Hot days and heatwaves became more frequent in the latter half of the twentieth century. Soil moisture deficits increase the probability of hot extremes, according to global analysis of observational data.
Brigitte Mueller and Sonia Seneviratne, of the Institute for Atmospheric and Climate Science, ETH Zurich, used observational and reanalysis data to examine the relationship between precipitation deficits — a proxy for surface moisture deficits — and hot extremes across the globe between 1979 and 2010. According to their analysis, surface moisture deficits in the three months leading up to the hottest month of the year were associated with an increased number of hot days during that month. A correlation between dryness and heat was apparent in most regions, but the strongest relationships were found in North and South America, Australia, Southern and Eastern Europe, and parts of Asia.
Surface moisture deficits appeared to increase the probability of an above-average number of hot days by 30–60%, relative to wet conditions. Incorporation of this relationship into long-range forecasts could help in the prediction of extreme weather events.