CRASH COURSE:

Atmospheric Dynamics For Dummies.

In the process, I realized that knowing the fundamentals of how air works makes everything around you so much clearer. We just don’t teach these lessons in plain English, or in a remotely fun way.

After understanding the rules I’ll go over, you’ll be able to use them all the time and see their applications all around you. Or, you could end up getting ostracized out of your social circle while you try explaining the concept of pressure gradients to your friends. From personal experience, that could be a real possibility.

Hey, What Are You Even Talking About?

Before talking about the technical details behind a term I just threw into the air, it might help if I went over what it actually meant. Well, to describe things in the simplest way possible without oversimplifying things, atmospheric dynamics is the study of how air moves around.

A Little Thought Experiment

Imagine you took Earth out of the constant chaos of space and gently let it levitate in a special container. Inside, no object or force would have any influence over it. No sun, moon, space rocks, atmosphere, or even its own spin from the big bang.

Without some sort of external factor forcing the atmosphere to move, it won’t. Even if something did force it to move, it would eventually come to a standstill. As expected, force is where all motion starts. A lack of force is where all motion ends.

The only question left is where these forces come from.

A Light Review ¯\_(ツ)_/¯

Okay, so we know how force leads to motion, and how motion tends to disappear over time without force. But, the atmosphere never stops moving, and that can only mean one thing. There’s something out there that forces things to move 24/7 here on Earth.

The full range of a photon’s possible oscillation rates forms the electromagnetic (EM) spectrum. That’s where all the colours of the rainbow come from — along with lots of other things we can’t see at all.

On the right end of the spectrum, you’ll see the low-energy/high-λ radio waves that let you listen to your favourite FM channel in your car. To the very left, you’ll see high-energy/low-λ gamma rays. They’re the wavelengths you don’t want your lightbulbs to emit, unless you want to develop cancer in the near future.

When they fall back down, they release their energy by releasing photons. This results in a distribution of photons emitted at different wavelengths and intensities.

If you plotted those intensities and wavelengths, you’d get what we call a Planck curve. Since every object’s made of atoms, everything radiates light in unique curve-like distributions depending on how it’s energized.

The Sun’s Planck curve matches pretty closely to the 5000K curve. Most of its radiation is visible.

A Balancing Act

Anyway, what do you think happens to this absorbed light? How does it end up changing how air moves around? Wanna guess?

“The warmer object (the coffee) would lose its energy to the cooler one (the bag of ice) until they’re at about the same temperature. You’d be left with useless ice and a sad excuse for iced coffee. “

You get the same sort of phenomenon happening on Earth all the time. But you have to remember, zonal heating from sun continues to mess this equilibrium back up. We’re dealing with a never-ending game of order vs. chaos here. You’d get a similar effect if you modified the coffee-and-ice analogy a bit:

This time, picture the coffee boiling on a stove and the ice sitting in a freezer. You’d still see heat transfer between the two, but a new source of external energy prevents either object from losing its original temperature.

Now that’s a lot more realistic.

Atmospheric “Gears”

Let’s try visualizing something else. Let’s go back to our Earth-in-a-box and try adding an atmosphere and a sun into this experiment. Since there’s still have no sense of spin here, you’d end up with a zonal spot of radiation and heating — with the other side of Earth staying completely dark and icy.

Think of those paradoxical gears as the Polar and Hadley Cells. The Ferrel Cell would serve the same function as a third gear you’d put in between the two. It would spin the opposite way and keep things running smoothly.

Unlike the Hadley circulation model, the addition of rotation leads to a circulation system with three distinctly moving air cells per hemisphere. You’ll be able to see this effect at different degrees on any planet that rotates over its own axis. As expected, Earth’s a great example of this.

If Only Earth Stayed Still…

A couple hundred years ago, the French army hired a mathematician named Gustave Coriolis to figure out why their cannonballs kept veering to the right of where they wanted to shoot them.

Now that you’re not part of your Earthbound reference frame, it’ll look like the Earth’s spinning while the universe is dead calm. Notice how nothing’s really “moving” unless you specify what its moving relative to.

Alright, then. We already know what happens when we try aiming “straight” at a target in an Earthbound reference frame. Thats’ what happened to the French army’s cannonballs that never seemed to hit their targets — at least from their point of view.

The object in question can be literally anything . That includes air itself. In atmospheric science, we call a chunk of air with distinct properties an air parcel. As it turns out, air parcels are at the mercy of Coriolis, too.

That means the entire atmosphere is.

We’re also working with volume here. Any increase to that variable has a cubic effect on the amount of air in our parcel. With parcels that clock in at 100’s of km³, the force gets multiplied billions of times over, and that’s where you’ll really see deflection working its magic.

And with that, we’ve unlocked half the puzzle of atmospheric dynamics.

Pressure Systems

Yeah that’s right. We’re not done yet. Not even close.

This tendency for high pressure gases to move towards low pressure ones is called the pressure gradient force. Why? Because this force is exactly what allows nature to balance pressure when there’s an inequality at different locations (AKA, a gradient).

We‘ve got a law to help us work out where these gradient are, and it’s called the Ideal Gas Law. I like to call it the IGL for short.

That’s mainly because the deeper you go, there’s more water above you and pushing down on you. It’s that simple.

The weight of pushing down on you is proportional to the pressure you experience. What I just went over is the the core of what’s known as the Hydrostatic Law. It’s the same story for how air above us exerts pressure on us, but it’s too tiny for anyone to notice.

The Final Piece.

Geostrophic balance.

If we translated the roots of that word from Greek to English, we’d see that it means “constant pressure”. If you were anywhere on that line, both the weight and air pressure on you would stay the same.

If you moved to another isobar, you’d experience a different pressure from the first one, but it would be constant as long as you stay on the line.

But, as the parcel gets pushed faster and faster by the pressure gradient, it’ll boost the intensity of the Coriolis force.

Remember, the direction always stays 90° to the right of our parcel’s motion vector in the Northern Hemisphere. As our parcel moves in new directions from this tug-of-war, you’d update this formula and see that the Coriolis Force ends up pulling the air in a new direction every time the parcel changes its trajectory.

Over time, the Coriolis force acts in such a way on the air parcel so that its magnitude matches the pressure gradient force, but tugs the parcel in exactly the opposite direction. That’s geostrophic balance.

That leaves the parcel in an interesting situation. The pressure gradient force was originally pushing it, but the Coriolis effect didn’t let it move that way. But, since your parcel’s still experiencing lots of force, it has to move. And so, it end up moving left in this pressure field — parallel to the isobars.

Isobars that are tightly packed together will have a higher pressure gradient vs. the same isobars spaced further apart. That’s because you’ll get that same 12 mBar pressure change over a shorter distance.

Since the pressure gradient’s much higher in tight groups of lines, that means the pressure gradient force’s going to rise along with it. A stronger pressure gradient force means faster geostrophic winds that run along our isobars. That’s exactly what we see.

Making Connections

Over the past few sections, I covered how you needed a strong Coriolis force for large cyclones to form, since weak Coriolis forces can only cancel out weak pressure gradients. Scrolling back up to our Coriolis formula, you might make the guess that there should be lots of cyclones at the Poles.

Wonky Winds

I have a feeling we’re good with cyclones now. Let’s go back even further to another application of the Coriolis Force.

When cool air at Earth’s surface moves toward the equator to complete the Hadley Cell, it gets deflected.

That’s one of the perks of living on a spinning planet, and I absolutely hate it.

When an air parcel cools, its maximum holding capacity for water vapour decreases, and any excess vapour condenses into a cloud. This whole process of air cooling when it rises is called adiabatic cooling.

The exact opposite happens when air sinks. That’s right. Adiabatic warming. Cool, sinking air above high pressure systems usually means clear skies and nice weather. That’s because air parcels compress as they get closer to the ground and the environment transfers energy into its particles — making them vibrate faster and get warmer.

Sea…horses?

Speaking of latitudes, that reminds me of a fun story. If you want to hear about one of the rare occasions where an unexpected, creative name came out of science, I’ve got a classic for you.

A couple of hundred years ago, when European tradesmen relied on sail-ships to travel across oceans, there was one place they absolutely hated. It was the boundary between the Hadley and Ferrel Cells — better known as 30°N.

Usually, sailors relied on the power of the Westward winds in the Hadley Cell to get to islands in the Caribbean. Sometimes, though, they ended up drifting off to its boundary along the Ferrel Cell without knowing it. Here, we get an area of high surface pressure from the sinking air between the two cells:

Well, everyone had the bright idea of getting rid of the horses they kept on board. After all, horses went through a lot of food really quickly and weighed the ship down quite a bit. That’s exactly why they threw them off into the Atlantic Ocean.

I mean, that’s not exactly the first step I’d take if I was stranded at sea, but who am I to judge?

Constant Flux

That was pretty grim. For a change in mood, let’s revisit how the sun and Earth stay in their infinite balancing act.

That’s because, as more water weighed down over it, the pressure at the drain increased and the water’s outflow rate increased, too.

Just for fun, what’d happen if you set the sink to spew out water as fast as it could? Depending on the sink, the water would rise even higher, or even overflow. If you had a large enough sink to contain it, though — even that water would reach a steady level. It would be a level where water’s flowing in as fast as it’s draining out. We call that a steady-state.

This whole scenario makes a really huge assumption, though. It assumes that Earth’s a blackbody. This means we’re assuming Earth absorbs and re-emits all the light that hits it. That’s not entirely accurate

For one, a lot of photons get reflected off Earth before they ever get absorbed. That’s because Earth isn’t a blackbody. It has an average albedo (or reflectivity) of about 33%, from formations like clouds, ice and deserts. For every hundred photons you’d track, only about sixty-six or sixty seven would ever contribute to heating Earth up.

It’s no surprise that Earth’s cooler than the Sun. With a quick calculation using Wien’s law, we can tell that it radiates most of its energy as infrared light at a wavelength of about 10 μm.

Greenhouse gases (GHGs) are compounds that have the ability to absorb IR light. When you have enough of them in the atmosphere, they stop Earth from getting rid of its excess heat energy. Isn’t that interesting? GHGs let light from the sun pass through since its mostly visible, but at the wavelength where Earth radiates it back out, they absorb it.

“Are You Done Yet?”

Yes.

Even so, what I’ve covered is the essence of the atmosphere. If you understand these concepts well enough, then you’ll be surprised at how intuitive any extra information is to learn. It doesn’t even have to be about the atmosphere at all.

Air’s a fluid, and that means you also have a good idea of how water behaves now, too. Now, you even know about the sun’s radiation, wind patterns, and global warming, too. Not to mention, you’ve got some powerful equations up your sleeve.

I write about things every week(ish).

I write about things every week(ish).