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Section Z Z index 1-9 of 44 terms previous 1 2 3 4 5 Next z-less scaling—See local similarity. Z–R relation—A relationship between radar reflectivity factor Z (mm6 m−3) and rain rate R (mm h−1). Empirical relationships of the form Z = aRb are often used, with a ≈ 200 to 600 and b ≈ 1.5 to 2.0. See radar reflectivity, Marshall–Palmer relation. Z time—Same as coordinated universal time (UTC). See zone time. Z0—The internationally accepted symbol for the elevation of mean sea level above chart datum. Zanzibar Current—(Also known as the East African Coastal Current.) A western boundary current in the Indian Ocean that flows permanently northward from 10°S along the east African coast. During the northeast monsoon the Zanzibar Current flows against the wind, meeting the southward flowing Somali Current at 1°N in December, 4°S in February (when the Zanzibar Current is weakest), and at the equator in April. Throughout this period it continues across the equator as an undercurrent that feeds into the Equatorial Countercurrent. During the southwest monsoon season the current strengthens considerably, attaining speeds of 2 m s−1 and a transport of 15 Sv (15 × 106 m3 s−1) and feeding into the Somali Current, which flows northward during that season. zastrugi—Variant spelling of sastrugi. ZDR—See differential reflectivity. Zeeman effect—A shift in the energy levels of an isolated atom or molecule as a consequence of an external magnetic field. The energy levels of an atom (or molecule) depend on the internal forces that electrons and nuclei exert on each other as well as on any external forces (e.g., an electromagnetic field). An observable consequence of the Zeeman effect is the shifting and broadening of spectral lines. The Zeeman effect is the magnetic analogue of the Stark effect. Zeldovich mechanism—Chemical mechanism for producing active nitrogen from molecular nitrogen in the gas phase. The pair of reactions O + N2 → NO + N and N + O2 → NO + O were first proposed by Russian scientist Y. B. Zeldovich in 1947. The reactions are thought to account for much of the active nitrogen formed in hot exhaust gases from combustion sources and following the rapid heating of the air during lightning discharges.           Zeldovich, Y. B., P. Y. Sadonikov, and D. A. Frank-Kamenetskii, 1947: Oxidation of nitrogen in combustion (M. Shelef, Transl.). Acad. Sci. USSR, Inst. Chem. Phys., Moscow–Leningrad. zenith distance—The angular distance of any celestial object from a given observer's zenith, measured along the great circle of the celestial sphere from the zenith to the object; the complement of the elevation angle. previous 1 2 3 4 5 Next