This wasn’t supposed to be a post on Earth’s atmosphere. I was trying to write about a common but underappreciated misconception about orbits, gravity and the atmosphere (post to follow!). But I kept writing phrases like “above the atmosphere”. And it became clear I was being dishonest. I mean I know that there is no hard ‘edge’ to Earth’s atmosphere, and that it tails off gradually. But the way I think about the atmosphere intuitively isn’t like that at all. I’m a sucker for the ‘the atmosphere is equivalent to the skin on an apple’ explanation. It’s hard to get away from the mindset of ‘atmosphere = something; outer space = nothing’.
So I had to write about this before I could carry on with the orbits stuff. With any luck, this will give you some new insights (if you’re an atmospheric scientist, it might not). There will be a lot of facts, and they can be hard to process, so I’ll display it visually too. It might give the words more impact if you continually flick from the text to the diagram and back (although that may be annoying on a mobile…).
Let’s start with a really ‘busy’ diagram of the atmosphere, and point out its features, imagining what happens in a fictional journey from the ground up. Note the layers ‘troposphere-stratosphere-mesosphere-thermosphere…’ differentiate regions that have different temperature gradients. So in one layer, the temperature may increase with altitude, and it will then decrease in the next.
At sea level, we are at zero altitude. As we zoom up, we pass the top of Mount Everest at nearly 9 km up, where the density of air is one third that at the surface. Everest is still in the region of the atmosphere called the troposphere. In the troposphere, the temperature of air decreases with increasing height, since it is primarily being heated by re-radiated energy from the ground. This temperature gradient causes turbulent mixing, because the lower, warmer air tries to rise through the colder, higher air by convection. And turbulent mixing means… weather!
As we continue up, at an average of about 12 km we hit the tropopause – the boundary between the troposphere and the next layer of the atmosphere, the stratosphere. ‘Hit’ is misleading – we don’t notice it. In the stratosphere, solar heating of the ozone layer causes an increase in temperature with height. Convection currents do not flow, and there is virtually no weather. So the tropopause is just the point at which the temperature gradient reverses direction.
As we pass the tropopause, 75 % of the mass (not the volume!) of the atmosphere is below us. An airliner typically flies in the high troposphere or low stratosphere. When we reach an altitude of 16 km, already 90 % of the atmosphere is below us.
At an altitude of approximately 50 km, the temperature gradient reverses direction again, and we enter the mesosphere. At this point the density of air is 1/1000 that at the surface. In the mesosphere we pass the highest weather balloons, and at about 80 km we enter the thermosphere. Shortly after, we get bored of constantly reversing temperature gradients and ask for some other kind of boundary. And as if by magic, we pass the Karman line…
The Karman line denotes an altitude of 100 km. It is the official boundary between the atmosphere and outer space (I think the US Air Force say that space starts at 50 miles, but the Karman line is used by most organisations, including NASA). The Karman line is also the boundary between aeronautics and astronautics. Fly below the Karman line and you are a pilot. Fly above it and you are an astronaut. The idea is that the Karman line is the critical air density below which wings get enough traction on the air to provide lift, and above which wings are useless and you need a spacecraft, not a plane.
As we pass the Karman line, presumably swapping a plane for a spacecraft, 99.99997 % of the atmosphere is beneath us. Very close to the Karman line, we pass the turbopause. Before we reached the turbopause, we saw an atmosphere of constant chemical composition – constant with height because of gaseous mixing. Above the turbopause, the mean free path of the molecules is so high that collisions are rare, and mixing does not work in the same way. Above the turbopause, the gaseous components of the atmosphere (remember, it’s only about 0.00003 % of the atmosphere…) stratify out according to their molecular weight.
At an altitude of 150 km we can point our spacecraft in the right direction, turn off whatever engines we are using and settle into a circular orbit with no propulsion. If we had tried this lower down, atmospheric drag would have caused us to spiral down toward the surface. If we arrange an elliptical orbit instead, we can get away with a perigee (closest approach to Earth) of altitude 130 km.
And it is around here that the sky goes completely black, although for a few dozen kilometres now, we could have debated the difference between ‘black’ and ‘very dark blue’. We are half way to the International Space Station, by which point the air is so thin that the mean free path of molecules is around a hundred kilometres.
There’s a problem with the diagram above – it isn’t easy to draw it to scale because the inner layers are so much smaller than the outer layers. It’s much like the ‘problem’ of drawing/making a scale model of the Solar System, because of the enormous distances involved. If you draw the planets to scale, you can’t put them the right distance apart (they don’t make paper that big). And if you draw them the right distance apart, they are each microscopic (literally) on your paper. Likewise, the diagram above is fine up to about 150 km, and then has to compress the scale to fit in the exosphere. If we had drawn it to scale, we wouldn’t have had room to put on the facts, most of which apply to the lower atmosphere. So let’s strip away the facts, leaving the ISS, and show it to scale… The blue-black gradients in the two pictures correspond to each other.
Although we have given facts like ’90 % of the atmosphere below 16 km altitude’ and ’99.99997 % below the Karman line’, it’s still not that easy to grasp. So below is a visual representation of how the density of the atmosphere tails off with height. It’s on a completely different altitude scale from the two diagrams above, and only extends to about 30 km high. Remember the tropopause – the boundary between the troposphere and the stratosphere? Find it in the picture above, and then locate it in the graph below, just to orientate yourself with the scale change. Once again, the blue gradient is meant to help…
The red line shows the air density as a function of height. If the temperature of the atmosphere were uniform, this red line would be an exponential decay curve (see here for what that really means). The temperature variations with height make the drop-off with height a more complicated function than a pure exponential. In any case, look at how thin the air becomes 20 – 30 km up. In the first diagram we said that the density at 50 km is 1/1000 the surface density. Look at what that actually looks like in graphical form (in fact, the graph stops well before 50 km, and is almost indistinguishable from zero at this scale!).
Going back to the ‘atmosphere is to Earth as skin is to apple’ analogy, imagine for a moment that the atmosphere didn’t tail off gradually. How high would it be if it’s density were constantly at the value of the surface density. Well, as shown in the graph the ‘edge’ would be at a height of 8.5 km. Mount Everest would poke through it into the pure vacuum of space. You can think of this as follows:
- The actual density profile is the ‘light green plus yellow’ region under the red line.
- If you could ‘pick up’ the distant part of the atmosphere shown in yellow, it would ‘fit’ in the dark green region
- You would then have the green (light plus dark) region, which represents an atmosphere 8.5 km high and of uniform density 1.2 kg m-3.
Just to try to reconcile the different altitude scales we have used, here is the same graph, but with a scale that goes ten times as far, up to the ISS, well into the thermosphere.
The red line seems to hit zero pretty quickly. In fact, the value of density is always falling from left to right, but that fact is obscured by the thickness of the red line, because we are at such a small fraction of the surface value…
Right, I think that’s it – I’ll crack on with ‘orbits’ now. That’s what I meant to write about anyway…
Disclaimer:
This article is trying to paint a ‘big picture’. It collates lots of data from lots of sources. Please enjoy this article. Please DON’T use it as your primary source of atmospheric science data!