I received an e-mail from someone a few weeks ago asking about "ball lightning" as during a mountain thunderstorm, he thought he saw balls of light flash instead of typical lightning flashes.
Ball lightning is as mysterious as it is rare, and he's right in that it really doesn't look like lightning at all. Instead, it's a glowing ball of light that can range in size from about a grapefruit, up to the size of a small car.
They usually form from the most violent thunderstorms. They are a very strange sight to behold, as these balls of light can either roll along the ground or atop fences, or float thousands of feet in the air -- sort of like electrically charged beach balls. They can either last several seconds, or several minutes.
Usually, they don't cause any damage, but have been known to leave burn marks if they pass through windows or screens.
I had a personal experience with ball lightning when I was a kid in Oregon. During a thunderstorm, a group of us kids were in the dining room working on a project (I think it was a Boy Scouts thing) when all of a sudden, a small ball of light just popped out of the TV and exploded in the living room -- brightest and loudest thing I had ever heard. But it only left a few scorch marks on the carpet (and, of course, fried the TV). I surmise the lightning traveled down the antenna -- this was in the early 80s and it was the old fashioned metal-antenna on the roof.
I also spent a few years in North Carolina and would see balls of lightning ignite in the sky a few times -- sometimes you'd see a faint ball drop from the sky a la Times Square on New Year's Eve, then a brilliant flash, then gone.
But the most mysterious thing about ball lightning is, to this day, scientists can't explain what causes it.
Here are some examples of it I found on YouTube: