With the summer months on approach–and maybe your summer breaks if you’re in school–you might be setting up to go on a camping trip. Maybe you’re planning on just having a bonfire and drinks with your friends. However you decide to set up your fire gathering, the experience is basically universal. You know the one, it’s what brought you here: the smoke from the fire somehow always makes its way directly into your eyeballs. So what’s up with that? Why does the smoke of your friendly little fire always seem to follow you around?
Air and Density
You’ve probably heard this assertion before: hot air rises. You might also know why this happens, or at least a very quick version of it. Hot air is less dense than cold air, and less dense things rise.
Okay, but what does that actually mean? When you unpack it, we’ve really just defined “hot air rises” twice. Anyway, we have to start with talking about temperature and molecules. Everything is made up of molecules, and those molecules are always vibrating. If you’ve heard of absolute zero being the lowest possible temperature–where molecular vibration is at its bare minimum. Conversely, the more molecules are vibrating the hotter something is. This is why hot things melt and eventually turn into gas. When something is solid, its molecules are still enough to keep a shape. When it’s a gas, those molecules are vibrating so much they’re basically just bouncing all over the place. Temperature, then, can be thought of as a measure of how much molecules are vibrating in a given sample.
Back to air (this applies to other things too), hot air is less dense. Got it. This is because as the air heats up, its molecules begin to move more–and they take up more space. The same amount of molecules take up more space by virtue of their movement and thus, the air is less dense.
Remember this as we continue: while less dense stuff rises, that also means more dense stuff sinks. The reason why this occurs is because everything is always pushing back up against everything that is above it (and vice versa). Think about being deep underwater and feeling the water pressure squeeze on your ears a bit. All the water above you is crushing you, and your body is pushing back to maintain its shape. So you can think of it like all the air on top is compressing the air on the bottom, making it more dense. This also pushes dense air down, making it sink.
The Hot Air Around Your Fire
What a concept, the hot air around your fire is also less dense than the air around it. Which means it has to rise, since your fire is low to the ground (probably). When this air rises, new air has to take its place and immediately fills the space. This should be familiar if you know how a convection oven works (if you don’t that’s basically how it works).
Because new air is filling space where the heated air rose, this creates a little air current. That’s what blows the smoke from your fire upwards.
So Why Does the Smoke Like Me?
Turns out, it’s actually 100% your fault the smoke follows you. Because cooler air has to replace the rising hot air, you can imagine your fire is sucking in all the air around it. Unfortunately for both you and your fire, you get in the way. Your fire is taking in air from all sides equally until you stand near it. It’s like a circle. With you in the way, you create a sort of “wall” that keeps air from getting to your fire as quickly (in relation to the places you’re not standing). The air behind you can’t get to the fire. This difference might not seem like a lot, but it’s enough to create a low pressure area that affects the rise of the fire.
Remember the circle thing we mentioned earlier? Well if your fire has no low pressure areas, the hot air (and thus the smoke) will go straight up. This is because at every point, the air being pulled towards the center of the fire has an equal force going in the opposite direction. With you standing in the way, you disrupt that equilibrium, and there’s now nothing keeping the air from the other side getting into your face.
You might not have started the fire, but the smoke definitely got in your eyes. Still a good song though–see if you know all the things Billy Joel namedropped here.