Density of Water

Vertical motions in the ocean are driven by small differences in water density due to differences in salinity (salt content) and (or) differences in temperature. Increased salt content increases density of water, and generally cold water is more dense than warm water, with one profound exception. Figure 7 describes the density of pure water as a function of temperature. It is apparent that density goes up as temperature decreases from $20oC\; on\; the\; plot.\; This\; trend\; is\; terminated,\; however,\; at\; about$ 4oC\; where\; the\; curve\; peaks\; and\; indicates\; that\; the\; density\; of\; water\; decreases\; with\; temperature\; below\; this\; point.\; To\; visualize\; the\; impact\; of\; this\; concept,\; consider\; an\; idealized\; body\; of\; water\; at\; uniform\; temperature\; of$ 20oC.\; When\; it\; is\; cooled\; at\; the\; surface,\; dense\; water\; from\; the\; surface\; will\; sink\; to\; the\; bottom\; and\; be\; replaced\; by\; warmer,\; less\; dense\; water.\; This\; process\; continues\; until\; the\; surface\; (and\; hence\; the\; entire\; water\; body)\; reaches$ 4oC.\; Any\; further\; cooling\; at\; the\; surface\; creates\; less\; dense\; water\; at\; the\; top\; which\; will\; stay\; there.\; Ultimately\; the\; surface\; will\; freeze\; if\; cooling\; continues,\; with\; the\; lower\; levels\; remaining\; at$ 4oC.\; Fish\; and\; bottom-rooted\; plants\; therefore\; are\; protected\; from\; freezing.\; (Every\; Canadian\; fisherperson\; ought\; to\; keep\; this\; graph\; in\; his/her\; tackle\; box\; as\; a\; reminder\; that\; lakes\; in\; that\; region\; support\; fish\; because\; of\; this\; peculiar\; effect!)$$$$$

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