It has been wet in the UK. Nothing unusual there, it may seem, although compared to the tropics, the rain is mostly mild. Temperatures are moderate and therefore the air does not contain as much moisture. Major deluges are rare. But not this winter. Especially the south of England has seen widespread flooding. France too has been hit. Further north, though, it has been a lot drier. The abundant rain has not been universal.

Other countries are also known to have had excessive rain on occasion. An anonymous, exasperated tourist visiting the West Coast of South Island, New Zealand, left a poem dripping with dry humour:

It rained and rained and rained
The average fall was well maintained
And when the tracks were simply bogs
It started raining cats and dogs
After a drought of half an hour
We had a most refreshing shower
And then the most curious thing of all
A gentle rain began to fall
Next day was also fairly dry
Save for a deluge from the sky
Which wetted the party to the skin
And after that the rain set in.

One of the obvious consequences of global warming is that the air contains more moisture. When it rains, the rain is heavier. That is now seen worldwide, but remember that the total amount of rain in a particular location depends on the local climate and this will change as well. For instance, the Middle East is drying out – while the UK is getting wetter.

Volcanoes are blamed for many things. Hector has pointed out that after large eruptions, Spain can see enhanced rainfall. It will take many eruptions before the evidence becomes statistically significant. But is it possible there is a link? Do volcanoes cause rain? Grab yourself a beer and join VC in our quest for water. It is Miller Time!

Frankenstein’s rain

Geneva (Switzerland) in 1816 was cold and rainy. Lord Byron and Mary Shelley spent the summer in the area (“Head for the Mountains” to quote an appropriate beer slogan), with great plans for all the activities they and the rest of their group wanted to do, but the weather was so gloomy that they started writing ghost stories instead. Shelley’s Frankenstein became the master piece of that year. Lord Byron wrote a proem ‘Darkness’ which is remembered mainly for one line:

Morn came and went—and came, and brought no day

Indeed, from April to September there was rain on 130 out of the 183 days. The incessant rain was interrupted only by tremendous thunderstorms. Shelley wrote about the travel toward Geneva, during spring:

We slowly ascended, amidst a violent storm of wind and rain, to Champagnolles [..] The spring, as the inhabitants informed us was unusually late, and indeed the cold was excessive; as we ascended the mountains, the same clouds which rained on us in the valley poured forth large flakes of snow thick and fast

In June she wrote

The thunderstorms that visit us are grander and more terrific than I have every seen before. ..] One night we enjoyed a finer storm than I had ever before held. [..] The thunder burst at once with frightful loudness from various quarters of the heavens. I remained, while the storm lasted, watching its progress with curiosity and delight. As I stood at the door, on a sudden I beheld a stream of fire issue from an old and beautiful oak, which stood about twenty yards from our house; and so soon as the dazzling light vanished, the oak had disappeared, and nothing remained but a blasted stum.

(Quotes from Phillips, 20026, Atlantis, 28, pp59-68: Frankenstein and Mary Shelley’s “Wet Ungenial Summer”)

But to quote another beer slogan, Out of the darkness comes light. Now we know that this horrendous weather had been caused by a volcano. Tambora has erupted in April 1815, the largest explosion for almost a millennium. The link between the weather and this distant volcano was made only a century later. (Byron did in fact refer to a volcano in his Darkness poem, but in entirely the wrong context:

‘Happy were those who dwelt within the eye
Of the volcanos, and their mountain-torch’)

But while Europe drowned in a chilly rain, the eastern US suffered in a chilly drought. The cold was widespread but the rain was more local.

So what is the relation between volcanoes and rain? Let’s have a sip.

Water injection

Some volcanoes eject not only magma and rock, but also water. Magma may contain up to a few per cent water. The water remains in the magma until the ascending magma becomes quite shallow: we can take the water release by an erupting volcano as a percentage of the magma that reaches the surfaces. Submarine eruptions don’t count, obviously: adding water to the ocean is like carrying coal to Newcastle. (Newcastle Brown Ale. The Other Side of Dark.) Subaerial eruptions currently produce 1-2 km³ of lava per year. However, looking back further in time, this number becomes larger, because of the impact of the very large but rare eruptions. The long-term average has been estimated at 4 km³ per year (Papale et al., 2022, https://doi.org/10.3389/feart.2022.922160). If 5% of that is water, the total amount of water added to the atmosphere is 0.2 km³ per year on average.

Volcanoes can also borrow water from the environment. An example is seen in fumaroles, which spout steam into the atmosphere. This is mostly rain water which soaked into the ground, was heated by volcanic gasses and blown back out. There is no net gain for the atmosphere, although the region around the fumaroles may become quite humid. Geysers are a more extreme case, but their water has a similar origin. From experience I can confirm that they are quite wet: people standing downwind from one of them, on a quite windy day, were moving out of the way very quickly – but not fast enough. In extreme cases, Maar eruptions may blow large holes in the soggy ground. But as for fumaroles, the humidifying effects are mainly local.

Rain forests are also famous for this. The trees constantly evaporate water through their leaves, making the air humid and oppressive – not helped by the lack of wind below the canopy. During the wettest seasons (which for equatorial forests tends to be twice a year) the rainfall runs off rapidly through the swollen rivers (such as the Amazon), but at other times the evaporation rate fairly well balances the rainfall. When it is wet, the trees source their water from the upper 50 cm of the soil, but at drier times they go deeper, taking in water that came down as rains months earlier. The evaporation becomes the main source of rain. The forest makes the rain, not the rain the forest! Take the trees away and the region becomes drier.

But geysers and fumaroles are small fry compared to a complete rain forest. The rain around them is limited to the surrounding acres (and tourists). There is no impact further afield.

Water budget

How do these amounts compare to normal amounts of rainfall? Let’s take the UK as an example. A typical year may show 70cm of rain on average. The surface area of the UK is 245 thousand square kilometer. That gives an annual volume of rain of 170 km³ – a VEI 7 if it were a volcano. And that is just one small country on a very large globe. It is clear that the Earth’s rain cannot live on volcanoes alone. It comes from evaporation from the seas and oceans, and from the land, whilst any water injection from volcanoes adds little. To paraphrase inappropriately another beer slogan, Everything you always wanted in a volcano…and less.

Let’s put some more numbers on this. Evaporation from the oceans is estimated at 577 000 km³ per year. Most comes from the tropical oceans, driven by a combination of strong sunshine and trade winds: strong winds increase evaporation. The tropical Indian ocean around the tropics has the highest amount of evaporation. Evaporation from land is much less, at 72 000 km³. (These numbers date to the 1980’s, and it will have increased since then because of the warming of the oceans and the air.) Evaporation takes energy: around a quarter of solar energy that falls on Earth is used for this. The evaporated water carries solar warmth around the world.

The water does not remain in the atmosphere for the full year. It takes on average only 8-10 days before evaporated water falls back to Earth as rain. At any one time, the atmosphere will contain roughly 15 000 km³ of water.

The total annual rainfall around the world must be equal to the total annual evaporation, plus of course any volcanic input. But from the numbers above, it is clear that volcanoes have no significant effect on the global water vapour budget, and hence would not be expected to increase average rainfall worldwide.

Hunga Tonga

Apparently, one particular beer can refresh the parts other beers don’t. Some volcanoes can reach the parts other volcanoes don’t. There are two things to consider. First, some eruptions don’t just eject their magmatic water, but tap into surface water. An explosion under water can eject a large column of water into the atmosphere. Second, the ejecta can put water in regions which are normally very dry, such as the stratosphere, and the impact there can be significant.

Both effects showed in the Hunga Tonga eruption of 14 January 2022. it erupted some 500 meters below the water surface, and explosively ejected of order 2-5 km³ of dense rock (double that volume in tephra) in the largest eruption since Pinatubo and the loudest since Krakatau. (We take some pride that the VEI value assigned by VC within days of the eruption (5.9) is still the accepted value, being spot in the middle of the range of estimates.) It is worth mentioned that only around 10% of the ejecta entered the atmosphere: the remainder formed thick deposits and distant flows on the sea floor. This is also the reason that little sulphur was ejected into the atmosphere: over 90% of the SO2 emission dissolved into the sea water.

The depiction shows the various outputs of the eruption and where they ended up. The debris and gases dumped into the ocean are not relevant to us. The rising column which entered the atmosphere contained some SO2 and some water that came from the magma. The magma had around 1-5% water content (by weight), so this amount was not large. The figure above gives 25 Tg (tera-gram) magmatic water entering the atmosphere (and much more which did not) which comes to around 0.025 km³ of water. The amount of seawater captured by the plume (and incidentally providing much of the buoyancy) was much larger: it is given as a minimum of 2900 Tg, or 2.9 km³ of water. (Other authors argue for a lower value of 1500 Tg: Suzuki et al 2025, https://link.springer.com/article/10.1007/s00445-025-01919-9). A fraction of this water will have remained liquid rather than vaporising.

In normal explosions, water thrown up into the air immediately falls back. But where the explosion is volcanic, much of the water is heated by the interaction with the magma and is raised up as steam and vapour. The heat from the eruption causes the explosion column to rise, carrying the water vapour with it. The eruption should not be too deep, not only that the eruption can reach the surface, but also because at depth of a few kilometers or more, water can’t turn into vapour. Hunga Tonga met these requirements.

This was rather more water than in normal eruptions. But worldwide, the amount was not significant. On a given day, 1500 km³ of water may evaporate from the oceans, and Hunga Tonga added a measly 0.2% to this. Locally though, it would have more of an impact. Assuming this water rains out within 100 km of the volcano, that would give some 300 mm of rain – 6 inches in the US – which is flooding levels. But there is little more than ocean around Hunga Tonga and we have no weather reports from the region.

Krakatau

We do have such reports from a somewhat similar (though larger) eruption, that of Krakatau in 1883. The ship ‘Gouverneur Generaal Loudon’, located some 70 km northwest of Krakatau, reported a rain of mud, half a meter deep, which came after the initial fall of pumice. Two other ships in the region also reported a fall of dust and water. The ash had turned to mud from the ejected water. In Batavia, 150 km from the explosion, ‘watery particles’ began to fall, followed later by ash which contained about 10% of water.

Captain Watson of the shop ‘Charles Ball’ provided a detailed report of which the relevant part is reproduced here (from The Atlantic, https://www.theatlantic.com/magazine/archive/1884/09/the-volcanic-eruption-of-krakatoa/376174/):

These reports show that the ash that rained down following the main explosions had been thoroughly wetted. This lasted for some two hours. Had the ejected water been directly absorbed by the ash, or had it caused rain which later wetted the ash? Wet ash is heavy and would have fallen quite fast. Perhaps both happened, with the early mud being wet from the start, and the later ones wetted by rain. The watery particles in Batavia are perhaps best explained as the latter, given the distance from the eruption. The role of water in the ejecta of Krakatau appears to be poorly studied, but it seems that much of the ejected water returned to earth (or sea) quite quickly.

Stratosphere

Hunga Tonga (“probably the best beer in the world”) did more than wetting the ocean. The eruption column, driven by the steam, reached the highest recorded altitude for any Earth-based eruption at 56 km. It entered the stratosphere and spread out as an umbrella cloud at 30 km. A small part of the plume kept rising, reached through to the top and entered the mesosphere. And the water rose with.

The water input into the stratosphere has been measured by NASA’s AURA satellite, at around 140 Tg. This is only 5-10% of the total amount of ejected water (the remainder staying lower down), but it entered a very dry region. Around 0.1 Tg of the water may have reached the mesosphere. There is significant loss of water during the ascend, because part of it will freeze out in the frigid layers of the atmosphere and the ice will fall down. In case you worried, very little chloride (HCl) reached the stratosphere. The entire stratosphere contains some 1400 Tg of water. Hunga Tonga increased the water content here by 10% worldwide! Pinatubo also blew water into the mesosphere, but this is estimated at less than 40 Tg. (Pinatubo did put much more SO2 here than did Hunga Tonga.) Some extreme weather events can humidify the stratosphere, but these are rare and the strongest ones have been measured at only around 20 Tg.

From February onward, the water spread around the southern tropics at an altitude of around 25 km. By June it has reached Antarctica and from January 2023 it also spread into the northern hemisphere. Four years later, perhaps a quarter of still remains in the stratosphere.

Water in the stratosphere cools the stratosphere. The effect on the surface is less clear. Early modeling predicted a slight cooling of the southern hemisphere, by around -0.03C, which could offsetting a few years of global warming. This has since been updated to -0.05C lasting 1-2 years, but it is still undetectable against normal year-on-year variability. After two there is a very small warming effect from stratospheric water, but it is again undectable.

A long and comprehensive report on the impact was published in December 2025 (https://juser.fz-juelich.de/record/1049154/files/Hunga_APARC_Report_full.pdf). It finds a significant effect on the circulation in the stratosphere for the southern hemisphere which lasted throughout 2022 and caused changes to the ozone hole. After that, the effect on both hemisphere became smaller than the normal annual variations, making it hard to isolate the impact from the Hunga Tonga eruption. The cooling in the middle and upper stratosphere is clearly visible in the data and remains present in 2025. In contrast, the El Chichon and Pinatubo eruptions warmed the stratosphere. Hunga Tonga also caused a strong cooling in the mesosphere, but here the recovery was faster.

The mesosphere does not affect our weather. The stratosphere does, by affecting the jet stream, but we can’t easily attribute jet stream changes to Hunga Tonga. Our current rainy weather does not have a clear link to a volcano. It is just weather.

Rain and Tambora

Tambora’s major impact was cooling of the climate. The sulphate aerosols were shielding the Sun. People didn’t readily notice the problem: the aerosols were colourless and we tend to avoid looking at the Sun. The cloud cover would not have helped! I only found one mention in a Boston newspaper that the Sun did not have its usual strength. But the climate noticed, and snow could fall even in the summer of that year; the story of Frankenstein is also a story about dealing with the cold: cold weather and cold hearts. Crops grew poorly or not at all. Three years of famine ensued.

But why the excessive rain? As the beer slogan says, If I wanted water, I would have asked for water. In fact, much of Europe was unusually wet and rain was never far away. But rain comes from evaporated water, and that requires sunlight. In colder weather, shouldn’t there have been less rain?

Weather records from the time show that the weather was for from uniform across the globe – not everyone shared in Shelley’s wet summer. It was cold in Western Europe, Eastern North America and Asia. Eastern Europe and parts of Russia were warmer, as probably was the Western US. The southern hemisphere was much less affected, probably because it has much more water which cools very slowly. The rain was also not universal, with Western Europe being soaked, the Eastern US was dry and parts of Eastern Asia were wet, while the monsoon in Asia may have weakened.

Putting the various weather record together for the summer of 1816 gives the picture shown here. The top diagram shows the temperature deviations compared to normal, with Western Europe being particularly chilly. The bottom panel shows the precipitation, with Spain being dry and Western Europe and Scandinavia being much wetter than normal.

Models have difficulty reproducing the details. It appears that the developments may be quite sensitive to the initial conditions, which in this case were already affected by the 1809 eruption – which may also have been a large one. There are also suggestions that an El Nino occurred after the 1815 eruption. Observations from ships indicate that the air pressure was low around Iceland in 1816, while the Azores high was weaker than usual. This causes a strong westerly to northwesterly flow, bringing low-pressure systems to Western Europe. In Geneva, low pressure systems brought cold rain and the high pressure systems hardly ever came – the only week with above average temperature in Geneva was in early April 1816. Hence the gloom of Frankenstein.

What could have caused this? A stronger polar vortex may have been involved: this is something which is also suggested by climate models. It set up a stable pattern, with Geneva in the firing line. It was cold in many places, due to the fainter Sun, but Western Europe was also on the receiving end of a long line of low pressure systems which lasted all summer. If the eruption had happened at another time, with different starting weather, the pattern may have been different. A new Tambora would not necessarily give the same weather.

Our rain this winter is the same. There is no volcano at fault. We just became caught in the wrong stable pattern.

Volcanoes and rain

So do volcanoes cause rain? It is a mixed bag. It is plausible that the water they put in the atmosphere may rain out nearby, but at larger distances volcanic water is negligible compared to what is already in the atmosphere. Large eruptions cool the climate and this will reduce rainfall worldwide. But locally the effect can be very different, though hard to predict. Western Europe was drowning in rain after Tambora. But it may not be the same for the next major eruption: every volcanic eruption is different. Ou weather may be cold – and dry.

Did you finish that beer yet?

Albert, February 2026