Several severe convective storms occurred at the beginning of August 2013 over western and central Europe.
On 6 August, a severe supercell (Figure 1) formed downstream of the Black Forest near the city of Villingen-Schwenningen and moved further over the Swabian Jura. Supercells are rotating thunderstorms accompanied by severe weather phenomena such as heavy rainfall, severe convective gusts, hail or even tornadoes. They can reach a diameter of 50 km or more and persist over several hours. The supercell on 6 August passed along the northern border of the Swabian Jura, where it already produced hailstones with diameters of up to 8 cm close to Balingen (BW). At around 15:40 close to Undingen south of Reutlingen, several large hailstones as large as tennis-balls or even oranges fell to the ground – among them also the recorded hailstone with a diameter of 14.1 cm. It is reasonable to assume that the hailstone may have been even larger, before local inhabitants found it and put it into the freezer.
During the Munich hailstorm in 1984, the largest recorded hailstone had a diameter of 9,5 cm, but eyewitnesses report on even larger hailstones of up to 12 cm. Hailstones of the severe Villingen-Schwenningen hailstorm on 26 June 2006 were reported to have diameters of 9,5 cm. So far the largest documented hailstone worldwide was found on 23 July 2010 in Vivian, South Dakota (USA). It was the result of a supercell with estimated vertical speeds of over 50 m/s. The hailstone had a diameter of 20 cm, a circumference of 47,3 cm and weighed 0,88 kg (NOAA, 2010).