Positive Lightning Ask Dave a weather question

Dave Thurlow, Host

 
Here on The Weather Notebook, we like to focus on the positive, and now we have a good excuse to do so. It's a story about positive lightning. Hi, I'm Dave Thurlow from the Mount Washington Observatory.

Most lightning flashes are classified as negative, because they lower a negative charge from cloud to ground. Thanks to a national lightning-detection network, we now know that about 90% of all cloud-to-ground flashes are negative. But there *are* those occasional rebels, the strikes that bring down positive instead of a negative charge.

Last year, an interesting thing happened on the way from spring to summer across the southern Great Plains. The plains were withering in a severe drought. To make matters worse, huge clouds of smoke were blowing in from Mexican forest fires. There were still the usual spring thunderstorms across Texas, Oklahoma, and Kansas. What was unusual was the nature of the lightning. Instead of having the typical 10 percent ratio of positive flashes, some of these storms had positive counts of 60 or 80 percent. The change was so dramatic and widespread that the lightning detection network checked to make sure their instruments weren't malfunctioning.

It turns out that smoke from the Mexican fires seems to have changed the way that cloud droplets and ice crystals formed inside the storms. It's when the droplets and crystals bump into each other that an electrical charge gets generated. Scientists think the smoke's impact on the size and number of the cloud particles had something to do with the bumper crop of positive flashes. The same thing has been observed near pollution filled big cities, and near forest fires like the Yellowstone fires in 1988. Assuming there's no big fires this spring, the storms should be back to normal across the southern Plains, so the news we bring about lightning will be mostly negative.

Thanks to contributing writer Robert Henson. The Weather Notebook is underwritten by Subaru with major support provided by the National Science Foundation.