Rainfall in a ‘Cloudburst’

Abstract

APART from an apocryphal record of 12.48 in. at Portree, Isle of Skye, on December 5, 1863, the largest amount of rain known to have been collected by a rain-gauge within a single day in the British Isles is 9.56 in. at Bruton, Somerset, on June 28, 1917. It has long been recognized, however, that the occurrence of far greater falls than this within much shorter periods than twenty-four hours must be invoked to account for the observed effects of those extremely heavy localized downpours which go by the name of ‘cloudbursts’. Raindrops cannot fall through air rising at a vertical velocity exceeding 8 m./s. Upward currents having a higher order of magnitude of ascent than that are often engendered during severe convectional storms, and thus it may happen that, under conditions of prolific condensation in a well-developed cumulo-nimbus cloud, large quantities of water are carried to an altitude far above their level of origin. If for any reason the convectional movement suddenly ceases or diminishes rapidly, as it may do when the storm system reaches an escarpment of hills, the water accumulated aloft descends more or less en masse. At such times the surface soil beneath may either be washed away to rock level or be deeply excavated and heaped as by a giant plough over areas ranging from a few thousand square yards to 100 acres or more in extent.