Squall

Squall, the name given to any sudden increase of wind to gale force. Generally speaking a squall is understood to be of short duration, but the word " gust " would be used to indicate an increase of wind force of more transient character than a squall. Gusts may succeed one another several times within the compass of a minute. A squall may comprise a succession of gusts, with intervening partial lulls, and would last with varying intensity for some minutes at least. The distinction between gusts and squalls is best illustrated by the traces of a Dines pressure-tube anemograph. The trace reproduced in fig. i for an ordinary steady wind shows that the force of the wind is constantly oscillating. The general appearance of the trace is a ribbon which has a breadth proportional to the mean wind velocity. The breadth of the ribbon is also dependent upon the nature of the reference; the better the exposure the narrower the ribbon; for an anemograph at a coast station the ribbon is wider for a shore wind than for a sea wind.

From the records obtained at Scilly and Holyhead, Dr G. C. Simpson concluded that a wind of mean hourly velocity v was composed of alternations of gusts and lulls ranging on the average between limits .5 +I. 2V and - .5+. 76v with occasional recurrences to extreme velocities of I. 5+1.3v and - 1.0+ 65v. In other words, the average range of the ribbon is -5 + -45 v for the two stations during the hour when the mean velocity is v, and the extreme range within the same period is 2 '0+ 68v.

The differences of gust velocity at stations with different exposures may be illustrated by quoting the breadth of the ribbon for a 30 m. wind at the following stations: Southport (Marshside) io m.

Scilly 15 „ Shoeburyness 20 „ 10 » Holyhead 15 „ Pendennis Castle (Falmouth) .

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Aberdeen Alnwick Castle Kew.. .. .. ... 30 Fig. 2 represents a succession of squalls occurring in an ordinary gusty wind; the squalls succeed one another with fair regularity about every twenty minutes and last in full force for a few minutes. A 10 20 ' '0  vi FIG I.

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0 ' '0  0 8 „ 16 „ 0 „ 2 „ (from W.) (from E.N.E.) (from S.) (from W.) (from N.W.) 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 of 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 travelling 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 10 events at the same time, and the line of action sweeps across the 200 country as a front advancing nearly uniformly throughout its length. This march of a linear front gives the impression of a wave (Redrawn by permission from the or bore with an advancing front British Association Report, 1908.) hundreds of miles long, sweeping FIG.'S. - Variation of Meteover the country with a velocity orological Elements in a Line 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 carefully 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 is a local violence of the wind bearing no relation to the isobaric distribution of the final wind.

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Endeavours have been made to explain the phenomena of line squalls as due to vortex motion of particular character. The violent wind blowing out in front of the storm is part of the circulation of a vortex with horizontal axis. It supplies the air for the rainfall of the stations in front. Its place is taken by descending air at the back, which becomes in its turn the surface supply for stations farther in. But such an explanation ??I¦ -__ xxv. 24 a seems in many ways incomplete. Although perhaps if the wind velocities in a vertical plane were plotted there might be some evidence of circulation in the mathematical sense by integrating round a closed curve, yet the idea of circulation in a vertical plane as suggesting the primary constitution of the phenomena is very inadequate. The change of air which takes place during the passage of the line squall is altogether different from that which we would get by passing the surface air through a complete vertical cycle and condensing a large quantity of water vapour on the way. If vertical circulation were complete the air would FIG. 4. - Times of occurrence of sudden meteorological changes, and isochronous lines showing the advance of the squall. The hours are numbered consecutively from 1 to 18, starting at 1 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, however, 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.

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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 August i 1906 r  301050 (Redrawn by permission from the Quart. Journ. Roy. Met. Soc.)  FIG. 5.-Distribution of Pressure (Feb. 8, 1906). Isobars are shown for each 0.1 in.

in the localities on the line of advance very steep pressure gradients for which there are corresponding winds. The violent winds may therefore be attributed to the breakdown of the dynamical system under the stress of these local differences of pressure.

From this point of view the phenomena of the line squall are to be regarded as a development of the ordinary phenomena of the V-shaped depression. A sudden change of wind and a line of rain that pass over the country with the velocity of the same order as that of the following wind are quite common features of the S.W. quadrant of a cyclonic depression, and they, too, seem to point to the juxtaposition of currents of different temperature coming from different regions but forming adjacent components of supply for the depression.

Examples of all degrees between the comparatively unimportant rain line and the most violent tornado-like squall could be put side by side with cases in which the typical pressure, temperature and weather changes are accompanied by a sudden lull in the wind, as in the example quoted in the Life History of Surface Air Currents  (M. 0. publication, No. 174, 1906). An example of a line squall in its most violent and destructive form is shown in the records for the st of June 1908. In the record for Kew the squall of wind which destroyed a number of the trees of Bushey Avenue is shown as lasting for a very long period (fig. 6).

A line squall of historic interest is that which capsized H.M.S. " Eurydice " off the Isle of Wight on the 24th of March 1878. The occurrence is discussed by the Hon. Ralph Abercromby in 1884 (Quart. Journ. Roy. Met. Soc. x. 172) and previously by the Rev. Mr Clementhey (Symon's Met. Mag., April 1878). The shift of wind in this case appears to have been from west to north, and the change in the wind was accompanied by the transitions from fine blue sky to snow. The records at the seven observatories belonging to the Meteorological Council are reproduced in the Quarterly Weather Report, from which fig. 7 is taken.

Whatever explanation may be given of the cause and origin of the phenomena of line squalls, it must take account of the fact that a first squall is often succeeded by others of a similar character but often of less intensity than the first. After the sudden shift of wind, with accompanying weather changes, the conditions seem to revert more or less to the original state. The warm southerly wind reasserts itself, but is driven out again by another attack, and ultimately the cold wind holds the field. It is easy to suggest, but at present not easy to verify, the course of replacement of the warm wind. Upper air observations in such circumstances with kites or manned balloons are dangerous, both for the apparatus and the observer; but it may be possible to trace the actual course of events by the records of rounding balloons supplemented by observations of the motive balloon by means of theodolites.

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Little has been said about the actual force of the wind in gusts or squalls, and in the present state of anemometry it is difficult to (By permission of the Controller of His Majesty's Stationery Office.) FIG. 7 .-" Eurydice " Squall, from the Quarterly Weather Report.  Shepherds Bush Shepherds Bush t0 Kew 10 (Redrawn by permission from the British Association Report, 1908.).

FIG. 6.-Records of wind velocity on June I, 1908.

50 40 30 Q 20 g 0 Brighton 30 20 0 13 regard the figures hitherto obtained as final; moreover, the large wind force in squalls is probably subject to large local variations, the difference between the record of the squall of the 1st of June 1908 at Kew and Shepherd's Bush suggests that it may have been much stronger at Bushey, where the damage was done. The highest velocity in a gust hitherto recorded upon instruments belonging to the office is 106.5 m. per hour at Pendennis Castle on the 14th of March 1905. Gale force is defined for the purposes of the meteorological office as that of a wind which has an average velocity during an hour of 38 m. per hour. According to Simpson's results at Scilly or Holyhead, where the exposure is good, a wind that just got within the reckoning of gales would reach 44 m. per hour in the ordinary gusts, with occasional records of 51 m. per hour. Squalls with velocities reaching 55 m. per hour are not uncommon, and the range of wind velocity which constitutes a squall may be anything between 40 m. an hour and upwards of 100 m. an hour. (W. N. S.)