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498. STORMS are violent commotions of the atmosphere, occurring in all climates, and differing from other atmospheric disturbances in their destructive power and the extent over which they spread. There is, perhaps, no question in physical science in which there has been so large an admixture of speculation with fact as in attempts made to reduce the phenomena attendant on storms to general laws; the reason being, that meteorological observations were, for long, too few in number, and too far apart, to enable any one to give the atmospheric pressure, the general course of the winds, the temperature, and the rainfall, without drawing largely on conjecture. Now, however, owing to the growing popularity of meteorology, and the countenance happily given to it by most civilised nations, sufficient data may be obtained for a fuller and more satisfactory statement of the question. I shall first state the chief facts of observation regarding storms as obtained from synchronous charts of the weather over a considerable portion of the globe. Since such charts present the principal elements of the weather at a given instant, they may be regarded as successive photographs of storms in their passage across the earth's surface. The Scottish Meteorological Society has been for some time in a very favourable position for examining the storms of Europe. The sources of information are the following :-Observations (1) from the Society's stations on the mainland and in the outlying islands; (2) from Farö and Iceland ; (3) from several places in Norway, from Christiania round the coast to Vardo, east of the North Cape, sent monthly by Professor Mohn ; (4) from sixteen places in Austria sent monthly by Dr Carl Jelinek; to which may be added (5) the daily Weather-Telegrams published in The Times ;' and (6) the Observations and Chart published daily in the ‘ Bulletin International.' Through the courtesy of Dr Buys Ballot, any of the observations published in the 'Jaarboek' are available when required in particular discussions. There are thus ample materials for investigating European storms. The most valuable of these observations are those from Iceland, Farö, Scotland, and Norway ; indeed, without these, most European storms could not be satisfactorily investigated, since the position, extent, and nature of the area of least pressure, and hence the drawing force of the storm, could not be certainly known.
499. In discussing storms, the most important by far of all the observations are those made by the barometer. Three different methods have been adopted to represent the relations of atmospheric pressure to storms :-First Method. By iso barometric lines drawn through places of equal pressure, for every tenth or two tenths of an inch of pressure. Lines for every two tenths are very suitable, since the disturbing force is sufficiently represented by them, and the eye is not distracted by the overcrowding of lines, which would be the result were smaller differences represented on the charts. This method of charting the barometric observations really leaves nothing to be desired, and no doubt it will soon be the only method used in investigating storms.
500. Second Method.-In place of lines of equal barometric pressure, some Continental meteorologists draw lines of equal barometric disturbance—that is, the difference between the barometer on the particular day, and the mean height of the barometer for the place and season. As this method can only suit a limited space of the earth's surface, it ought to be discarded. Thus, suppose on the 15th January the barometer in Western Europe was the average of the month-viz.,
in Iceland, 29.510 inches ; at Aberdeen, 29.737 inches ; in London, 29.956; at Algiers, 30.119 inches, &c.; in other words, suppose the pressure to be exactly the mean of January, as represented in Plate II., then, by the method of equal barometric differences, these pressures would all be reckoned as 0—that is, no barometric disturbance would be indicated. But under such circumstances there would be considerable barometric disturbance, which would occasion a general flow of the atmosphere from the very south of Europe northward towards Iceland. Hence no method but that of reduction of barometric observations to sea-level represents the whole of the disturbing force at any time prevailing over wide areas. Except at very great heights, reductions to sea-level by the usual methods may be accepted as close approximations. The pressures which prevail at great heights must not be mixed up with pressures which prevail at lower levels in examining storms. The only use to which pressures at great heights can be legitimately put is in their relation to the upper currents, or to the winds which prevail at these heights, and not to the winds which blow on the earth's surface. On this point R. Russell, in his Climate of America,' has made some ingenious and valuable remarks.
501. Third Method.—By this method account is taken only of the line of minimum and the line of maximum baro. meter; in other words, a line is drawn through all places at which at the moment the barometer has fallen to the lowest point, but has not yet begun to rise; and another line through all places where the barometer has risen to the highest point. By this method only one feature of the pressure is given, which is included in the first method. It needs scarcely be said that this method is very defective, since by it no information can be got as to where the greatest differences of pressure are,—the most essential element of the storm, as pointing out where the wind is strongest,—the shape and position of the storm as defined by differences of pressure, and the exact direction towards which the body of the storm is moving. This is the method which was adopted by Espy in examining the storms of America, and it is at present advocated by Signor Matteucci and R. Russell.
STORMS OF EUROPE.
502. I have charted a very large number of European storms, laying down in all cases the isobarometric lines, or lines of equal atmospheric pressure, and the direction and force of the winds; and, in a great many instances, the lines of equal thermometric disturbance, and the rainfall, cloud, and clear sky. I have also completed many of the daily charts of the weather published in the ‘Bulletin International,' by filling in the observations from Iceland, Faro, Norway, Scotland, England, Austria, and other places. Hence the results which will be stated as applicable to the storms of Europe are based on a sufficiently large number of instances. Plate VII. is given as illustrative of the general features of these storms. It is a synchronous or synoptic Chart of Europe, giving, from observations made at about one hundred and forty places scattered over this continent, the atmospheric pressure and direction and force of the wind at 8 A.M. of the 2d of November 1863, by which are shown the position and character of two storms passing over Europe at the time. The isobarometric lines, or lines showing where at that hour the height of the barometer reduced to 32o and the level of the sea, was the same, are given for every two tenths of an English inch in the difference of the pressure. They are the red lines of the chart. Hence, where these lines approach near each other, or crowd together, the difference of pressure, or the atmospheric disturbance, was greatest ; and least where they are most apart—a distinction of the utmost importance in its relation to the force of the wind. The force of the wind is shown by arrows represented as flying with the wind. A plain arrow —> shows that the pressure does not amount to 1 on the scale 0 to 6; → that the pressure is 1 on the same scale; that it equals 2; and so on up to ma >,