Page:EB1911 - Volume 25.djvu/766

Rh

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 â€” 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 :—

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