THE RANDOM NATURE OF DROUGHT - MATHEMATICAL AND PHYSICAL CAUSES

Annual and monthly rainfall records for three regions of the United Ki ngdom and five locations in Australia have been examined with specific regard to the frequencies of droughts and dry spells. It is shown tha t plots of cumulative totals of precipitation tend to obey arc sine la w behaviour and the excursions above and below the x axis fall within the limits of patterns to be expected by chance. Runs of successive va lues of negative anomalies from the monthly median were counted and th eir mean and theoretical recurrence times computed. Chi-square tests s uggested that the frequencies of runs were no more nor less than might be expected from chance expectation in the United Kingdom. In Austral ia the frequencies in the Perth, W. A., Adelaide, S. A., and Hobart, T asmania records showed similar characteristics to those in the United Kingdom. However in Melbourne and Sydney the frequencies of runs of ne gative anomalies from the monthly median showed high significance > 1% that they were more frequent than would be expected. This higher than expected frequency of droughts and dry spells is attributed to the EN SO factor. It is suggested that overall the occurrence of precipitatio n events is a chance mechanism although some regions, notably southeas t Australia, northeast Brazil and the Sahel are affected by climatical ly forced mechanisms such as sea surface temperature anomalies, blocki ng, over grazing etc. A discussion is offered of the meaning of ''natu rally'' occurring drought in terms of physical interactions described by nonlinear mathematical equations and alternatively in terms of casi no probability.