May 28, 1998 transcript #: 235-4
Subject(s): lightning & tornadoes
Title: Lightning & Tornadoes

Two of the scariest parts of thunderstorms-lightning and tornadoes-have an intriguing connection that scientists are now looking into. Hi I’m Dave Thurlow. Many people have pondered whether the electrical activity in a thunderstorm might have something to do the formation of tornadoes. But lightning is a difficult thing to pin down, and so are tornadoes, so it's been hard for scientists to connect the two.

In the past few years, however, a new lightning detection system has come to the rescue. It uses a whole bunch of ground-based receivers that pick up the electromagnetic energy emitted by cloud-to-ground lightning strikes. By comparing the signals from each receiver, the system can identify the time and location of each strike, and also whether that strike is negative or positive -- that is, whether it brings a negative or positive charge to the ground. There are two kinds of lightning. Only a small fraction of all lightning strikes are positive, but they do tend to occur in strange places. Scientists at the National Severe Storms Laboratory have discovered that some storms change their lightning patterns in midstream. They start out producing mostly positive strikes, then switch to negative in the middle of their lives. What's even more intriguing is that this switch tends to happen just before a storm produces a tornado. Lots of storms produce mostly negative flashes for their entire lives, but when a storm starts off positive, it's very likely to produce severe weather, including large hail and tornadoes. Knowing this, forecasters may soon use lightning data to help identify threatening storms before they produce tornadoes -- certainly for all of us, a positive step.

Our show is written today by Bob Henson. The Weather Notebook is produced by the Mount Washington Observatory , funded by The National Science Foundation, and underwritten by Subaru -- the beauty of All-Wheel Drive.