Adverse weather significantly influences safety and operating efficiency of air traffic, particularly in the terminal area, resulting, generally, in a reduced operating efficiency and in particular in delays, diversions, and cancellations of flights. In this paper the impact of thunderstorms on landing aircraft at Frankfurt Airport (Germany) during 1997 and 1998 is analysed. In Germany, thunderstorms are among the major weather phenomena affecting airport operations. Weather and operating conditions permitting, the Frankfurt airport is operated at more than 100% of its nominal capacity, which in turn lowers the actual capacity during periods of adverse weather. Here we present the first quantitative study on weather impact on aviation in Germany. To identify thunderstorms as the unambiguous cause of delays, days without thunderstorms were used for reference and the difference in delay time was determined. The study shows that thunderstorms increase the accumulated delay time by a factor four to 13 in 1997, respectively one to four in 1998. In average, a thunderstorm typically caused about 740 delay minutes additional to the 280 regular delay minutes for a sum of 100 arriving aircraft within four hours of average impact time.