Bill Burton: It's time for us to take a look at the Science Behind the Forecast as I am joined by WAVE 3 meteorologist Tawana Andrew. Good morning, Tawana.
Tawana Andrew: Good morning. Today's topic is a little bit shocking, you could say.
BB: Shocking, hair-raising, all of those things would be appropriate, because we're talking about lightning today, and one very specific type of lightning, negative lightning. What do we need to know?
TA: So for lightning to happen, of course, you need a thunderstorm. And within a thunderstorm, you have updrafts carrying tiny, little water droplets from the lower regions of the cloud all the way up to about 35,000 to 70,000 feet. Now, at the same time, you have down drafts, bringing hail and ice from above. Now, eventually these are going to collide, and once that happens, those tiny little water droplets freeze and release heat. This is what creates soft hail, or graupel.
BB: Ooh, graupel. Anytime we get to talk about graupel, I'm happy.
TA: It's one of my favorite weather words, but yeah. So as the graupel interacts with the water droplets and the ice particles, electrons are sheared off and eventually accumulate on those particles that are falling within the cloud. That creates a negatively charged cloud base and a positively charged top. So when the positive and negative charges in a cloud separate, that actually creates an electric field between the top and the bottom of the cloud. This separation strengthens the electric field, and it takes a lot of energy before lightning can ever happen, because the atmosphere is an incredibly strong insulator.
Once that electric field is strong enough, it breaks through the atmosphere, that is what's causing lightning. And about 70-80% of lightning actually occurs within the cloud itself. It never actually leaves the cloud. So for these storms where you have that negatively charged storm base, a positive charge accumulates on the Earth's surface. It travels along with the storm, which actually contributes to cloud-to-ground lightning.
A typical cloud-to-ground lightning strike begins, of course, within a storm where you have what's called a step leader that develops. As it moves towards the ground, it travels about 160 feet in a microsecond. So it's moving very far, very quickly.
Now keep in mind that this step leader has about 100 million volts within it, so it is a lot of energy in a short amount of time. And we are watching it as it's going closer and closer to the ground. It pauses for about 50 microseconds between each step, searching for its target. So if it doesn't find one, it just keeps going and it keeps going and it keeps going. And a single leader can consist of over 10,000 steps.
As it approaches, a positive charge will accumulate in the ground and within other objects, and that creates a channel called a streamer. When these connect, that is what causes a lightning strike to happen. So the streamer is positively charged, and as it connects with that step leader, that happens around 100 to 300 feet above the ground, and then the leader's negative charge will flow down the channel, creating a return stroke. The electricity in that is about 30,000 amperes that shoots in just a microsecond. So it's a lot of energy, and the return stroke accounts for over 99% of a bolt's brightness. So this is what most people think of, typically, as lightning. So it actually goes from the ground to the cloud, not the other way, but because of the way our eyes work, it looks like it's going from the cloud to the ground.
BB: Can't see something going that fast.
TA: Exactly. So fun fact, when you're talking to friends: Lightning doesn't quite always connect to the top of the cloud, since the cloud itself isn't conductive, it just kind of branches like a tree down towards the ground, and that is what we know as negative lightning. Now there's also positive lightning, which, if you want to know about that, you got to tune in next week.
BB: Ooh, a cliffhanger. To be continued. But now we understand negative lightning thanks to this edition of Science Behind the Forecast with WAVE 3 meteorologist Tawana Andrew. Thanks for the knowledge, Tawana.
TA: Of course.
