that ice floats. Pure water reaches its maximum density at 4 °C. Seawater does not reach maximum density until −3.5 °C (Vogel 1981), which is below its freezing point at −1.9 °C and ice crystals have already begun to form. When seawater freezes, the salt is excluded as a brine; the ice itself is fundamentally salt‐free.