Beyond the folklore saying, the connection between rain and lightning has intrigued scientists since the 1950s when it was calculated that storms that produce a little rain are unlikely to have lightning, and in the 1980s when another scientist calculated that total rainfall is proportional to total lightning in a storm. Most interesting was the finding that lightning peaked a few minutes before the rain. Studies of isolated thunderstorms in the American northeast and southeast (the latter is especially prone to thunderstorms), and around Darwin, Australia, showed that storms produce the most lightning flashes when a large mass of water is at high altitude. This corresponds with the model that compares storms to a large Van de Graaff generator like those used in science demonstrations. A moving belt scrapes across material and carries and electrical charge to the upper dome where it stays until something - like a volunteer - touches it to complete the circuit. (You get the same effect shuffling across a rug on a cold day.) In a storm, the moving belt is cold water, often ice crystals, carried aloft by winds. This generates an electric charge until the potential (the voltage difference) between cloud and ground is great enough that the electricity pushes through the insulating air to form a lightning bolt. The complex, fascinating process is described at the GHCC web site. The LIS teams theorizes that stronger updrafts carry more mass to high altitude where they can generate more lightning. This may also account for why storms over land - which have stronger updrafts - produce 10 times more lightning than storms over water. When its data are matched to rainfall data from TRMM and ground instruments, LIS will help scientists test their theories about relationships between rain and lightning. Back to the Lighting Imaging Sensor story.

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