In the eye of the storm

We live in scary times. As I write this, one hurricane has left destruction in the Caribbean and is bearing down on Florida. A second hurricane is scheduled to clean up what the first one left standing for some of the worst affected islands. Paradise can come to a devastating halt – and was it ever paradise for the poor people who lived there in the shadow of the tourism industry, and who are now left with nothing? A third hurricane is heading for Mexico. And Houston is still recovering from the previous hurricane. Air can be a dangerous substance.

The link between hurricanes and volcanoes is a tenuous one. Of course, both feature regularly in papers which make a living by selling stories of doom. But apart from that, can the events of the air and the ground have anything in common?

There is an obvious link – and a less obvious one. Let’s do the obvious one first.


Pinatubo, shortly before its eruption. Source: wikipedia

This was another paradise. Luzon is the northernmost island of the Philippines. Mount Pinatubo was a heavily forested, eroded mountain. Nearby were two American military airbases, as well as 6 million people. The mountain was known to have a volcanic history, but then, there are many extinct volcanoes. However, people were beginning to recognize that this one was far from extinct. There was debris around the mountain of many eruptions, the most recent one only some 500 years old. An older one had left pyroclastics 100 meter deep.

Earthquakes began on March 15, 1991. Activity may have started with a large quake a year before: a relation is plausible but unproven. Phreatic explosions began two weeks later, and danger levels quickly increases. This was the time when scientists began to draw attention to the older debris and began to realize what this mountain was capable off. It led to one of the most successful evacuations in history. Magmatic eruptions started early June. By June 15, 30,000 people had been evacuated. The USGS had done much of the ground research, and the PHIVOLCS made marvellous use of this. The Clark airbase was evacuated just in time. By the time the main eruption happened, June 15, it had just been cleared. These actions saved many thousands of lives.

The track of Yunya towards Pinatubo

But one thing could not have been foreseen. The Philippines have regular visits from typhoons, and one was just on the way. As misfortune would have it, typhoon Yunya passed 75 kilometer north of the mountain just when the main explosion happened. One result was that the eruption was heard but not (well) seen, hidden by the weather. The pyroclastic flows which destroyed the area mixed with the huge amounts of rain. The rains destabilised the volcanic debris and huge mudflows came down. In total, over 800 people died. Without the immense pre-eruptions efforts, it could have been many tens of thousands. But the destructions continued for a long time. Lahars continued to come down the river valleys for years afterwards.

Clark airbase, destroyed by Pinatubo and Yunya working together. The weight of soaked ash exceeded the limits of the buildings.

Yunya was a weak typhoon when it reached Pinatubo. Out at sea, it had peaked as a category 3 but the winds has subsided from this peak. Air pressure in the core had been down to 950 mbar, consistent with the powerful storm it was at that time. But typhoons, as hurricanes, get their danger not just from the wind. The storms take in tremendous amounts of water from the warm ocean. This water comes down as rain – lots of it. They bring flooding, and this typhoon rain is what added to the destruction of Pinatubo.

The eruption was one the largest of the last 200 years and the ash punched through the clouds, some to a height of well over 30 kilometer. It came down over a large area, spread further by the wind. The rain soaked the ash, adding to the weight on the roofs. It was a singular case of a bad coincidence, and the only reason that the disaster was not much worse was the work done by the scientists, politicians and military personnel in the weeks leading up to the eruption.

The eye of the storm

Tropical storms behave like very large thunderstorms. Convective cells rise up above the warm water, and keep rising and rising. As they go up and expand, the air cools and water vapour condenses: clouds form, rains comes and lightning begins. The air flows round the rising air. But what goes up must come down. The risen air flows over the top of the storm, and descends again around it.

But once the circulating air reach a certain speed, something funny happens. Of course when air goes round, the centre doesn’t take part. The air here doesn’t know what to do. As the winds increase, this confused region expands. Finally, it becomes larger enough that the risen air on top sees it is chance, and begins to descend right at the centre of the storm. Descending air warms up, rain evaporates and cloud disappear. A cloud-clear eye has formed. The formation of the eye is a sign of a hurricane.

They eye of the hurricane

The central air is clear and sunny, but is surrounded by a circular wall of clouds with violent winds. The air pressure here is low. Extremely low, in fact. As I write, Irma has a central pressure of around 925mbar, and it has been lower. Some storms have reached values below 900 mbar, 10 per cent below normal. For Irma, people mentioned that they could feel the change of pressure in their chests. Hurricane Wilma in 2005 was reported to have reached a central pressure of 880 mbar. That is low enough that that cup of tea you needed in the eye is notable cooler than it should be, because of the lower boiling temperature of water. The effect of hurricanes on the temperature of tea is overlooked in many studies of the impact of hurricanes.

Horror vacui: air and pressure at the Puy de Dome

And this brings us to a less known relation between hurricanes and volcanoes.

Evangelista Torricelli

The story begins with Evangelista Torricelli, in 1643 in the Italian city of Pisa. The Pisa connection is not entirely accidental as he was a student of Galileo. Torricelli was interested in whether it was possible to create a vacuum. A popular notion at the time, dating back to Aristotle, was that nature did not allow ‘nothing’ to exist: if there was a vacuum, something would immediately move in and fill it. This was called ‘horror vacui ‘ (fear of emptiness), and it is still in use to describe art where every part of the frame is filled.

Galileo had looked at the problem of pumping up water. A suction pump can bring up water about 10 meter, But try higher, and it fails – the water refuses to rise any further. The top end of the pump stays empty. To pump higher you need several pumps, working step wise. Is this a vacuum, or something else? Galileo failed to answer this question. Torricelli had the bright idea to try it with a denser liquid, to avid having to work with a ten-meter tall tube. He used mercury. To simulate a suction pump, he used a 1-meter long glass tube, open at one end and closed at the other. Do the same, and fill it with mercury, and close off the open end. Make sure no air bubbles are trapped. Now turn the tube upside down, so the opening is at the bottom. Put the bottom into a bowl which is also filled with mercury, and re-open this end. There is no air in the tube! But the level of the mercury in the tube will fall, and an empty space opens at the top. The mercury level drops to a height of 76 centimeter above the mercuy of the bowl. How can this be?

Torricelli thought that the reason was that the empty space above the mercury was a vacuum, filled with nothing. In contrast, the air around was ‘real’ and its weight pushed down on the mercury in the bowl. This weight of the air pushed the mercury in the tube up. The reason that mercury could only rise to 76 centimeter, and water to 10 meter, was because mercury was much heavier. It is 13.5 times more dense, and indeed had risen up 13.5 times less high. In other words, the weight of the column was the same for both mercury and water. Torricelli had discovered the weight of air – air pressure. And as an aside, he had shown that above the column was vacuum. Horror vacui was wrong – nature did allow for nothing to exist.

Blaise Pascal

News of the discovery reached Blaise Pascal, a famous French physicist. He repeated the experiment, and confirmed the finding. But it also became clear that the height of the column was not exactly the same all the time. It changed with time. So perhaps it wasn’t the air? Pascal realized that there was one experiment that could settle the issue. If it was the air, if you could do the experiment at high altitude, with less air above, the column should be lower.

There were no mountains at Rouen, near Paris, where he lived. But Pascal had grown up in Clermont Ferrand, further south, where there were impressive hills. He contacted his sister and her husband Florin Périer who still lived there, to ask whether they were able to help. A year later, in September 1648, Périer carried glass tubes and mercury to the Puy de Dôme, the volcanic cone near Clermont Ferrand, a kilometer above the surrounding country side. It must have been a difficult journey, trying not to break the fragile glass or lose the liquid mercury! But the experiment was well prepared. First, he and his friends measured the height of the mercury column at the garden of the local monastery – 71.1 cm. They had two such instruments. One was left at the monastery, and was checked all day by one of the monks. The second one they took up to the peak. And indeed, at the top of the Puy de Dôme, the height of the column was only 62.7 cm! At the monastery, the level had not changed.

The difference was so large that they decided to test it in much easier way. Périer took the less-arduous climb to the top of tower of the cathedral of Clermont-Ferrand, 50 meters tall. The mercury level dropped by 4 mm, easily measurable. The trip up the volcano had been overkill.

This experiment settled the issue. Torricelli’s instrument indeed measured air pressure. And this pressure changed with altitude, but also with time – sometimes the air pressure dropped for no obvious reason. Low-pressure weather systems had been discovered.

We still use this instrument, now called the barometer. It measures something that affects our weather tremendously, and also limits to what altitude we can live, but which we are ill equipped to detect any other way. Physics had finally entered the realm of measuring things which our senses could not directly detect.

The Puy de Dôme is now a major tourist attraction. It is one of the more famous volcanoes of Europe (although being monogentic, it will never erupt again), which you can climb to admire the view from the top, or of you want you can even cycle up. Next time you visit, remember how this volcano led to the discovery of air pressure – and the association of low pressure with poor weather.

Puy de Dome


68 thoughts on “In the eye of the storm

  1. Re: Pinatubo, the central core of a Hurricane evidently can pull down the level of the tropopause… this probably augmented the sulfur load that Pinatubo could inject to the stratosphere.

    • I’m not sure that explanation can be quite right. If any significant volume of cold dry air entered the hurricane system it would snuff it out due to lack of moisture to power the system. Some of the incoming now-dry high level air will sink, but the overwhelming majority streams away to the top of the nearest high pressure system because at that elevation its warm and (relatively) high pressure so it ‘blows away’. I think this is what NOAA refers to as outflows(?).

      We tend to forget that fluid systems are in a sort of dynamic equilibrium due to conservation of energy. This can be modeled with a variation of Bernouill’s equation which is quite hard to solve analytically but quite easy to understand in a handwavy way which is very useful for comprehension.

      For example take a little packet of air of constant mass full of moisture.
      Its too big/moving too fast, to exchange heat with the adjacent packets.

      (point A) It goes up, pressure is lower at altitude so it expands.
      Expansion cools the packet (it does work to expand).
      Water condenses out, but this has very little effect on the volume.
      But it liberates a LOT of energy of evaporation (720cal/gm if 50 yr old memory works)
      This heats up the packet of air which increases the pressure.
      So it expands some more till its now less dense than the surrounding air.
      So it rises some more (goto point A)

      Its a very simple linear motor powered by latent heat of evaporation.

      This is why rain is cold, it formed high up where the air is cold!

      So the point is that at ground level the pressure is low (because the air is going away – upwards) but at high level its (relative to altitude) pressure is high so it blows away. You have no accumulation but a net flow.

      So at low level air is drawn into the lower pressure but at the top the pressure is high; conversely away from the low, typically an area of “low level high pressure”, the air above is dry and cold so is locally dense so it falls bringing cold dry air to the ground (ie deserts) but at the top the pressure is locally low, sucking in the adjacent cooling air into the ‘high pressure’. Note that as the cold air descends it get compressed so it warms as it falls, powered by gravity in this case.

      The packets of air are too big/move too fast to exchange significant energy to adjacent structures so its a simple conservation of energy with a known loss due rain. Would be easy to model in fact.

      Its really really simple but I have never seen anyone, anywhere, spell out this trivially straightforward mechanism.

      • Before someone else comments note that air has a molecular weight a bit above 28 (ie two nitrogen atoms N2) but water is only 18 (H2O) so water vapour slighly decreases the density of air so moist air is less dense than dry air (very slightly) so there is a slight negative feedback to the mechanism.

      • Noted. But the tropopause does tend to dip a bit over the core.

        The upper level outflow regime that you stated is spot on. That’s part of the secondary circulation that comes into being as the storm gains strength.

  2. Half OT, half not. The local grocers had run out of water by yesterday. Today I counted at least 12 new pallets of water at the closest store

    , but they were running shy on bread products. According to the stocking clerk, all the flour was going to the Houston area bakeries. This is one the extreme end of Florida away from Miami. All of the hotels are packed. To this stores credit, according to the clerk, they have more water coming in. At $10 per 4 slabs, they are making their money off of volume and not gouging. That’s actually about half of what I’ve seen for normal prices for bottled water around here.

  3. “the temperature of tea is overlooked in many studies of the impact of hurricanes” – we have to doubt the validity of any academic paper that doesn’t mention its effects on tea 😉

    • My tea is usually kept on ice in a 40 oz insulated steel mug. (Unsweetened with a bit of lemon). Great for florida heat.

    • It’s not hurricane related; but for only $28 you can read my paper on called “The Effects of Acidification of Volcanic Soils on the Production and Flavour of Blueberries and Tea.” 😉

      • (or .edu) is best avoided, in my opinion. It is more of a parasitic site without quality control and with dubious adverts. Researchgate is a similar repository but appears better behaved and is often a very useful resource. It has annoying pop-ups asking you to join but you can just dismiss these and still get the paper.

  4. Snicker, weather twit messed up his analogy. He was trying to explain te venturi effect with the wind driven surge. “It’s like blowing across the bathtub”

    • Thank you for that. I discovered that I have nerves off goo. Wound up buying a new generator and chainsaw. Just felt naked without an operable saw.

      Edit: Changed “functional” to “operable” since the previous word could easily be misconstrued as cursing. It would be a conceivable use, but I’m not that angry yet.

    • Now all I have to is wait for my grandson to bring the truck. (The generator was an impulse buy.. but, it’s got twice the capacity of the one I had.)

    • … and another gaff by the weather twits. I just saw one of ’em state that the west coast of Florida has a negligible shelf.

      Idjit. The shelf off the west coast of Florida is almost as wide as the entire peninsula.

    • I agree. I was standing there in line with chainsaw in hand looking at the generator on the pallet and mulling it over in my head. Little voice said “Yeah, remember how hard they were to find post Ivan?” The decision sort of made itself.

  5. Does anyone know of a good cam around Venice, Florida and Port Charlotte, Florida? I have family that lives in the area.

  6. No, I don’t. is pretty decent for traffic info and has a few highway cams linked there. I think it’s affiliated with florida DOT.

    Currently watching weather twit on tv out on key largo. (Fairly close to mainland) just south of there the core of the storm is showing 125 mph on nexrad… aimed towards key west apparently.

    • That’s gonna leave a mark… I’m seeing indications of hail in the northern eyewall on nexrad. Cloud tops are touching 54kft.

    • I’ll check it out. I’ve been keeping an eye on the weather in motion radar map that’s on It’s showing it’s a category 4 again, 130 mph. My cousin told me earlier the day before her husband went into afib again in the middle of packing up their house. He got to go back home, but are now at one of their sons place. He has hurricane windows, etc… She said they don’t want patients if not life or death at the hospital. I guess I ought to go to bed. Tomorrow night will really be hard to sleep.

      • Interesting advice from a law enforcement representative in s florida for those who are trying to shelter in place in mando evac areas. If water starts flooding, move to second floor. If forced higher than that, take an axe when you flee to the attic, that way you can cut your way out.

        • Personally, I think it’s a matter of knowing how to breach a roof. Short decisive chops are the preferred way to keep the cut under control. I’ve gone through the side of steel clad buildings using that method… but then, I wasn’t in a cramped space frantically trying to get out. The other tool suggested was a chainsaw. But in that circumstance, you had better be efficient or the carbon monoxide will take you out before you can get an opening. I used a chainsaw to lop off the legs of my desk in New Jersey so that the movers could get it through the door… I was so eager to leave that I didn’t have time to wait for building maintenance to get around to eventually showing up to remove the door. (The hinges were painted to the point that you couldn’t remove the pins) If I remember correctly, I set off the carbon monoxide detector doing that. I tossed the desk legs into a drawer and put them back on when I got moved in here. You should have seen the look on the movers faces when I cranked that saw.

          A bit extreme? Yeah, but I was not gonna spend another night in that state.

        • Yes, that advice, to only enter the attic if there is a clear way up on to the roof, is standard in areas at risks of deep floods, such as the Dutch polders.

  7. Its looking like the,southern eyewall fell off or went poof. No mention of that in the nhc discussion yet, but thats never a good sign for a storms health.

  8. While the media is frantically hopping up and down, I still want to know where the other half of the storm went to.

    • Satellite shows the cloud, but the rain radar shows a dry-up. That’s…weird. Now Cat 2. Might it be the influence of the drier air mass bounding Irma off to the west?

    • Likely. I think the front that dropped through here a few days ago started tangling with it.

  9. And the Romans build a temple dedicated to Mercuri on th Puy de Dome. Coincidence?

    • Interesting point! Of course the two are completely unrelated, the temple being for the god Mercury (also the planet), and the element being known as hydrargyros at the time (hope I spelled that right), ‘silver water’. The name of the planet is the older one. I don’t know when the current name of the element was adopted, but I do know the origin of the planet names and have little doubt the planet came first.. But one could wonder whether Pascal had this temple in mind when deciding this experiment should be done at the top of Puy de Dome.

  10. And now we get aftermath stories laden with as many superlatives as can possibly fit into a sentance. The only one that has amused me so far is the one about a guy spending all morning trying to catch a baracuda in his swimming pool.

    • What “everybody” is waiting for; “Did he catch the baracuda?” How well does the baracuda do, in what I assume now is brackish water?

      Our thoughts, wishes and prayers are with the victims of first Harvey and now Irma, and it really looks like Jose is going to stick his nose into the soup.

      Best of luck to all; even those of Homo stultus.

      • I don’t know the disposition of he Barracuda. The story was related by a younger person being interviewed who stated that he had spent most of the morning watching the escapade. Since the fish obviously got there via over-wash or surge in a wave, it’s a really good bet that the water was at least brackish. How this mixed with normally chlorinated pool water affected the barracuda is unknown. Salt water pools are not widely used, though some companies are pushing that method of pool maintenance and advertise it. Whatever the composition of the water was, it couldn’t have been overly detrimental to the fish or else there would have been no problem retrieving it. Dead/sick fish don’t move very fast.

    • Sad indeed, but utterly stupid and preventable. Volcanoes are not to be trifled with – they bite.

      • Terrible. You don’t know what happened. Young children can do impulsive things and parents walk a tightrope between allowing freedom to explore and keeping them safe. One moment inattention can lead to accidents. And how could parents not run out to help their child in trouble? This was not like a 20-year old trying to show off. It was an accident with awful consequences.

        • Yes, I agree Albert. Things can happen so quickly & parents will do anything to save their child. Even when you prepare for things there’s an odd chance something just happens. Parents can warn there children & sometimes kids don’t think things will happen to them. So very sad. 🙁

        • At the same time, there is a level of ‘Well, what did you expect would happen’ to be had here. At best, having a young child in an area where danger can appear rapidly and from unexpected sources is highly irresponsible. But running around with an 11 year old and a 7 year old, both of whom could very well end up being rebellious idiots in an area with the potential for sudden exposure to lethal volcanic fumes is appalling parentship. These kids had little to no understanding of just how dangerous the fenced off area was, and it’s possible that they might not have cared either. It becomes a question of whether or not minors should be allowed anywhere near this stuff, really.

          • I have been apprehensive at commenting on this for that very same reason.

            The best analogy I can give, is would you take kids of this age down into main control where the ships boiler is at? If not, then it’s probably not a good idea to take them into a natural environment where the same hazards exist.

            Engineering plant – Smelly, Noisy, Hot, and some stuff down there can kill you before you know it.

          • It depends.
            I was exposed to potential hazards from an early age, and I’ll bet many of you were. After being stung by stinging nettles, fallen down slopes or walls, burned myself on hot things I quickly learned to take heed of what I was told. The thing is to warn kids when need be, and not when the risk is modest or small. That way they listen to you.
            I have been to NZ where there are hazardous areas, as there are all over the place and kids do behave. If a tiny number behave stupidly, well it happens in all sorts of circumstances where less publicity results.
            Molly coddling kids leads to an inability to judge risk as and adult, where the consequences are often much more dire.
            I’m quite surprised at Geolurking’s comments though….

          • How so? I didn’t wish to be abrasive or trite with a tragedy, so I moderated my tone as best I could. The event was instigated by someone who had little chance of knowing better, the follow-on deaths are similar to how many family members would respond. You do what you can to save your family. It’s not the first time that a family member has rushed into extreme danger to save a child. It’s about the only aspect of human behavior that still makes us human. In a way, the Stutus tendency has us throwing caution to the wind in order to save family… or to at least try.

          • You can’t answer this based n what we have heard. The images show a good fence: children can’t skip underneath, and it is high and dense enough to be an effective barrier. But clearly there was a way over it which an 11-year old somehow managed. If his parents lifted him over, that would be irresponsible on their part. But it may well be that the child climbed over by himself, in which case it is not ‘appalling’ parentship, but the kind of impulse action that every parent has nightmares about and which could have happened to anyone.

            Gelurking’s point is correct: there are places you would not take children into as they are not safe. But here the fence did seem adequate, and the security better than at Yellowstone.

            A younger child survived. It would be much better for her to read that her parents died trying to safe her brother, rather than them being the people causing his death. And I think it is probably true.

          • 11-year olds can climb almost anything. They can’t necessarily get down, but climbing it is usually no problem.

            Looking at this from a disjointed psuedo-objective point of view. Solfatara probably formed as a result of the First Phlegraean Period. That event likely stressed the Neanderthals to the doorstep of extinction. So, here we are, 40000 years later, and Campi Flegrei is still killing people.

            As for the 11 year old motivation to climb, at about the same age, I made a leap to a limb from one tree to another. I successfully grabbed the target limb, but when my body had pivoted to horizontal I lost my grip and went splat 8 feet to the ground. I remember it well since that was one of the object lessons I learned that day. *don’t do that.

          • You just have to brief them carefully and watch them properly. I’ve taken my kids to places a lot of parents wouldn’t when they were quite a bit younger – and they’ll never forget it! (in a good way!) Care – not cotton wool!

        • Not blaming the kid or the parents – surely it is blooming ridiculous to allow such easy access, in exactly the same way that people can’t normally just wander into a lion’s cage.

          • To make it clear, I agree with eruptionchaser, lurk (this time) and Albert and disagree with eddie. Kids need to learn, and going safely somewhere potentially (but not actually, done properly) lethal is part of life’s training. Not doing so results in greater danger, later.

            This is unlike the Sicilian mud volcanoes at Maccalube (which are actually tiny) and completely unfenced that killed a child a week after we visited it. I think the first ever reported death.

    • Too much current and the fish will just explode… or fly out the window.

    • “…all of these studies rely on some type of remote sensing, and none were able to directly investigate the physical products of subglacial volcanism. Here, we present the first geochemical and volcanological evidence of subglacial volcanism occurring beneath WAIS obtained directly from associated tephra layers.”

  11. Mauna Loa has reached cumulative energy levels equal to those prior to the last two eruptions, USGS says eruption in the coming weeks and months is “not likely” at the moment though.

    Gunung Agung alert level has been increased to yellow due to increased vt-earthquakes. 3km exclusion-zone has been put into place. That’s a volcano to watch closely….does anyone have more information on Agung? Wikipedia doesn’t do it justice, I’m afraid.

    • Agung is a nasty volcano. Hopefully no eruption will happen. It seems the VT quakes aren’t super vigorous right now and this is mostly a precaution, so that’s probably a good sign.

  12. Being it a Friday I suppose I can post these two off topic links.

    Besides being able to be both dead and alive cats seem to be able to be a aolid and fluid at the sam etime too.

    On another note chek the gallery of Cassini images in the Guardian, they are beautiful with lots of detailed images.

    • Cassini has been a marvel. I remember the landing on Titan well. What a journey that was. And almost a failure because they had forgotten that the spacecraft and lander had different speeds – in the original plans the signals from the Huygens lander would have been lost because the frequency was shifted outside of the range that Cassini could detect.

  13. Just some random info, but Grimsvötn is currently in it’s most quiet period since summer 2014

    • Why did the petrol companies push tetraethyl lead instead of ethyl alcohol?

      This was the USA. In the 1920s. The era of Prohibition Some linkage, possibly?

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