Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/166

Rh METEOKOLOGY [WIND VELOCITY. The Inclination of Winds to the Isobars, The vorticose motion of the winds in a cyclone towards and in upon the centre has been already pointed out. One of the more important practical problems of meteorology is the deter mination of the angle of inclination of the winds to the isobars in the different segments of the cyclone, not only from the application of the results of the inquiry to the theory of storms but also to practical navigation. The first real contribution to the subject, based on accurate measure ments, was made by Clement Ley in 1873. 1 From the observations made at fifteen places in north-west Europe examined by him he showed that the winds incline from districts of higher towards those of lower pressure at a mean angle of 20 51 ; that the inclination is much greater at inland than at well-exposed stations on the coast, the respective angles being 28 53 and 12 49 ; and that the greatest inclinations are with S.E. winds. Then follow S.W., N.E., and 1ST.W. winds, the last showing the least inclination. Whipple has recently compared the winds at Kew with the barometric gradients for the five years ending 1879, with the result that the greatest inclination is 63 with S.E. winds, the least 35 with N.E. winds, and the mean for all winds 52. As regards the open sea, Captain Toynbee has shown, from a careful investigation of the great Atlantic storm of August 24, 1873, that the mean angle of inclination calculated from one hundred and eight observations was 29, the mean at the three selected epochs examined varying from 25 to 31. Barometric Gradient and Velocity of the Wind. In inquiring into the relation of the velocity of the wind to the barometric gradient, it is necessary to have some definite information as to the increase of the velocity with height above the ground. Stevenson recently made observations on this point on winds varying from 2 to 44 miles an hour from the surface up to a height of 50 feet, from which he has drawn the following conclusions : (1) the spaces passed over in the same time by the wind increase with height above the ground ; (2) the curves traced out by these variations of velocity from 15 to 50 feet high coincide most nearly with parabolas (fig. 20) Fig. 20. having their vertices in a horizontal line 72 feet below the surface ; (3) between 15 feet and the ground there is great disturbance of the currents, so that the symmetry of the curves is destroyed ; (4) the parameters of these parabolas increase directly in the ratio of the squares of the velocities of the different gales. If x be the velocity of the wind 1 Journal Scottish Meteorological Society, vol. iv. p. 66. at height H above the ground, the parameter of the corresponding parabola is x 2 /(H. + 72) ; and as x varies the parameter will vary as x&quot; or as the square of the velocity of the gale. It follows that, to render wind observations comparable, it is necessary that anemometers be placed at one uniform height above the ground, and that standard height not lower than 15 feet above the. surface. It is very desirable that the inquiry were prosecuted up to a height of 1 00 feet ; and it is of the utmost importance that the variation in the diurnal velocity be at the same time determined at different heights from 15 feet upwards. Stevenson also made wind observations on the Calton Hill, Arthur s Seat, and the Pentland Hills, in the vicinity of Edinburgh, up to a height of 1600 feet above sea-level. It is from observations made at stations on knolls and peaks at different heights above the sea, and at different heights above the surfaces of their summits, that the problem of the variation of the wind s velocity at different heights with the same barometric gradient can be ascer tained. In carrying the inquiry to considerable heights, the results cease to be comparable with those obtained at lower levels, unless in those cases where neighbouring heights are available for data from which the barometric gradient at the observed height can be calculated. The results of observations as to the velocity of atmospheric currents at very great elevations in the atmosphere deduced from the apparent movements of the higher clouds arc altogether incomparable with the winds near the surface of the earth, for these among other reasons : the heights of the clouds can be at best but imperfectly ascertained; the motion of the clouds, particularly the higher clouds, may be only apparent, it being sometimes difficult to distinguish between the formation and dissolution of clouds and their motion ; and above all, since the higher clouds are usually the accompaniments of the greater weather changes, their movements are the result of barometric gradients towards a knowledge of which we are absolutely powerless to take a single step. As regards surface winds, Clement Ley in 1881, and Whipple more recently and with greater fulness, have calculated the mean wind velocities for twelve gradients,- the gradients being derived from the daily weather charts of the Meteorological Office for the five years 1875 to 1879 at 8 A.M., and the corresponding wind data being obtained from the hourly readings of the Kew anemograph. The barometric gradient is for 15 nautical miles, and the following are the velocities for the twelve gradients on the mean of the year : Gradient. inch. 0-002 0-005 0-007 o-oio 0-012 0-015 Velocity. miles. 5-0 7-0 7-5 9-2 11-6 12-6 Gradient. inch. 0-017 0-020 0-022 0-025 0-027 0-030 Velocity, miles. 15-0 16-5 19-1 22-0 22-0 25-5 The influence of season is very strongly marked. The velocities for the same gradients in order are October to December, 12 5 miles; July to September, 12 G miles; January to March, 14 8 miles; and April to June, 17*2 miles. From those observations of Whipple it follows that during the six months when the temperature is falling the velocity for the same gradients is least, while the velocity is greatest during the six months when the temperature is rising, and absolutely greatest during the three months ending June, when the greater part of the annual increase of temperature occurs. It is evident that the observed increase in the velocity of the wind for the same gradients is to be referred to the same cause that brings about the diurnal increase in the wind s velocity, viz., the wind blowing over a warmer surface than itself.