+CG flashes typically have only a single return stroke, and they are more likely than -CGs to have a sustained current flow. Less commonly, a downward traveling positive leader followed by an upward return stroke will lower positive charge to earth, referred to as a positive CG (or +CG). The net effect of this flash is to lower negative charge from the cloud to the ground so it is commonly referred to as a negative CG (or -CG). The more common cloud-to-ground flash has a negative stepped leader that travels downward through the cloud, followed by an upward traveling return stroke. The actual diameter of the lightning channel current is one to two inches, surrounded by a region of charged particles. We see lightning flicker when the process rapidly repeats itself several times along the same path. A negative CG flash consists of one or perhaps as many as 20 return strokes. This return stroke current of bright luminosity travels about 60,000 miles per second back towards the cloud. When the oppositely-charged leader and streamer connect, a powerful electrical current begins flowing. As it nears the ground, the negatively charged stepped leader causes streamer channels of positive charge to reach upward, normally from taller objects in the area, such as a tree, house, or telephone pole. This stepped leader is invisible to the human eye, and shoots to the ground in less time than it takes to blink. In the most common type of cloud-to-ground lightning (CG), a channel of negative charge, called a stepped leader, will zigzag downward in roughly 50-yard segments in a forked pattern. If the plane is below the cloud, then a CG flash could result. Lightning can also be triggered by aircraft flying through strong electric fields. Upward triggered lightning usually occurs in response to a natural lightning flash, but on rare occasions can be “self-triggered”-usually in winter storms with strong winds. Triggered lightning starts at the “ground,” which in this case may mean the top of a tower, and travels upward into the cloud, while “natural” lightning starts in the cloud and travels to ground. Artificially initiated lightning is associated with things like very tall structures, rockets and towers. Charge distribution in a typical storm cloudĭoes lightning go up or down? There are two ways that flashes can strike ground: naturally downward (those that occur because of normal electrification in the environment), and artificially initiated or triggered upward.
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