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SQUALL

745

August a 1906

(Redrawn by permission from the British- Association Report, 1908. J


succession of squalls of this kind is a common experience with • westerly wind at Scilly, and the onset of squalls is generally associated with the veering of the wind to the north-west. Changes in wind velocity, either in the form of gusts or squalls, are generally associated

with some change in direction ot' the wind, but the relation between the changes in gusts have not yet been studied.

A characteristic of squalls is the suddenness with which the increase of wind velocity occurs. At sea the ruffling of the surface can be seen travelling over the water, and the wind producing it and travel- ling with it strikes a sudden blow when it reaches a ship. If squalls are of sufficient violence to do damage to trees or buildings their progress can be traced in a like manner over the land.

These phenomena are exhibited in their most striking form in " line squalls." The characteristic feature of a line squall is that a number of places arranged, roughly speaking, in a straight or slightly curved line across the country experience a similar sequence of events at the same time, and the line of action sweeps across the country as a front advancing nearly uniformly throughout its length. This march of a linear front gives the impression of a wave or bore with an advancing front _ ,. . . e ,, hundreds of miles long, sweeping

o^^SETi? a"^ °™ the country with a velocity Squall. that can be identified from the

time of occurrence of the various changes at different places. The associated events are very well marked by those recorded for the line of squall of the 2nd of August 1906 (fig. 3). They comprise a sudden increase of wind with a veer of direction of 45° to 90 , a sudden rise of pressure known in France as the crochet d'orage, and in Germany as the Gewitter Nase, a pronounced and permanent fall of temperature, and a shower of rain, hail or snow. While these various phenomena are indicated all along the advancing line their intensity may be very different at different points along it. The squall often exhibits greater violence in the middle portion, and it becomes more intense as the whole line advances. In the most fully developed portions the weather phenomena take the form of thunderstorms with violent wind and rain. The course of events in a typical line squall has been most care- fully worked out by R. G. K. Lempfert in a paper on the " Line Squall " of the 8th of February 1906 (Quart. Journ. Roy. Met. Soc. vol. xxxii.). Fig. 4 (reproduced from the papers) shows the successive positions of the line of the front from which its rate of travel can be estimated. The line of advance of a line squall is generally from some point between south and north on the western side, the change of wind being from a warm southerly or westerly wind to a colder westerly or northerly one. So far as is known to the writer there is no case of a line squall exhibiting a backing wind. The date and direction of advance appear to be, generally speaking, those of the final wind, but in cases where the thunderstorms are developed there »



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    Fig. 4. — Times of occurrence of sudden meteorological changes, and isochronous lines showing the advance of the squall. The hours are numbered consecutively from I to 18, starting at I a.m. (Feb. 8), and the minutes are expressed as decimal fractions of an hour. Hail and thunder storms occurred in the region to the east of the dotted line.

    return to the surface warmed and dried. A few revolutions would produce a very considerable elevation of temperature. The air which remains after the passage of a line squall is, how- ever, distinctly colder, of an entirely different kind from that which it replaces and, in those cases which have been investigated, can be traced back to a different point of the compass. Moreover, the smallness of vertical dimensions in the atmosphere as compared with the horizontal dimensions makes it difficult to allow that there is really room for an effective vortex with a horizontal axis. To carry air up 5 m. and bring it back again would practically deprive it of all its moisture and raise its temperature 72° F. Yet 5 m. would be a very small allowance for the horizontal spread of the phenomena of the squall.

    The sudden replacement of warm air by cold with a change of wind seems much more likely to be associated with the flooding of the country by an advancing sweep of cold air. The pressure changes are continuous in the old layer and in the new layer, but discontinuous with varying degrees of discontinuity along the line of junction, where instability of the upper air may be set up. Fig. 5 •shows the discontinuity of pressure in the example discussed by Mr Lempfert. It is clear that as

    the discontinuity of pressure becomes accentuated there arise

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