KABOOM! Lightning is Spectacular!

Here's a time lapse photo of a lightning storm over boston, taken by the National Oceanic and Atmospheric Association.

Hello there! I’m sorry for the long delay in new content. I’m in the beginning stages of constructing a Van de Graaff generator. Don’t worry – there will be video, and it will be awesome.

Anyway, I wanted to start a new series of posts. The subject of this series is “amazing stuff right here on Earth.” I think when we have something we are commonly exposed to, we often forget the really remarkable characteristics that it may have. This is the case with water, a ubiquitous but really wonderful chemical, and this is why I wrote that series on water. And so I want to do the same thing with the planet Earth! People get all worked up about the volcanic activity of Io or the great storms of Jupiter or the rings of Saturn. But our own Earth is home to a huge variety of astounding and even mysterious phenomena! To prove this point, I’m going to start off this series with everyone’s favorite electrostatic discharge, lightning! Everyone has experienced lightning, but how many of us really know what it is, and how it forms? Judging by this site’s traffic, that number might just increase by 1-2 dozen! Let’s get to it!

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A bolt of lightning is a spark that connects the negatively charged bottom of a storm cloud with a positively charged surface (the Earth!). So let’s step back a few levels.

You’re probably familiar with the structure of the atom. An atom consists of a positively charged nucleus orbited by negatively charged electrons. Well, when two atoms or molecules collide, there’s a chance that some of these electrons will be ejected from the nuclei, resulting in a separation of positive and negative charge, where before you had a both in one neutral atom. The positive charge rests among the protons in the nucleus, and the negative charge with the now free electrons that have been bumped out.

In a cloud, molecules of water vapor are constantly rising and falling. As water on the ground gets heated up (whether by the sun or by you lighting a fire under a potful), molecules of water escape the liquid and fly through the air. This escape from liquid form is what we call evaporation. Now, once the molecules have evaporated, they travel upwards. In fact, the whole mass of air and water vapor just above the ground moves upward in a convection current. A convection current is just the movement of fluid (that’s liquid or gas) that you get when you apply heat from one side. The heat causes the fluid to expand, decreasing its density. The fluid in the surrounding area is not heated (or is heated much less), and so it stays the same density. So you have a parcel of air that is hotter and less dense than its surroundings. If you know how buoyancy works, you know that things which are less dense than their surroundings float, and this is just what happens with the heated air and water vapor. Now as the vapor rises with the air, the vapor molecules bump into other molecules. During these bumps, they may lose their electrons, becoming charged ions!

So does the water vapor just continue to rise into the air until it escapes from the Earth? No! And thank goodness, because otherwise there wouldn’t be much water around for us to survive off of! No, what happens is the water vapor continues to rise until its concentration in the air is greater than its solubility in the air solution. Let me explain what I mean by that.

Solubility is the measure of how much “stuff” you can dissolve into a uniform fluid like air or water. So for example the solubility of simple table salt, sodium chloride (NaCl) in water at 25°C is 35.9 grams per 100 milliliters. What that means is that a glass with 100ml of water could dissolve exactly 35.9g of salt. If you put 35.9g of salt into that glass, you would see it all disappear. You could also put 359g of salt into 1 liter, or 3590g of salt into 10 liters, and all the salt crystals would disappear. But if you added even one gram more in any of these examples, you would see salt crystals that would never dissolve. This is because the concentration of salt has exceeded its solubility in the fluid. We say the fluid has reached its saturation point, and so any further addition of solute will not dissolve. Now here’s something really important: if you increase the temperature of the solution, the solubility of the thing being dissolved usually increases (there are some exceptions). Likewise, if you decrease the temperature of the solution, the solubility usually decreases. For example, while salt’s solubility at 25°C is 35.9g/100ml, its solubility at 100°C is 39.1g/100ml and its solubility at 0°C is 35.6g/100mL. In practice this means that you can dissolve more salt in hotter water. So if you were to put 39.1g of salt in 100ml of water and raised the temperature of the water to 100°C, all of that salt would disappear – dissolved. Now if you cooled that water to 25°C where the solubility of salt is 35.6g/100ml, some of that dissolved salt would come out of solution as salt crystals. In fact, 3.5g would come out of solution since that is the difference between the solubilities of salt at 100 and 25°C.

Well like I said before, air is a fluid, and it has a saturation point. And water vapor has a solubility in air just like salt has a solubility in water. And guess what? Temperature affects the solubility of water in air just like it affects the solubility of salt in water. Raise the temperature and you can get more water dissolved in the air. Lower the temperature and you can get some of the water to come out of the solution as water droplets. So let’s get back to water vapor. When we last left it, it had been heated by the sun and was rising along with a column of air due to convection.

As the water vapor gets higher and higher into the atmosphere, the temperature of the air decreases. As the water vapor gets into the colder air, some of the water molecules come out of solution, just like salt crystals coming out of solution when we chill the hot salt water. As they come out of solution, they form hydrogen bonds with their neighboring water molecules.  The formation of these bonds is exothermic, which means that when the bonds form, energy is released into the surrounding air and water vapor.* This energy heats up the air and vapor and sends it further up into the atmosphere, where the temperature is even colder! Now even more water vapor comes out of solution, forms hydrogen bonds, and boosts the parcel of air and vapor still further up into the atmosphere.** Eventually, at cold enough temperatures and high enough altitude, you run out of water vapor and this condensation-boost activity ceases. At this point gravity takes over and starts to pull the water droplets and ice particles that have formed back down to Earth. As they fall, they pass through warmer and warmer air, and are able to dissolve again as the temperature rises. This sends them back up through the cloud for another spin. As they come back down, they will dissolve and head skywards once more. Because of this cycle, each water droplet makes many trips through the cloud. The result is many opportunities for collision, electron ejection, and the production of positively charged nuclei and free electrons.

This diagram demonstrates not only the cyclic transit of water droplets in clouds, but also my enviable ms paint skills! Make sure to click for bigger.

Once this breaking apart of electrons from nuclei has occurred, the charges are separated in the storm cloud, with positive ions collecting at the top of the storm cloud and electrons gathering on the bottom. Why exactly this happens (and indeed even how the charges separate in the first place) is still up for debate. Isn’t that cool? Phenomena that occur all around the world and that have been observed by humans for a long time are still not understood! The world is still full of mysteries waiting to be solved, and you could be the one who solves them! Ahhh!

Anyway, the charges separate. And they separate so that there’s a lot of positive charge at the top of the storm cloud and a lot of negative charge at the bottom of it. So you can imagine the bottom of a thunder cloud brimming with electrons, while the top part of it has lots of atoms without their electrons (positive ions). Now, you probably know that like charges repel, and this physical law is very important to how lightning works. The big bank of negative electrons at the bottom of the cloud pushes the electrons in the surface of the Earth downward, deeper into the crust of the Earth. The result of this is that you now have a huge bank of negative charge at the bottom of the cloud and a huge bank of positive charge at the surface of the Earth.

Another stunning masterpiece illustrates the seperation of charges that makes conditions right for lightning to strike!

Now, whenever you have a huge difference in charge like this (or if you like, a high voltage), it will be resolved if the charges have a path that they can meet through. For instance, if I ran a copper wire from the base of the cloud to the Earth, the electrons in the cloud could flow down into the Earth and neutralize the difference in charge, and in fact this is one tested way of triggering lightning. Metal is a good conductor, meaning electrons can flow through it very easily. It’s a good conductor because the electrons in a metal are not tightly bound to their nuclei – they can move about a bit, and if there’s a reason that they should be moving (say a big positive charge attracting them), those electrons can flow right through the metal. But air, like the air separating the cloud from the Earth, is a very bad conductor (we call it an insulator for this reason). The electrons in the molecules making up the air are bound very tight, and they resist being moved around even when there is a positive charge attracting them. Add to that the fact that gasses at room temperature are about 1000x less dense than liquids and you should be able to understand why air is a poor conductor.

However, even air has its breaking point. If you get enough of a charge difference (high enough voltage), the air can become ionized as the massive banks of charge rip electrons away from nuclei and send them careening into other nuclei, often freeing more electrons. Once the air is ionized, it becomes a much better conductor, and electrons can flow through it just like electricity through a wire. This is what happens when lightning strikes. First, the huge difference in charge causes the air to ionize in the immediate area of the cloud base. Then, the air continues to ionize in a branching path down to the Earth, searching out the positive ground. You might expect the lightning bolt to travel straight down to Earth (as we know a straight line is the fastest path between two points), but instead it forks and branches and gives us the lightning pattern we all recognize. This tortuous, branching path is the path of least resistance, the path with the most ionized and best conducting air. So when you look at a forked lightning bolt, you can tell that wherever the bolt goes is the path that best conducted electricity at that moment in time. Now this ionized air with electrons flowing through it (called a stepped leader because it proceeds in discrete little “steps” of movement) continues to branch and fork and wind its way toward the Earth and those positive charges it so longs for. At the same time, that positive charge in the Earth begins reaching up towards the clouds, and for the same reasons. The air becomes ionized and the positive charge can climb upwards. These upward reaching paths of ionized air are called positive streamers, and they look like small, purplish lightning bolts. You can actually see them! They usually form at the top of tall, conductive objects like trees, lightning rods, or even humans.

Here you see the stepped leaders coming down and the positive streamers headed up. When they connect... BOOM!

Now check out this high speed photography video of a lightning strike. You’ll see the stepped leaders coming down, branching a lot. Eventually one reaches the bottom and KABLAMMO! The charges drain and you get the massive flash.

Finally, after much searching, one of the branches of the ionized air touches a positive streamer! The circuit is completed, and electrons in the cloud and in the rest of the branches flow rapidly down the path of ionized air in a blinding flash of light and heat. This electrical discharge, this super fast transit of electrons from negative cloud bank to positive ground, is what we call a lightning bolt. It is a force to be reckoned with! In a single strike of lightning, about 30,000,000,000,000,000,000 electrons rush from cloud to Earth, superheating the ionized path of air that it travels through and dealing tremendous damage to anything it touches.*** Sand, for instance, is fused into glass. When trees are struck by lightning, the flow of electricity superheats the sap of the tree, which expands rapidly, causing the tree to explode spectacularly!

Look at this tree! Lightning is POWERFUL!

Check this out! The scars on this unlucky victim's back are a permanent record of lightning's deadly power.

But wait a minute. Haven’t you heard of people who have survived being struck by lightning? How could they possibly survive if lightning is powerful enough to fuse sand into glass or explode a tree? The key is in the constitution of a person. See, in a tree, the bark of the tree is a poor conductor but the inner sap is a good conductor, so the bolt travels through the inside of the tree, superheating the sap and exploding the tree. But the outside of a human, especially a human that has been coated in water by a thunderstorm, is a very good conductor. As a result, the bolt can travel around the human and into the ground. Still, the effects are very damaging, and the moisture on the person’s skin quickly superheats and boils, giving the person severe steam burns. If the person has a lot of moisture trapped anywhere on their body, for instance in a waterlogged hat or shoes, these items can explode due to the superheating from the lightning bolt, causing terrible damage. But just because a person can survive lightning, doesn’t mean it isn’t very, very dangerous. Indeed, though you have a chance of having a painful but not lethal experience with lightning, you also have a good chance of sudden, painful death! If you’re lucky enough for the bolt to travel along the outside of your body, you still might die from the burns. Or, if it passes straight through you, it will simply incinerate your internal tissues. Another fun option is that sufficient current could pass through your brain or heart, disrupting their electrical flow and stopping your vital functions!

Luckily, there are plenty of ways to stay safe from lightning! Watch weather forecasts and avoid being outside during storms. Don’t wave golf clubs or umbrellas around in the air. If you get caught in a storm, take shelter inside a house with a lightning rod or in a car. If lightning strikes either of these, it will travel down the path of least resistance (the lightning rod or the metal shell of the car) and pass harmlessly into the ground. Never take shelter under a tree, as lightning could hit the tree and cause it to explode right in your face! Yikes. And if you ever find your hair standing up while outside in a lightning storm, you need to take immediate action. If this happens, it means you are giving off positive streamers and are about to be the last segment of the path of least resistance! Immediately crouch to the ground (don’t lie down) and make yourself small. If you’re lucky, the bolt will find another way to the ground and you will live to see another day.

So you see, lightning is really awesome and amazing, and it happens all the time on our little Earth. So next time you’re in a thunderstorm, think about what is really happening each time you hear that rumble, and appreciate the simple and elegant physics of the lightning bolt!

Now, I leave you with fun facts about lightning.

-Lightning travels at 130,000 mph.

-Lightning heats the air it travels through to 54,000°F (30,000°C). That’s hotter than the surface of the Sun!

-Over eight million lightning strikes occur each day on Earth. That means one hundred strikes every second!

-The dust and gas given off by erupting volcanoes rubs together in a similar way to water molecules and can produce lightning as well! Here’s an awesome picture!

I told you it was awesome!

-Lightning bolts have been observed giving off small amounts of gamma radiation!

-There are all kinds of cool and exotic lightning bolts, such as 50km tall “sprites” that reach high into the atmosphere from the top of clouds!

Hooray for lightning!

-Neil

*I’ve searched and searched, but I cannot figure out why bond formation is exothermic. I mean, the given explanation is that when bonds form the molecule that forms is more stable than the atoms or molecules it formed from, and therefore heat needs to be given off to account for this. But no one so far seems to know exactly what happens to the energy released, or what form it takes. If anyone knows this please email me and I’ll add the explanation (dangthatscool@gmail.com).
UPDATE!! Thanks to collaboration with amazing comrade “thisisntcody,” aka the “Hero of the Gardens,” it has been determined that the energy released when a hydrogen bond forms is due to infrared photon emission! When the bond forms, the movement of electrons from higher energy to low causes the production of an electromagnetic wave in the infrared spectrum. That is where the energy goes! These infrared photons strike nearby particles and heat them up, causing the continued upward motion of the air mass. Booyah.

**You may be interested in knowing that the point where the water starts to come out of solution is the lower border of the clouds you see. Below this area there is still water vapor, but it is dissolved in the air instead of out of solution in the form of liquid water droplets. So now if you please, let me stretch the salt water analogy further. If you had a tall glass of water and you could heat it so that you had a temperature gradient with cold water on top and hot water on the bottom, and uniformly distributed a certain amount of salt, you would see that at a certain point, salt crystals form, but below them, despite the presence of dissolved salt, no visible crystals appear. The same thing happens with water vapor, with the clouds being analogous to salt crystals coming out of solution.

***The superheated air expands extremely rapidly, producing a pressure wave that our ears perceive as thunder! You hear a long rumble instead of a single noise because sound waves from the bottom of the lightning bolt hit your ear before waves from the top. In addition, the landscape and varied air densities between you and the bolt distort the wave. The result is a series of noises instead of a single one. Still, if you are very close, you will probably hear a very loud CLAP! instead of that roll of thunder since most of the wave will hit you at the same time.