The dark side of nighttime warming.

If global warming strikes mostly at night, as it has in recent decades, it could pump up the destructive power of summer storms, according to a French scientist.

Meteorologists noted several years ago that daily minimum temperatures have climbed faster than daily maximums during the 20th century, indicating that the effects of global warming in this century have occurred mostly at night (SN: 1/4/92, p.4). To see what this might mean, Jean Dessens of the Universite Paul Sabatier in Campistrous looked at statistics on hail damage gathered by French insurers since 1946. He compared the annual losses caused by hail to a record of average summer minimum and maximum temperatures.

The hail losses showed substantial correlation with the nighttime temperatures, Dessens reports in the May 15 Geophysical Research Letters. The rise in nighttime temperatures could account for much of the 45 percent increase in hail damage since 1981.

While most researchers consider global warming a threat to society, some meteorologists have proposed that a hike in daily minimum temperatures could actually yield benefits--prolonging the growing season and reducing the need for home heating. Dessens' study, however, suggests that even a nighttime warming can cause problems.