Online Encyclopedia

SQUALL

Online Encyclopedia
Originally appearing in Volume V25, Page 744 of the 1911 Encyclopedia Britannica.
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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 distinci n 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+1.2v and — •5+ •76V with occasional recurrences to extreme velocities of 1.5+I.3V and —I'o+•65v. In other words, the average range of the ribbon is •5+.45v for the twostations during the hour when the mean velocity is v, and the extreme range within the same period is 2.0+ .685. The differences of gust velocity at stations with different exposures may be illustrated by quoting the breadth of the ribbon for a 3o m. wind at the following stations: Southport (Marshside) to m. Scilly 15 „ Shoeburyness 20 „ (from W.) . . . 10 „ (from E.N.E.) Holyhead 15 „ Pendennis Castle (Falmouth) 8 „ (from S.) 16 (from \V.) Aberdeen 30 „ (from N.W.) Alnwick Castle 25 „ 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 1~ ;'l ird 9a "k IT r P1 1 , riff I~r ~lu'lO L~~dtiIi, 11~4i Vi Nou 5p.m. 6 5p.m. 6 T 8 9 10 11 Midt. 1a.m. 2 FIG. 2.
End of Article: SQUALL
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