Page:EB1911 - Volume 25.djvu/766

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744
SQUALL


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 +.isv and --5+761; with occasional recurrences to extreme velocities of 1-5 + 1-32; and — i-o+-65i>. In other words, the average range of the ribbon is -5 + -451; for the two


stations during the hour when the mean velocity is v, and the extreme range within the same period is 2 0+ -68"j.

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) 10 m.

Scilly i§

Shoeburyness 20


Holyhead 15

Pendennis Castle (Falmouth) ... 8

.... 16

Aberdeen 30

Alnwick Castle 25


(from W.) (fromE.N.E.)

(from S.) (from W.) (from N.W.)


Kew


30


5p.m. 6


Hours

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



Fig. 1.

5p.m. &


Midi. 1a.m. 2 Fig. 2.