None for the road.

When it comes to understanding what electrifies clouds and triggers lightning, "we're still where Benjamin Franklin left us," says physicist Peter Handel of the University of Missouri in St. Louis. Most current theories involve falling water droplets that polarize clouds by carrying negative charges down to their bases. But these ideas leave much unexplained, says Handel. For example, they cant's account for slightings of lightning before rainfall and they can't explain how stratus clouds, without ever shedding a raindrop, can leak negative "dark currents" to the earth.

In the June 20 JOURNAL OF GEOPHYSICAL RESEARCH, Handel outlines a new theory which he says can account for most observations associated with lightning. His proposal is rooted in the recently discovered, unique dielectric properties of ice and water. In bulk, ice has not net polarization, but in small enough groups, the water molecules in ice are free to cluster together so that their dipole moments are aligned in the same direction. According to Handel's theory, once in the number of such small aggregates and the number of molecules in each group reach a certain limit within a cloud, a "polarization catastrophe" occurs in which all of the clusters spontaneously align--typically along the direction of a very weak fair-weather field between the earth and the ionosphere.

As the cloud becomes more polarized, positive charges at the bottom of the cloud and negative ions at the top attract free charges of opposite sign from the air. Lightning is triggered both inside the cloud and from cloud to ground, Handel suggests, when the individual aggregates of ice grow too large to hold a strong dipole. This rapidly annihilates the polarlity of the cloud as set up by the ice and suddenly releases the free positive and negative ions from their respective bonds at the cloud ends. Without having time to slowly slink away, the now rudely unanchored charges discharge via a lightning bolt.

Handel says this theory is strongly supported by past experiments and can provide a mechanism for the dark currents. It also can explain horizational, helical and positive lightning, he says.