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Imagine: You’re having a good time around a fire when out of nowhere,
the smoke blows straight into your face.
Naturally, you just get up, you move to get out of the way.
You go back to having a good time again,
and the smoke is suddenly blowing right back in your face!
It was over there when I was over there!
Now I’m over here, it’s over here!
You and the people near you, you move out of the way, everything’s fine for a bit.
Until, once again, your eyes start to sting.
Eventually, everyone gets huddled in the one place
that seems like it’s been smoke free.
But you know what happens next.
Your friends say that you are cursed
because you brought the smoke with you as you moved.
There is no curse, which is honestly too bad.
Because curses can be broken, but laws of physics cannot.
So all that smoke starts with fire.
And fires release a lot of energy as heat,
and air molecules nearby absorb some of that heat and start to move faster.
Since temperature is just an average measurement of how much
energy molecules have, that means the air starts heating up too.
Those speedy molecules collide with their slower neighbors
and send those neighbors flying off in the opposite direction,
freeing up space for the hot air near the fire to expand.
This whole chain of events results in the air near the fire
becoming less dense than the air around it.
Now, you may have heard that less-dense air rises, and that is true.
It's usually better to live on the top floor in the winter
and in the basement in the summer.
You also may have heard that less-dense air rises
because it weighs less than the air it replaces,
which is also true, but it's more of a definition than an explanation.
It just means that less-dense air rises because it's less dense.
The reason dense stuff sinks is because as you get
farther above the ground, there is less and less air above you.
So air down at the ground has to push with a lot of force
to support the weight of all the air above it.
And it's not just air!
Everything down here at the ground pushes against the weight of the air above it.
Now, if you were light enough that the force of gravity on your body pulled you
down less than that air pushed you up, you would lift off like a hot air balloon.
But you’re not: gravity pulls you down more than the air pushes you up.
So you sink.
The hot air around a fire, on the other hand, is less dense than the air around it.
Gravity pulls it down less than the air around it pushes it up,
and it rises into the sky.
This is called convection. It happens in the Sun; it happens in the Earth;
it happens in your oven, whether or not you have a convection oven.
And, most importantly, it happens around a fire.
And the real key to understanding why smoke keeps blowing in your face is that
the rising air doesn’t just rise on its own.
It is pushed by the air around it.
Because when hot air moves away from the fire, the air that pushed it
keeps right on going and fills the gap that the hot air left behind.
And this is where you come in.
You might think that you’re just sitting by the fire roasting some marshmallows.
The fire is affecting you, but you’re not affecting the fire.
But you are.
You form a wall that creates a kind of vacuum.
If you were not there, air that comes from where you’re sitting
and air that comes from across the fire would meet up at the fire and stop there.
But you are there, so your body blocks
the air behind you from moving toward the fire.
So there’s nothing to stop the air coming toward you from moving right on through
the fire and into your face, bringing any smoke or soot from the fire along with it.
And since it’s you that’s blocking the air, you block the air wherever you move to.
Even more annoyingly, the vacuum is stronger when you have
more people in one place because more people block more of that airflow.
So you’re better off on your own than huddled with your friends
if you want to avoid getting all that smoke in your eyes and lungs.
Or you could just get rid of the smoke in the first place.
Then, the vacuum would just send warm air your way.
And that’s the idea behind smokeless fire pits.
Smoke is mostly compounds in wood that didn’t burn completely,
either because the fire isn’t hot enough or because there isn’t enough oxygen.
Smokeless fire pits solve the second problem by creating additional paths for
incoming air, reducing or even eliminating how much smoke a fire produces.
If that’s not an option, your next best bet is to bring a protractor.
And the more protractors, the better the party.
That’s what I always say.
Without any outside wind, having everyone spaced out equally
means that the same amount of air is blocked from all directions,
and nobody’s vacuum is stronger than anyone else’s.
It’s really like a perfect pandemic activity: you’re outside,
you’re breathing into a fire plus, if you get too close to
your friend, you get punished with a bunch of smoke in your eyes.
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Thanks for watching!
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