It’s thunderstorm season in northeast Ohio and western Pennsylvania and that means you see us showing a lot of graphics that look like this:
You are no doubt familiar with the “traditional” radar, with the colors corresponding to the intensity of the precipitation. Modern weather software can also display lightning data on top of the radar and this is a valuable tool in our arsenal. I am frequently asked questions such as “What do the (+) signs mean?” and “What’s the difference between positive and negative lightning?”
The fact is, there is a significant difference between the two. Knowing the charge of lightning can tell us about the nature of a thunderstorm. We also know that positively charged lightning is much more powerful and dangerous than lightning with a negative charge.
HOW DOES LIGHTNING FORM?
In mature thunderstorms, the bottom of the storm contains rain and melting hail. The middle of the storm contains hail and small ice crystals and the top of the storm is comprised of mostly small ice crystals. The middle of the storm is a pretty violent place with precipitation and ice flying around and crashing into each other. This causes the precipitation to become charged,
The lighter ice crystals pick up a positive charge and are carried to the top of the storm by updrafts. Meanwhile, the heavier items get volleyed around the middle and lower portions of the cloud and get a negative charge. In response to this negative charge in the middle of the storm, a small positive charge develops at the bottom.
On the ground, underneath the anvil (outer edges) of the storm, a negative charge can develop.
When the difference in the charges reaches a certain critical level, there is a rapid discharge of electricity…..LIGHTNING.
Negative lightning refers to the “polarity” of the lightning strike. The sign depicts the type of charge that is transferred from the cloud to the ground.
90-95% of all lightning strikes are negative. Most lightning strike victims are, not surprisingly, struck by positively-charged bolts.
While positive lightning occurs much less frequently, it is MUCH more powerful and dangerous. Positive lightning mostly originates from the top of a storm. When conditions are right and a big charge difference between the anvil and the ground develops, a positively-charged bolt can develop. The strike has to travel a much larger distance and so can last much longer than a negative strike. In addition, positive strikes carry a dramatically higher voltage than negative ones.
While only 10% of all lightning strike victims are killed, the percent killed by positive strikes is much higher.
Positive strikes are also usually the cause of forest fires.
Some studies suggest that there is a connection between tornado formation and spikes in positive lightning strikes.