Page:The American Cyclopædia (1879) Volume XI.djvu/450

 438 METEOROLOGY of wind force, it is difficult to convert these empirical scales into actual wind velocities, and they should be discarded whenever a good anemometer is available. Empirical Scale of Winds. BEAUFORT SCALE. SMI THSONIAN SCALE. Number. Designation. Description. Number. Designation. 0... Calm

Calm. 1 2 Light air Light breeze .... Just sufficient to make steerage way. With which a ship with all 1 1 to 2 knots. 1 2 Very light breeze. Gentle breeze. j > sails set would go in smooth > 8 to 4 " 8 Fresh wind. water j 5 to 6 ** 4 5 Fresh breeze . . . f royals, &c. 5... High wind. 6 Strong breeze . . in whiVh hP 8in " le reefs and topgaUant sails. 6.... Gale. 7 8 Moderate gale. . Fresh gale jsijsrs, gg?,3?* to 7 8 Strong gale. Violent gale. 9 Strong gale. [ close reefs and courses. 9 Hurricane. 10 Whole gale j In which she could just bear close-reefed main- ) 10 Most violent hurricane 11 Storm j topsail, and reefed foresail. f Under storm staysails or trysails. 12 Hurricane Under bare poles. The movements of the upper currents of the atmosphere have been observed by means of the clouds, balloons, the transfer of volcanic ashes, and the occasional luminous trains that are left in the wake of bright meteors. These latter, being from 10 to 100 m. above the earth's surface, have given us the only knowl- edge we have attained with respect to the cur- rents at so great an elevation. With regard to the ascending and descending currents of air, no satisfactory method of observation has yet been put into execution, though such seems practicable by an aeronaut. So far as our present information justifies an opinion, it seems probable that at an altitude of above 10 m. the atmospheric currents are subject to variations as large, though perhaps not as sudden, as are the winds at the surface. In the lower portion of the atmosphere it is not uncommon to find two or three currents of air, from as many different directions, superim- posed upon each other. (See STORMS.) The general phenomena of the winds at the surface of the earth may be considered in reference to their diurnal and annual variations, and their geographical distribution. The daily period in the strength and velocity of the wind is due in great part to the unequal heating of the different portions of the land, and especially to the over-heating of the land as compared with the sea. This period is most strikingly manifested at the stations on the immediate coasts of continents, and on the borders be- tween mountainous countries and plains. In the latter case the elevated regions of the earth are cooled by nightly radiation, and the cold layers of dry air in contact therewith subse- quently flow down and slip under the warmer DIAGRAM V. Diurnal Change in the Direction of the Wind at Wallingford, Conn. and moister air of the lower lands. An excel- lent instance of the mutual influence of ocean and continents on the daily variations of the wind is shown in the accompanying diagram, V., which gives the direction of the winds each hour of the day, for the months of Janu- ary, May, and July, at- Wallingford, near New Haven, Conn., as published by F. E. Loomis