As Hurricane Matthew reminded us, cyclonic storms are a force to be reckoned with. What causes these storms, and how can they form across the Solar System.
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- Tropical Cyclone Climatology
- Barometric Pressure
- Polar Vortex
- Jupiter’s Great Red Spot
- The hexagon on Saturn
- Hexagon in Motion
Transcription services provided by: GMR Transcription
Fraser Cain: Astronomy Cast Episode 423: Cyclones. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain. I’m the publisher of Universe Today and with me is Dr. Pamela Gay, the director of CosmoQuest. Hey, Pamela. How are you doin’?
Dr. Pamela Gay: I’m doin’ well. How are you doin’, Fraser?
Fraser: I’m doin’ great. Happy Thanksgiving – Canadian Thanksgiving.
Pamela: Happy Canadian Thanksgiving to you and all of our Canadian fans out there. And our non-Canadian fans need to celebrate with you.
Fraser: Including your Canadian husband. You might want to –
Fraser: – remind him where his true loyalties stand.
Pamela: I know he’s planning spaghetti for dinner, so I think he is a bit lost.
Fraser: Yeah. I think so. You should remind him. See how long it takes him to figure it out. How is the eclipse going?
Pamela: You know, we’re already about half full. So, if you want to join us, get on the ball –
Pamela: – because we’re selling out fast.
Fraser: Yeah. And we got a fun Facebook group to go along with it. So you can join that and see sort of when there’s more availabilities and sort of where people are staying and coordinate rides and things like that. So check that out.
Alright. Let’s get on with it. Oh, just go to astronomycast.com and there’s a link to where the trips are. Okay.
So, as Hurricane Matthew reminded us, cyclonic storms are a force to be reckoned with. What causes these storms and how can they form across the solar system?
Alright, Pamela. So I think we’re all pretty familiar with the concept of a cyclonic storm: hurricanes in the Atlantic, cyclones in the Pacific. What’s goin’ on?
Pamela: It’s really nothing more than the pressure of things that have different temperatures creating really awesome gradients, and by gradient, I mean wind.
So, at their most – sorry, whoever’s editing this –
Fraser: Is it Chad?
Pamela: Yeah, I do too.
At their most simplistic level, a cyclone occurs when you end up with a low pressure over warm water, which evaporates the water, which then causes more low pressure as that water expands, gives its heat off to the atmosphere. And this warm water feeds the growth of this low pressure and, well, things spiral into low pressure just like things rolling down a hill except, because of conservation of angular momentum, you end up with this one-hell-of-a spiral.
And I don’t know of a better way to put it because these are the kinds of things that, when they get big enough, do horrible things like destroy all of the buildings on small islands or even in big countries.
Fraser: So, like, when I think about this concept of pressure, right? Let’s kind of go back to just explain what, like, atmospheric pressure is, right? When we’re walking around, we’re experiencing one atmospheric pressure – you know, the column of air that is pushing down on us from above. Thanks, gravity.
And if we go to, say, the top of the mountain, then the air pressure is less because we’re above a bunch of the air and we’re below a bunch of the air. And so, the amount of air that’s still left on top of us is lower, right? This is what this concept of air pressure is.
But the actual pressure –
Pamela: The Barometric Pressure –
Fraser: The Barometric Pressure.
Pamela: – that you measure with those little things –
Pamela: – you may have sitting on your desk.
Fraser: That changes. And as that pressure goes higher, it tends to make weather better and clearer; and when the pressure goes lower, the pressure –
Pamela: We get storms.
Fraser: Pressure tends to be worse. And so – but at, like, what – how large of a, sort of a pressure area are we looking at with these hurricanes? I mean, is that – is it really just a matter of scale?
Pamela: Well, it’s not just a matter of the diameter; it’s a matter of how big is that pressure gradient? So you can have a giant storm with a very small pressure gradient that – it gets a whole lot of rain somewhere but you don’t end up with these massive winds. The massive winds are driven by having a large change in Barometric Pressure and, by large change I’m talking sometimes like finding where the bottom of your scale on your Barometer is.
Fraser: And it’s not just a matter of pressure too, it’s – temperature plays part of this as well, right?
Pamela: And it’s, again, the gradient that’s the issue because you have the warm water that is evaporating up into the air and, as that water evaporates and heats the air as it gives off its temperature to the cooler air, that air that’s getting warmed; warm air expands, you end up with less pressure inside. That less pressure inside can – it’s what drives this entire process.
Fraser: And we tend to get – I know on the east coast of the United States or east coast of North America – you get those hurricanes forming in the wintertime, right? Over Hurricane Season.
Pamela: Right. So, we have a Hurricane Season that spreads from the summertime in the northern hemisphere, all the way out – we actually, a few years ago, went into January, which was, like, super-weird and not something that’s supposed to happen – but it happened.
In general, we’re starting to get towards the end of hurricane season but, in years where we have particularly warm oceans, it can spread further into the year.
Fraser: Yeah. We don’t get them on the west coast, though. So – I mean, we get some pretty big storms. We’ve had hurricane-force winds. In fact, we just had an awful storm just, like, a couple of days ago, where we were getting, like, 90 kilometer per hour wind gusts which was knocking down a bunch of trees. The place kind of looks like a war zone afterwards. But it’s still not the same and we don’t get that storm source.
So, why do you get them going into the east of North America but you don’t get them into the west of North America?
Pamela: Well, we actually do get them coming into the west. They just don’t make it as far north. So, it will be Baja, Mexico, that gets clobbered instead of San Diego or Los Angeles. But, if you look at a map, Florida goes so much further and you have the driving winds that go up the coast. When you’re on the other coast, you don’t have that same driving wind pattern.
So, in general, we have situations where hurricanes form – Gulf of Mexico, Bermudas, Caribbean – they go up the East Coast; they’ll sometimes make it, as they’ve become remnants of storms, all the way to England. You also get hurricanes forming off of the west coast of Mexico; sometimes going up California, sometimes going out to try and destroy Hawaii.
But if you skip over to the other side of the Pacific, here you start looking at what we call Typhoons, but it’s the exact same meteorology at play.
So we get typhoons that go out – Japan, Korea, the whole Pacific Northwest, gets hit – sorry, Pacific Northeast – gets hit with these. No, that is the Northwest – it’s the northwest of the ocean, it’s the northeast of the Asian land mass. So, pick how you want to call it based on if you’re talking about the land or the water.
We also get, by nomenclature, Cyclones in the Indian Ocean that then go and hit India. Again, it’s all the exact same thing but, due to different traditions, we have different names depending on which oceans have these massive lows.
Now, in cases of both the hurricanes that are in the Atlantic Ocean and some of the hurricanes, typhoons, cyclones that are on the other side of Africa, this is actually being created by extremely dry air moving out over the ocean. This extremely dry air will cause evaporation. This causes a storm – it’s actually called a tropical wave, where you’ll get a band of thunderstorms – and if you have just the right combination of still air, so these storms aren’t getting sheared off in the upper atmosphere, this still air will allow this evaporation to build up a centralized low that gets bigger and bigger, and more and more low, until you end up with a giant storm.
Fraser: I was going to say that, you know, when I’m sort of envisioning them – as you said, some do strike the west coast of North America but, in general, right? They’re all sort of going east to west, right? They’re going, you know – from the Atlantic, they’re going across and they’re going into the east coast of North America. Right? There are, sort of, larger patterns of weather that occur on the Earth, thanks to its rotation, right? And that plays into it?
Pamela: And, depending on which side of the equator you’re on, you end up with either the counter-clockwise or clockwise storms. And the direction they move is – in general, you do see this pattern of… they go, they go up the east coast of America; they go, they go attack Hawaii. But they’re driven, like everything else storm-wise, by where are the high pressures and where are the low pressures that give them the greatest path of least resistance to travel along.
And, with Hurricane Matthew, for the very first time, we actually had weather models that predicted that this particular storm was going to go in a complete circle. Now, for better or worse, Nicole, a new tropical depression that’s going to become another hurricane, it formed; it changed some of the dynamics out in the Atlantic. It now looks like Matthew is going to proceed to go off and be a storm that happily crosses the ocean.
But this ability for the storms to veer left, veer right, veer east, veer west, actually means that this is an extraordinarily complicated system where we can’t simplistically say, “If it forms here, it will consistently go there.” Instead, we have to say, “It formed here, now where are the highs and lows and how are they moving?”
Fraser: Right, okay.
One thing that’s kind of amazing to me is that NASA will send in airplanes into these hurricanes to sort of study them from the inside. Oh, what a terrifying –
Pamela: It’s awesome!
Fraser: What a terrifying flight that would be, to go and study a hurricane – in an airplane – from the inside. That would be pretty scary stuff.
Pamela: So, the amazing thing about hurricanes is they are related to a low-level low in the atmosphere. So these storms are fairly easy to fly over. You do get a lot of turbulence but they’re fairly easy to fly over because they are low-level storms. And then that eye, which, for things like Matthew, is huge – the eye is fairly still winds. So, you have a central low that’s kind of like the pit at the bottom of that gravity well game that you see in a lot of museums. And it’s in the gradient part where you get these massive winds, but that central part, which has consistent pressure across it, doesn’t have the physics to generate massive winds.
So, if you can just stay in that eye, you’re pretty good. You have clear skies above you. It’s when you end up hitting the eye wall, which is the inner wall of the storm, where you have massive winds, massive wall of clouds. That’s where you don’t want to put your plane and that’s not where they’re putting their planes.
Fraser: Right. Okay, okay. So it’s not that crazy.
Fraser: So, I mean, here on Earth, we’ve got sort of some understanding of the ingredients that go into a cyclonic storm. We’ve got the pressure differential, we’ve got the temperature differential, we’ve got ocean currents, sort of larger wind currents that are all playing into this. And then you’ve, of course, got the land fall – the places that they strike.
But this same kind of storm is really seen across the entire solar system. You know. So, where else do we see these kinds of storms?
Pamela: So, in general, cyclones are driven by thermal gradients. So, one of the things we haven’t really mentioned is, we have at our own poles, these polar vortexes – which aren’t cyclones in the sense of they’re going to move and take out Florida, but they are winds that are driven by a central low pressure.
And we see this similar kind of vortex developed on many other worlds. On Saturn, for reasons that we can’t quite explain, you end up with a hot polar vortex, which is kind of like something we only see on Saturn. But when we look at many of the other worlds, we see these polar vortexes which are occurring where you just don’t have things being heated up quite the same way, because that’s what happens at the poles. And so you end up with –
Sorry, we just had some really bad things happen in the chat and –
Fraser: Shouldn’t be watching the chat.
Pamela: Yeah. I’m gonna get rid of the chat and thank our moderators –
Pamela: – for being awesome human beings and do that set of statements over and apologize –
Pamela: – to Chad, who’s editing once again.
Okay. Starting that entire idea over. I’m so sorry, Chad.
So, we see on many worlds, including our own, these polar vortexes that we haven’t really talked about yet. And polar vortexes, just like the cyclones we were talking about, are generated by thermal gradients, where you have low pressure in the center, and that low pressure in the center leads to spiraling winds going around.
Now, here on Earth, when we have extreme gradients between the pole and then the higher latitude areas – so around 60 degrees north or south latitude – when we have a steep temperature gradient there, you can end up with a well-defined central vortex. Now, if you don’t have as large as a temperature gradient or if, for whatever reason, it just isn’t a well-defined temperature gradient, we actually observe that the polar vortexes here on Earth can actually fragment into multiple, not-well-defined polar vortexes.
But we don’t see that not well defined happening on other worlds. So, you go to Saturn, you go to Jupiter, you go to Venus, you see these beautifully well-defined polar vortexes that allow you to cut through, visually, the atmospheres of these other worlds. In all cases except for Saturn, which is weird, it’s cold polar air and a thermal gradient that’s just driving this low that spirals and lets us look down into the deep layers of the atmosphere.
Fraser: But we see on Jupiter, for example, right? You’ve got these bands on the planet that are going in opposite directions and where they are almost like rubbing against each other, you’re getting these cyclones across the edge. And then, of course, you’ve got the Great Red Spot, which is a giant cyclonic storm that’s –
Pamela: Anti-cy – It’s actually an anticyclonic.
Fraser: Oh, it’s an anti – it’s an anticyclonic storm –
Pamela: Yeah. So –
Fraser: – that has lasted for hundreds of years.
Pamela: Yeah. So the Great Red Spot is actually weirdo because it’s not an atmospheric low. It’s an atmospheric high. And we think that it has been sustained – well, let me rephrase that. We know it’s been sustained for over 180 years. We think maybe it has been sustained since the early observations of folks like Galileo.
But the problem we have is there’s this large gap in our observations and it’s like, how did no one notice Jupiter’s red spot for over 100 years?
Pamela: And so, either there was a gap in people paying attention, which – we’re humans; that might have happened.
Fraser: Telescopes were pretty bad back then.
Pamela: It’s true. But that red spot’s pretty easy to see. So, either it went away for a while or we simply had a human blindness to the red spot for a while.
So we don’t really know how persistent it is. We know its color changes and it is created by a high pressure in the atmosphere. So – it’s just awesome.
Fraser: I think I even remember this; that, like, when you watch the intro to Cosmos – you know, the new one –
Fraser: – with Neil deGrasse Tyson—and they show, sort of, flying over the Great Red Spot and it looks like this – this depression in the cloud tops of Jupiter, and sort of turning. The reality, I think, is that it’s actually bumped up; that it actually is raised up above the cloud tops around it. As you said, that it’s a – that it’s an anticyclone. It’s a – so – it’s a high pressure. Do we –
Fraser: We don’t get those on Earth, do we?
Pamela: We don’t get them the same way. We occasionally get winter storms that are anticyclonic and associated with these high-pressure fronts. But –
Pamela: On Saturn, we do see regular cyclones occurring. In fact, there was the Great White Spot a couple years ago, that – well, it started as a single white spot and then, as it became more and more chaotic as the low decentralized, you ended up with a storm that wrapped all the way around one of those convection bands that you see in the atmosphere.
Fraser: Right. Does the polar – the hexagon on Saturn –
Fraser: – have anything to do – That’s one of these polar storms, right?
Pamela: So, in the center of that hexagon – which we don’t fully understand – but in the center of the hexagon is a well-defined, well-understood, normal polar vortex.
Now, we see them on – I mean, we don’t get a chance to do a lot of close-up observations when we see them on Uranus and Neptune. There was a fairly famous storm on Neptune as well, right?
Pamela: And we think that these are fairly common on gas giants in general. So the thing with a gas giant is you don’t have land to disrupt the storm like we have here on Earth. So, once one of these temperature gradients, once one of these pressure gradients forms, there’s nothing to dissipate that energy frictionally.
Here on Earth, you have – when the storms go across whatever islands they’re busy destroying, whatever land masses they’re busy destroying – that interface with the land has two effects.
First of all, it’s just frictional. So it slows down the winds, it creates eddies… it’s disruptive.
The second effect is: Part of what’s driving the hurricane is the evaporation of that warm water. Well, if you’re over land, you’re probably not evaporating warm water.
So, this one-two punch of the friction dissipating the winds and the lack of evaporation to drive that low pressure really disrupts storms, for which we’re really grateful. But you don’t have that on a gas giant.
Fraser: Right. Right. So you don’t have that on a gas giant and so the storms could just go on and on and on, around and around and around. And you would think, then – you know, we could talk a bit about sort of water worlds, right?
Like, if we lived on a water world, not on a terrestrial planet like we have, we would probably – one of the downsides would be you’d have these cyclonic storms that would just go around and around and around.
Pamela: Well – and this is actually the real reason that we had that monster storm on Saturn that we haven’t seen replicated on Jupiter. On Jupiter, the little white spots – they don’t take over the entire band they occupy. But on Saturn, where you do have a lot more water vapor in the atmosphere, during the seasonal change, you can have that water vapor suddenly condense out of the atmosphere.
And this change – there’s a whole lot of energy getting shifted around and that creates that initial pressure differential that starts the storm going. And then, as you continue to have changes in the temperature, all of the different changes in phase state of the gasses in the atmosphere – as they condense out, as they vaporize – this all drives the increasing growth of the storm. Jupiter, with less water vapor in its atmosphere, just doesn’t undergo these same evaporative conditions.
Fraser: So, let’s kind of imagine, then, what we know about extra-solar planets and some of the super-weird extra-solar planets that have been discovered so far. What are some – I guess, some situations where you’re going to have these kinds of storm systems but maybe in sort of extreme environments, that we don’t have in our own solar system?
Pamela: So, here we have to worry: What’s the kind of situation that’s going to lead to the greatest thermal gradients? And you can kind of imagine that planet that is perfectly tilted; so that its North Pole and South Pole are exactly perpendicular to its star, so that they never really experience more than a glancing blow. Now, in this case, you don’t have any seasonal variation. So, you have consistent weather; you’re pretty safe.
Now, you also have to imagine: What about the case where you have the Uranuses of our galaxy; those worlds that go from having their pole pointed straight at the sun to having their equator pointed straight at the sun. Well, in this case, you go from having the pole getting all of the light to only getting a glancing blow. And that can cause a huge thermal shift.
With our own system, we actually have the poles go in and out, and so they can go from complete darkness to light. And this is part of what drives the storms. So, we’re kind of that intermediate between always perpendicular and occasionally head-on that allows us to have the storms. They’d get less if we were pointed up and down and they’d get stronger if our pole was occasionally pointed straight at the sun and occasionally pointed straight away.
Fraser: And they would be longer lasting if the world had no land masses.
What about a place that’s tidally locked to its star?
Pamela: So, tidally locked – you end up with a permanent, whatever-thermal gradient it is.
Fraser: Yeah, yeah.
Pamela: So –
Fraser: It’s stuck.
Pamela: Yeah. And this “being stuck” means that – saying it’s cyclonic behavior feels a bit like cheating because what you have is a side that’s permanently much hotter, a side that’s permanently colder, and the convective cells that are associated with that. So that’s more of a convective cell issue than a temporary gradient creating a storm.
Fraser: Right. So you’d have almost like a permanent weather system, where air is being heated on the – obviously, it’s being heated on the part that’s facing the star, and then it’s cooler on the part that’s away from the star, and then heat is transferring around the planet and then somehow getting cycled back in the other way.
I mean, I’ve heard that you would have winds that are hundreds, possibly even a thousand kilo – you know, more than a thousand kilometers per hour. You’d have winds like – like nothing we could ever experience here on Earth.
Pamela: And, you have to wonder, how long would an atmosphere on a world that was close to its star be able to last? And, if you’re dealing with a gas giant, it becomes more of a matter of how long can the planet last? But we’re seeing, as we find more and more of these hot Jupiters, that they can be fairly persistent and they are losing their atmosphere to their stars. But they’re persistent enough that maybe some day, as our technology increases, we’ll be able to start getting direct measurements of the wind speeds through the thickness of the atmospheric lines we’re able to observe.
Fraser: Yeah, that’s – that’s crazy.
The other thing that I find super-interesting is that even what we think are, like, mega-planets – like even brown dwarves – are – will probably have weather systems very similar to some of the large gas giants. That there’s –
Pamela: And that’s been observed, actually.
Pamela: Except there, you end up with, like, metals raining. So, it’s a –
Fraser: A cyclonic storm with heavier elements blasting around in the – in the atmosphere. That would be crazy.
And even, like, some of the – part of the problem with, like, the red dwarf stars, right, is that early on, they’re very energetic. Very – they throw a lot of blasts and – you know, they have almost weather systems in the – although it’s not sort of in the same concept –
Pamela: It’s magnetic field driven. It’s –
Fraser: Oh, okay. Okay.
Fraser: Still kind of a dangerous place to be around in the –
Fraser: – in the early days. Very cool.
Fraser: So the one thing that we haven’t talked about a bit is just what the future of our global warming is going to have on the kinds of cyclonic storms that we have here on Earth. So, you know, we are, of course, raising the temperature on the planet. What does this mean for the kinds of storms that we’re having?
Pamela: Yeah. So, they’re just going to keep getting worse. So, the issue that we’re running into is Climate Change, Global Warming – whatever name you chose to call it – the overall ocean temperatures are increasing. And so, we do things like – there is now a northwest passage that didn’t used to be there. So, if you want to ship your goods from Siberia to Alaska, you can do it.
But this increased temperature gradient is going to allow storms to get bigger and bigger and bigger as the water is hotter and has more energy to dissipate into the atmosphere, creating lower low pressures. This is bad. This is really bad. And bigger storms aren’t nothing – something that we necessarily want forming because they pack heavier winds and greater devastation.
It also means that, because of thermal lag, it takes longer for these thermal gradients to calm down when winter comes. And, if your thermal gradients are able to persist longer, it expands the hurricane season.
One of the things that we’re looking at in a lot of parts of North America, where we have this giant land block – Europe has a lot of weather moderation from being so surrounded by water; we’re just a larger block of land, here in North America – what we’re looking at is more extreme seasons.
The winters are going to get colder. That polar vortex is periodically going to decide it feels like visiting, as happened last winter, blasting that cold air down into, well, more southern parts of North America than it used to. We’re going to get hotter summers, which is something a lot of people experienced this summer. And we’re going to get much bigger storms. We’re going to get more tornadoes, more hurricanes; bigger weather gradients.
Fraser: So it’s gonna get weird.
Pamela: It already is. We can already see this.
Alright. Well, thanks a lot, Pamela.
Pamela: I love ending on a depressing note. Everyone, go study another planet. They’re more interesting.
Fraser: As opposed to our weird one. Okay. Alright. We’ll talk to you next week.
Pamela: Okay. Buh-bye.
Female Speaker: Thank you for listening to Astronomy Cast, a non-profit resource provided by Astrosphere New Media Association, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at astronomycast.com. You can email us at firstname.lastname@example.org. Tweet us @astronomycast. Like us on Facebook or circle us on Google Plus.
We record our show live on YouTube every Friday at 1:30 p.m. Pacific, 4:30 p.m. Eastern or 2030 GMT. If you missed the live event, you can always catch up over on cosmoquest.org or on our YouTube page. Our music is provided by Travis Serl, and the show was edited by Susie Murph.
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Duration: 31 minutes