Weather Blog

Why does lightning zig-zag?

Why does lightning zig-zag?
A nice simple topic for today's blog :) Despite lightning being around since the dawn of weather, there are many parts of it that are still not quite understood.

For instance, the general idea of lightning being formed as water and ice droplets move around inside a cloud and knocking off electrical particles (much like how you build up a static charge when you scuff your socked-feet on carpet) is pretty sound. But how those particles get knocked off and why is still somewhat of a mystery. But once you've knocked off the electrical particles, you now have some positive and some negative charged ones. The negative ones (electrons) tend to settle near the base of the cloud, and since opposites attract, it'll start to grab the attention of positively charged particles on the ground.

The bolt is created when the buildup in electrical charge is enough to create a channel between the two opposite charged particles. Typically a stream of the negative ions race to meet the positive ones, although the positive ones move toward the negatives ones too, only slower. ("Slow" is, of course, relatively speaking. We're still talking fractions of seconds to just a few seconds)

Once they meet, ZAP! You see lightning. Thunder is just the sound from the "explosion" of instantly heating air to several thousand degrees.

But why isn't lightning a straight line? Because in electricity, the goal is to take the path of least resistance, not necessarily the shortest distance. Particles in the air, or even water droplets, can affect the path of the channel the bolt will take.  That makes for the zig-zags.  

The forks in lightning are also formed from multiple paths that were created between the negative and positively charged particles.  Think of 100 cars heading from Salem to Longview. Some might take I-5, but you could also veer and take I-205. And some might find their path easiest from take I-84 in Portland to Maywood Park, then go north. And some might decide to detour and take the Sunset Highway to the coast.

This ties into the old debate of "Does lightning strike up or down?" when a bolt hits the ground.  It's actually both -- the positive charges start to "reach up" from the ground -- slowly as we mentioned earlier. They're then met with the electrons racing down from the sky.  So I guess you could say many times the process begins from the ground right before the strike (but then again, it's what's going on in the cloud that is what's making the conditions ripe on the ground to start the process.)  

But the transfer of electricity actually flows back and forth along the channel -- sometimes several times (that's why it sometimes flickers or has a strobe effect). So the answer is "both."

Also, while cloud to ground lightning is the most dramatic, it's not the most common. Lightning also occurs with a cloud (most common), or can even go cloud to just plain air.

The typical bolt can reach temperatures of some 50,000 degrees -- way hotter than the surface of the sun. It also moves about 1/3 the speed of light.

Speaking of which, many get this backwards, but the trick to figuring how far away a lightning bolt struck is to take the interval in seconds between the flash of lightning and the crack of thunder and divide by five.  The speed of sound is about 770 mph, which is 4.06 million feet per hour, or 1130 feet per second. A mile is 5,280 feet, so it's roughly 5 seconds per mile. A bolt that has a 10 second delay before you hear the thunder is two miles away.