It’s a question you may have never asked, or wondered about, but now I bet you’re wondering.
As it turns out, it’s an unexpectedly loaded question, too!
In order to answer, you have to know where the sky is. Where does it start? Where does it end?
You might say “that’s easy! the sky goes to the edge of the atmosphere, and then it’s space?
But then you need to know where the atmosphere ends! Is there an edge? At what point does it become space?
I’m going to attempt to answer every connected question in this post. Buckle up your brain buckets!
Where does the sky start?
That’s actually probably the easiest question to answer out of all of these. It starts at your feet! Weird, right? I mean, most of us use the word “sky” to talk about what we see above us, right? But another word for sky could be atmosphere, and the atmosphere is technically anything above land or sea.
But much like an onion, the atmosphere has layers.
The layer that starts where the ground ends is the troposphere, and it continues up somewhere between 5 and 9 miles, depending on how high or low the earth’s surface is any particular place. This is where we live, breathe, fly kites, experience most weather, and is the densest portion of the atmosphere, which should tell you just how thin the air is the higher up you go!
Where the troposphere ends is where the stratosphere begins, but that doesn’t mean there is some super obvious line in the sky that separates the two. It’s more like a slow transition.
Think about night and day, or moreso, dawn and dusk.
It doesn’t go from being sunny to dark in a split second. It isn’t like someone flips a giant light switch in the sky. It’s a slow fade.
The stratosphere continues up for another 31-ish miles.
The ozone layer, which you may have heard of before on account of the big giant hole in it, absorbs and scatters ultraviolet (UV) radiation from the sun.
From there, we’re in the mesosphere for another 53 miles. Ever see a movie where a meteor is entering the atmosphere but it burns up? That’s here! At this point, we are 93 miles above the ground.
And still going up.
Now we’re in the thermosphere which extends out for another 372 miles! This is where the northern lights put on their show and most satellites orbit.
And still, up we go.
Oh, I almost forgot!
There’s another layer, sort of a background one that shares its space with some others. The ionosphere is a layer of electrons, ionized atoms, and other molecules that starts about 30 miles up from the ground and stretches out to the edge of space nearly 600 miles up! It’s super important for keeping our relationship with the sun healthy and is also what makes things like radio communication possible.
Back to the edge of the thermosphere, we’re now entering the exosphere which continues on 6,200 miles above the ground you stand on. That’s nearly the distance from Houston, Texas to Tokyo, Japan!
So, where does the sky end and space begin?
This is another seemingly simple question with an oddly complicated answer in that there are multiple.
Some suggest it’s where normal planes can no longer fly, some say it’s only after you’ve left our atmosphere entirely, and some say it’s when you’re no longer affected by earth’s gravitational pull.
Let’s quickly look at each!
The first boundary to space: the Kármán line.
This “line” sits 62 miles above the earth’s surface. It’s the point at which pilots become astronauts. At this height, the air is too thin for normal airplanes which rely on lift to fly. To fly above that height you need a special propulsion system.
The line itself is located in the lower thermosphere. Reminder: the thermosphere ranges from 50 to 440 miles up!
NASA and the US military though consider the boundary to space to be at the border of the thermosphere 50 miles up.
If you don’t consider it to be space until you’ve left even the exosphere some 600 miles up, then that would mean the international space station isn’t actually in space, which is a major bubble burster for anyone who’s spent any time there or dreamed of going there.
Which brings up another weird point about the international space station. We’ve all probably seen footage of the astronauts on board floating around weightlessly. But they’re still in earth’s orbit, meaning they’re still affected by its gravity, so what gives?
Turns out, the astronauts aren’t in zero gravity at all. Gravity at that height still has 90% of its strength. Instead, they are falling…really fast! Literally speeds of 5 miles per second and since they, and the space station, are all falling at the same speed it looks like they’re floating weightlessly.
Think about being in a car driving down the highway at 70 mph and you drop your phone or toss a ball in the air and catch it. Neither goes flying out the back window, right? Because you’re shielded inside of a vehicle and you aren’t dealing with air resistance then everything is moving the same speed.
It’s all relative!
So where is space? It depends on who you ask.
What Makes the Sky?
Aside from when it’s windy, we hardly notice the air around us, and just how much of it there is! We don’t exactly feel the surprisingly tremendous weight of the sky, and for good reason.
The atmosphere is mostly a gas which means that its particles and molecules are far apart from each other most of the time. When they’re close together, we can more easily see they exist, like in clouds!
Before we can answer the question of how much the sky weighs, we have to think about what exactly weight is.
Everything that is made of matter has a mass, which is sometimes used interchangeably with weight, but the two are different.
Every atom has a mass. Every atoms mass is determined by how many electrons (small negatively charged particles), protons (positively charged particles in the center, or nucleus), and neutrons (no charge particles at the atoms center) it has.
An adult human is made of nearly 7,000,000,000,000,000,000,000,000,000 atoms.
So, their mass would be the combined mass of all of those atoms. No matter where that person is in the universe, even floating weightlessly in space, their mass stays the same.
In other words, mass a measure of the amount of stuff that makes something up.
Think about a bag of skittles which has about 56 pieces of candy inside. No matter where you go, that amount doesn’t change.
But, what does change? The weight.
That’s because weight is a measure of mass + the influence of gravity. Planets with less gravity are planets where you’d weigh less.
Your weight when you stand on a scale is your mass+the force of gravity pushing down on that mass.
So, if we’re going to think about the weight of the sky, we have to consider the mass of all the skies particles under the influence of gravity.
What does all that air add up to? 5 million billion tons.
Insane sounding, right? Think about it like a bed of nails.
Imagine I have a board with 200 nails. If I weigh 200 pounds, and lay down on the board with my weight distributed evenly, each nail only has 1 pound of pressure pushing down in it. No big, right?
Fortunately, that 5 million billion tons is distributed pretty evenly over the entire earth’s surface!
Even still, the average weight (the atmosphere+gravity) pushing down on you is 14.7 pounds over each square inch of your body.
You may have seen psi before, as in 14.7 psi. That’s 14.7 pounds per square inch.
An average adult’s skin surface area is roughly 2800 square inches, which means they have a total of 41160 pounds of force on them at all times.
Thing is, I’m assuming you’ve grown up in earth, meaning your body is adapted to that amount of pressure and shrugs it off like it’s nothing. Most of us don’t know anything different!
If you’re having a hard time grasping that idea, try to think about the ocean instead. Imagine instead of being under the air, you’re a fish underwater.
All that water over you weighs something, right? But you’re a fish! You can handle it! Your body has been forged by millions of years of evolution to adapt to your environment. A fish doesn’t feel the weight. It isn’t crushed by the weight, but a human can only swim so deep before the pressure becomes too much. So, how do we get deeper into the ocean? Specially designed and pressurized vehicles like submarines. For almost fish, entering the air is a lot like us being in the upper atmosphere without a special suit.
So, why is the pressure higher the deeper you go in the ocean? Because you have more mass over you being pushed down by gravity!
There’s simply more stuff above you.
The higher you go into the atmosphere, the thinner the air becomes and the less the pressure is, because there’s less and less mass above you!
Liquid is a lot like gas, but the molecules that make it up are much closer together, so it’s much thicker, or more dense. The reason the air is thinner the higher you go is because gravity isn’t packing particles together as tight.
Think about a bucket of sand. All the sand at the bottom becomes packed tight and firm, but if wind blows over the top of the bucket, sand will blow out, because the top sand isn’t under the weight of anything other than the air, while the bottom sand is under the weight of the air and each piece of sand above it.
Now, there’s a lot it also connects to temperature and density, as well as a couple of other things, but I’m going to save those conversations for another time!