SQUALL, the name given to any sudden large increase of wind velocity of less transient character than a "gust." A squall, usually lasting for some minutes at least, is attributable to meteorological causes, while gusts, which may succeed one another every few seconds, are the result of some mechanical interference with the direct flow of air, and are thus produced by turbulence. A squall may include a succession of gusts. A coefficient of gustiness has been determined for several stations by dividing the width of the ribbon (max. veloc.-min. veloc.) by the mean velocity. These coefficients show a wide range.
Pendennis castle (Falmouth) S. wind . Coefficient of gustiness -25 Southport (Marsh side) . . . . -3 Shoeburyness, E.N.E. wind . . . -3 Holyhead (Salt island) . . . . 5 Scilly (St. Mary's) . . . . . .5 Pendennis castle (Falmouth) W. wind. 5 Alnwick (roof of schoolhouse) . . -8 Shoeburyness W. wind . . . . -8 Kew observatory (roof) . . . . i-o Aberdeen (roof of King's college) . . io The most gusty exposure of the stations reporting to the Meteorological Office, is Dyce, near Aberdeen, with a coefficient of 1.3. At this station the mast of the tube-anemometer projects 15 ft. above surrounding tree tops, which are particularly liable to produce turbulences.
In the present state of anemometry it is difficult to give exact figures for the actual force of the wind during gusts, but io6.5 m.p.h. were recorded at Pendennis castle on March 14, 1905. Squalls with velocities reaching 55 m.p.h. are not uncommon, and the range of wind velocity during gusts in such a squall may be anything between 4o m.p.h. and upwards of 1 oo m.p.h.
The gradual veering of the westerly wind to the north-west is a common experience with squalls at Scilly. Changes in wind velocity are generally associated with some change in wind direc tion. The increase of wind velocity occurs suddenly. At sea, the ruffling of the surface can be seen travelling over the water, and, on land, the approach of a violent squall is frequently marked by falling trees, etc.
These phenomena are best exhibited in "line squalls." As the name suggests, a number of places arranged, roughly, in a con tinuous line, often hundreds of miles long, experience simultane ously a similar sequence of events. The line of action advances with an approximately uniform speed across the country, and somewhat resembles the dash forward of the water of a broken wave. Its velocity can be determined easily from the time of
occurrence of the various changes at different places. Though these changes normally occur at all places on the line, their in tensity varies considerably ; there is usually greater violence in the middle portion, and the disturbance becomes more intense as the whole line advances. The course of events in typical line squalls has been worked out by R. G. K. Lempfert and Richard Corless. The violent winds may therefore be attributed to the breakdown of the dynamical system under the stress of these local but sharp differences of pressure. In this respect a V-shaped depression (q.v.) may be regarded as a special case of a line squall in which the two currents are from approximately opposite direc tions, and the line of the trough which sweeps forward, keeping parallel to itself, represents the linear front. An example of a violent and destructive form of line squall is shown in the records for June I, 1908. The Kew squall of wind which destroyed a number of the trees of Bushey avenue maintained its violence for a considerable period. Many other famous line squalls, such as those of Feb. 19, 1907, Oct. 14, 1909, etc., have been examined. (See Lempfert and Corless, Line Squalls and Associated Phe nomena [Q.J.R. Met. Soc., vol. xxxvi., Iwo].) One of historic interest caused the loss of H.M.S. "Eurydice" off the Isle of Wight on March 24, 1878. It was among the first to receive special attention, and was discussed by Abercromby in 1884 (Quart. Journ. Roy. Met. Soc., x. 172) and previously by Clement Ley (Symon's Met. Mag., April, 1878). The events of the World War deprived Norwegian meteorologists of weather telegrams from a considerable part of Europe, and, in consequence, they were compelled to base their forecasts on the intensive study of a small region. This study resulted in much new light being thrown on the movements of air currents.
In addition to the references given in the text, see Napier Shaw's Life History of Surface Air Currents, Forecasting Weather (1923), The Air and its Ways (1923), Manual of Meteorology (1919, 1926, 1928) ; V. Bjerknes, On the Dynamics of the Circular Vortex with Applications to the Atmosphere (Christiania, 1921), together with various papers in the Q.J. Met. Soc. from 1910 onwards; J. Bjerknes and H. Solberg, Life Cycle of Cyclones and the Polar Front Theory of Atmospheric Circula tion (Oslo, 1922).