For the first time, scientists at ETH Zurich have examined globally the connection between soil moisture and extreme heat with measured data. Their study shows that precipitation deficits increase the probability of hot days in many regions of the world. The results will help to better assess heat risks.In July 2011, a heatwave broke all records in Texas. It did not rain in the region for months. Cornfields and grassland withered, and the soil dried out. For several weeks temperatures were above 38°C, in some places rising even to 43°C or 45°C. The Texan record summer was one example of what climate scientists have also been able to demonstrate in other regions in recent years: if there is a precipitation deficit in the spring, the probability of heatwaves in the summer increases. The water content of the soil is not only a decisive factor in ensuring that plants thrive, but it also influences the energy exchange with the atmosphere. If the earth is saturated with water, moisture evaporates from the ground, thus stopping the atmosphere from heating up too quickly. However, once the ground has dried out, solar radiation heats up the air unrestrained.