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Physical Processes in Earth and Environmental Sciences Phần 3

Chương 3 Ví dụ, một bề mặt đại dương hiện tại mật độ 1 có thể được cho biết "cảm thấy" lực hấp dẫn giảm vì sự nổi tích cực tác dụng lên nó bằng cách nằm dưới nước môi trường xung quanh có mật độ cao hơn một chút. | 54 Chapter 3 differing density does not feel the same gravitational attraction as it would if the ambient medium were not there. For example a surface ocean current of density P1 may be said to feel reducedgravity because of the positive buoyancy exerted on it by underlying ambient water of slightly higher density p2. The expression for this reduced gravity g isg g p2 p1 p2. We noted earlier that for the case of mineral matter density pm in atmosphere of density pa the effect is negligible corresponding to the case pm pa. 3.6.3 Natural reasons for buoyancy We have to ask how buoyant forces arise naturally. The commonest cause in both atmosphere and ocean is density changes arising from temperature variations acting upon geographically separated air or water masses that then interact. For example over the c.30 C variation in near-surface air or water temperature from Pole to equator the density of air varies by c.11 percent and that of seawater by c.0.6 percent. The former is appreciable and although the latter may seem trivial it is sufficient to drive the entire oceanic circulation. It is helped of course by variations in salinity from near zero for polar ice meltwater to very saline low-latitude waters concentrated by evaporation a maximum possible variation of some 4 percent. Density changes also arise when a bottom current picks up sufficient sediment so that its bulk density is greater than that of the ambient lake or marine waters Fig. 2.12 these are termed turbidity currents Section 4.12 . Motion due to buoyancy forces in thermal fluids is called convection Section 4.20 . This acts to redistribute heat energy. There is a serious complication here because buoyant convective motion is accompanied by volume changes along pressure gradients that cause variations of density. The rising material expands becomes less dense and has to do work against its surroundings Section 3.4 this requires thermal energy to be used up and so cooling occurs. This has little effect