Drought/Low Rainfall Watch - Summer 2018

FEBRUARY, 1922. MONTHLY WEATHER REVIEW.

THE DROUGHT OF 1921 IN THE BRITISH ISLES.
By C. E. P. BROOKS AND J. GLASSPOOLE.


The general rainfall in England and Wales was the least in 1921, so far as can be ascertained, since 1788. Individual long records indicated that over a considerable part of the southeast of England 1921 was the driest year for at least a century and a half.

The months of 1921 were not individually so remarkable as was shown by a comparison with the driest month known to have occurred in the British Isles generally. As shown by a map of standard deviation of annual rainfall 1881-1915, for the British Isles, the least fluctuations of annual rainfall occurred along the coast in the
northwest, increasing to a maximum in the southeast and center of the land masses. Constructing charts showing the distribution of, barometric pressure over the globe during and preceding droughts, beginning with 1864, it is found that the conditions which commonly prevail during dry spells are high pressure over the British Isles, the
greatest deviation from normal being usually over southeast England; low pressure over the Arctic regions, especially near Spitzbergen; and, generally, low pressure near the Tropics.

The first factor is related to the 11-year sun-spot cycle, occurring most frequently two years after the sun-spot minimum and three or four years after sun-spot maximum, so that it tends to recur every five or six years. Great droughts occur only when both of these factors are favorable. With pressure low over the Arctic, two or three months warning of as drought would be given by the development of high pressure over northern Russia.

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This story map shows the spatial extent and severity of three major historical drought periods in Ireland (1854-1860,1884-1896 and 1932-1933). Archived documentary evidence showing drought impacts are also presented.


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