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.
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.
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.
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.
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.
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.
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.