Hotter summers

At the same time, hotter summers are also more likely to bring severe weather events. Excessive heat leads to an intensification of convective processes in the atmosphere and there is a greater disparity between continental and maritime air masses, the latter not warming up so quickly because of the thermal inertia of the oceans.

Cold fronts, in which the cooler maritime air repeatedly surges onto the mainland and terminates or interrupts the hot weather conditions there, draw energy from these temperature differences between the two air masses and carry thunderstorms, hail, flash floods, and strong winds over the land.

An evaluation of the data that Siemens' lightning detection network recorded over a period of years uncovered a distinct exponential correlation with mean monthly temperatures (cf. Fig 4).

For every degree the temperature rises, the number of lightning strokes in Germany increases by around 50%. The extremely hot June of 2003 produced no fewer than 900,000 lightning strokes, which is way above the mean curve. The number is expected to increase even further, as are losses due to lightning and overvoltage: these have been a cause of concern to insurers for a long time now and are becoming even more 103 expensive due to the growing cost and sensitivity of electronic equipment.

As the intensity of summer storms increases, so too does the frequency of flash floods. Consequently, in spite of the trend towards less summer precipitation, extraordinarily large amounts of rain may fall locally in a short time.

In towns and cities they may overtax sewage networks, whereas in hilly or mountainous terrain they may trigger flash floods, mudflows, and landslides. In summer, therefore, a higher precipitation variability increases both the risk of drought and the risk of flooding.