The Hurricane Guide | Page 3

Berlin has suggested that in the temperate zones the compensating currents of the atmosphere necessary to preserve its equilibrium may be arranged as parallel currents on the surface, and not superposed as in or near the torrid zone. His views may be thus enunciated:--That in the parallels of central Europe the N.E. current flowing towards the equator to feed the ascending column of heated air is not compensated by a current in the upper regions of the atmosphere flowing from the S.W. as in the border of the torrid zone, but there are also S.W. currents on each side the N.E., which to the various countries over which they pass appear as surface-winds, the winds in fact being disposed in alternate beds or layers, S.W., N.E., as in fig. 3.
[Illustration: Fig. 3.]
The Professor also suggests that these parallel and oppositely directed winds are shifting, i. e. they gradually change their position with a lateral motion in the direction of the large arrow cutting them transversely.
In the course of the author's researches on atmospheric waves he had an opportunity of testing the correctness of Professor Dove's suggestion, and in addition ascertained that there existed another set of oppositely directed winds at right angles to those supposed to exist by the Professor. These currents were N.W. and S.E. with a lateral motion towards the N.E. He also carefully discussed the barometric ph?nomena with relation to both these sets of currents, and arrived at the following conclusions. The details will be found in the author's third report, presented to the British Association for the Advancement of Science (Reports, 1846, pp. 132 to 162). During the period under examination the author found the barometer generally to rise with N.E. and N.W. winds, and fall with S.W. and S.E. winds, and that the ph?nomena might be thus illustrated:--Let the strata a a a' a', b' b' b b, fig. 3, represent two parallel a?rial currents or winds, a a a' a' from S.W. or S.E., and b' b' b b from N.E. or N.W. and conceive them both to advance from the N.W. in the first instance and from the S.W. in the second, in the direction of the large arrow. Now conceive the barometer to commence rising just as the edge b b passes any line of country, and to continue rising until the edge b' b' arrives at that line, when the maximum is attained. It will be remarked that this rise is coincident with a N.E. or N.W. wind. The wind now changes and the barometer begins to fall, and continues falling until the edge a a coincides with the line of country on which b b first impinged. During this process we have all the ph?nomena exhibited by an atmospheric wave: when the edge b b passes a line of country the barometer is at a minimum, and this minimum has been termed the anterior trough. During the period the stratum b' b' b b transits, the barometer rises, and this rise has been called the anterior slope. When the conterminous edges of the strata a' a' b' b' pass, a barometric maximum extends along the line of country formerly occupied by the anterior trough, and this maximum has been designated the crest. During the transit of the stratum a' a' a a the barometer falls, and this fall has been characterised as the posterior slope; and when the edge a a occupies the place of b b, the descent of the mercurial column is completed, another minimum extends in the direction of the former, and this minimum has been termed the posterior trough.
It will be readily seen that the lateral passage of the N.W. and S.E. currents towards the N.E. presents precisely the same barometric and anemonal ph?nomena as the rotatory storms when moving in the same direction. If the observer, when the barometer is at a maximum with a N.W. wind, place himself in the same position with regard to the laterally advancing current as he did with regard to the advancing storm, i. e. with his face towards the quarter from which it is advancing--S.W., he will find that with a falling barometer and S.E. wind the current passes him from the left to the right hand; but if at a barometric minimum he place himself in the same position with his face directed to the quarter from which the N.W. current is advancing laterally, also S.W., he will find that with a rising barometer and N.W. wind the current passes him from right to left. Now the two classes of ph?nomena are identical, and it would not be difficult to show that, had we an instance of a rotatory storm in the northern hemisphere moving from N.W. to S.E., it would present precisely the same ph?nomena as