St Elmo's fire
Sailors out at sea would sometimes see a bluish glow seeming to shoot out of the ends of the masts of ships at night. The light wasn't hot and wouldn't burn anything on board. They took it to be a good omen and called it St Elmo's fire.
Atmospheric scientist Steve Ackerman at the University of Wisconsin-Madison in the US has been fascinated by St Elmo's fire ever since his brother encountered it.
I've still not seen it first-hand, but I'll keep looking for it
Ackerman's brother was working on some copper piping in his basement during some bad weather. "A thunderstorm moved into the area, and at one point many of the pipes had a blue glow over them," says Ackerman. "That's when I start looking into what caused it."
Thunderclouds create a strong electric field, because there is a strong difference in electrical charge between the cloud and the ground, which you can sometimes feel as static. The field can be intensified by pointed objects, like a metal pipe or the mast of a ship.
If this electrical field becomes strong enough, it can break air molecules down into electrically-charged particles. The gases become "plasma", and give off a glowing light.
A similar plasma glow can be created in the laboratory, using sharp or pointed objects to intensify an electrical field. Even so, Ackerman wants to see St Elmo's fire naturally. "I've still not seen it first-hand, but I'll keep looking for it."
Like St Elmo's fire, the will-o'-the-wisp is a faint light that has been reported for centuries. But unlike St Elmo's fire, in recent times people have reported it less and less.
As you might expect of a phenomenon whose name has come to mean something elusive, it has never been created in the laboratory.
It could be that the reports are fictitious
The will-o'-the-wisp is normally described as a light, flickering or constant, lying close to the ground, mostly in marshy areas of the countryside. It supposedly disappears after a couple of minutes.
Luigi Garlaschelli of the University of Pavia in Italy – best-known for recreating the Turin shroud with a few laboratory tricks – would like to study the will-o'-the-wisp in nature. But it is not clear there is anything to study.
"The risk is indeed that we are looking for something that does not even exist," Garlaschelli says. "We must trust or hope that all the sightings of will-o'-the wisps are those of a real phenomenon."
If will-o'-the-wisp really did represent a natural process, there are some possible explanations that Garlaschelli could test. For example, the association with marshy areas suggests that the light comes from burning marsh gas, which is mainly methane. However it is not clear what would set the gas alight.
Alternatively, it could be that the reports are fictitious; that the lights are imagined or hallucinated; or that the lights were reflections of the Moon or other lights that observers misinterpreted.
"You could be standing there in the middle of the ball of light," says Friedemann Freund of NASA's SETI Institute in Mountain View, California, US. "Maybe your hair would be electrified, you might have a halo like a saint. But it doesn't burn anything. You might feel a bit funny, but you wouldn't be harmed."
This is what it would feel like to find yourself in the middle of an earthquake light.
The shock waves were racing through, and as the later waves arrived there was an explosion of lights
These lights are a plasma discharge that happens when a particular type of rock is under stress and builds up an electric charge, Freund says. "We think that when we push the rocks together very fast, the charge is relieved through a plasma discharge up from the rock."
They can come in many different shapes, forms, and colours.
Coseismic earthquake lights, which happen during an earthquake, are bursts of light coming out of the ground over a space of a few kilometres. They rise 200-300m into the night sky in a fraction of a second, one after another.
In recent years, the abundancy of security cameras has led to beautiful videos of earthquake lights.
"Some of the best records are in Peru," Freund says. "A friend at a local university secured footage when a magnitude 8 earthquake hit the south of Lima. The shock waves were racing through, and as the later waves arrived there was an explosion of lights."
Often dismissed as a myth, ball lightning turns out to be quite real.
When an ordinary cloud-to-ground lightning bolt strikes the ground, it can vaporise certain minerals in the soil
In 2012 a team of researchers were measuring ordinary lightning in a storm-prone region of the Qinghai Plateau in China. Suddenly a ball of light about 5m across appeared in front of them. It burned white and then red for a few seconds before vanishing.
This was the first natural ball lightning to be studied. The researchers recorded the spectrum of light that the ball gave off, and analysed it to see if they could determine what this unusual lightning was made of.
It turned out to have a very earthly origin: soil. When an ordinary cloud-to-ground lightning bolt strikes the ground, it can vaporise certain minerals in the soil. Some of these contain silicon compounds, and under the extreme conditions they undergo chemical reactions to form silicon filaments.
These filaments are highly reactive, and burn with the oxygen in the air to create the orange glow that the researchers measured.
In the very last seconds before the Sun sets, its light can turn bright green. But the Sun has not changed colour: the flash is caused by a mirage.
These are the mirages that can make the Sun seem to move in shimmering waves
The atmosphere splits the Sun's white light into its separate colours, just like a prism: it bends red more than orange, orange more than yellow, and so on. Because red undergoes the strongest bending effect, it appears to fall past the horizon first, followed by orange, yellow and green.
The colours beyond green – blue, indigo, and violet – are strongly scattered by the gases in the atmosphere. That's why the sky appears blue. But as a result, the last coloured light that can be seen as the Sun falls below the horizon is green.
Normally this effect is very slight. To make the last green rays visible, there also has to be a mirage that makes the Sun appear much larger than usual. These are the mirages that can make the Sun seem to move in shimmering waves, and almost seem liquid as it pours past the horizon.
Ocean horizons often produce the best mirages for spotting a green flash.
Perching cameras on top of the Empire State Building in New York, US in 1935, Karl McEachron of the General Electric Company recorded something strange. Lightning was travelling not from cloud to ground, but shooting up from buildings into storm clouds.
Meteorologists now know around one in a thousand lightning bolts strike upward. But despite decades of research on upward lightning, its exact mechanism is still a puzzle.
Every time I flew through a thunderstorm, I reaffirmed that it was no place for a plane
Storm photographer Tom Warner is now researching how upward lightning is triggered, at the South Dakota School of Mines and Technology in Rapid City, US.
His and others' research has shown that there are two distinct forms of upward lightning. Both of them require a tall structure, such as a skyscraper or wind turbine, to happen.
The first kind requires a nearby ordinary cloud-to-ground strike first. The sudden disruption to the electric field causes a "lightning leader", a channel of positive or negative charge, to travel up to an area of thundercloud with the opposite charge.
The second kind doesn't require a downward lightning strike nearby, and can travel upward spontaneously.
Warner has studied and photographed these rare events since becoming fascinated by an upward lightning bolt in 2004. To get his data and images, he has piloted an armour-plated plane into the hearts of storms.
"Being able to experience storms up close and even from the inside was absolutely amazing," Warner says. "It was challenging and required intense concentration. Every time I flew through a thunderstorm, I reaffirmed that it was no place for a plane."
High above a thundercloud and its exchange of lightning with the ground, you might find a sudden reddish glow stretching for tens to hundreds of kilometres. It looks a bit like the straggling tendrils of a jellyfish.
Very large thunderstorms can produce these phenomena, which are known as sprites. "They're very intense," says Martin Fullekrug of the University of Bath in the UK. "The thunderstorm needs to produce a special kind of flash, and they're rare. Maybe only one in a thousand flashes produces a sprite."
You can get a low-quality picture of a sprite with a camera of a couple of hundred pounds
These flashes need to remove a lot of electrons from the thundercloud. A long, slow current is needed to make a sprite, and such currents can form in big storm systems reaching 100km across.
The elusiveness of these deep red flashes earned them their ethereal name, adopted from Shakespeare's A Midsummer Night's Dream. But as the price of powerful cameras has dropped, sprites are being caught on camera increasingly often.
An ordinary CCTV camera with good night vision can snap a low quality image. Amateur meteor observers are also collecting extensive data on sprites.
"You can get a low-quality picture of a sprite with a camera of a couple of hundred pounds," says Fullekrug. "With a little bit of guidance anyone can do it."
The term ELVES is a clunky acronym chosen to complement their cousins the sprites. It is short for "Emissions of Light and Very low frequency perturbations due to Electromagnetic pulse Sources", but that is something "hardly any scientist can spell out for you", according to Fullekrug.
It is very difficult to see an ELVE with the naked eye
Appearing around 80-100km above the ground, they look very different to sprites.
"They're expanding rings of light," says Fullekrug. "They look like a doughnut from space, with a dark hole in the middle, and they spread out for 1,000km or so."
ELVES are fleeting, lasting for less than a millisecond. The storm conditions necessary to make an ELVE (pronounced "elf") include a particular type of lightning, with a very sharp rise in current. Unlike for sprites, to get an ELVE the discharge has to be very sharp, so the two rarely occur at the same time.
ELVES occur more often than sprites, with about 1 in 100 lightning flashes producing one. Small storms are just as likely to make them as big storms, as a really fast current can happen in any storm.
ELVES are mainly white because they're so intense. "They're really, really fast," Fullekrug says. "It is very difficult to see an ELVE with the naked eye. I've not seen one myself, though I've been looking quite a bit."
Blue jets and gigantic jets
"Blue jets are a little bit of a mystery," says Fullekrug.
The first problem is that they're blue. Blue atmospheric phenomena are hard to study from the ground, because the atmosphere is so good at scattering blue light. They're also very narrow and rare.
There are marvellous examples of gigantic jets developing off the coast of Africa
"We don't know the ideal conditions for a blue jet to form," says Fullekrug. "One idea is that when thunderstorms get really tall, they pierce into the thinner layers of the atmosphere above." Storms have powerful updrafts, which can push them above normal altitudes. "When this happens it could generate a blue jet, but we really don't know for sure."
Researchers do know that there is another phenomenon called a gigantic jet, which seems to be a hybrid of a blue jet and a sprite. They are broader, wedge-shaped jets of light and easier to see. They can last 10-100ms, so they are relatively slow compared to other storm events.
"There are marvellous examples of gigantic jets developing off the coast of Africa," says Fullekrug. "But gigantic jets are rare. Perhaps only one in ten or one in a hundred sprites will combine with a blue jet to make a gigantic jet."
The green, blue and red shapes of the auroras, swirling over the two poles of the Earth, are a visible map of events that happened thousands of kilometres away. When the solar wind – charged particles from the Sun that brush past our planet – meets the Earth's magnetic field, the two interact.
The particles from the Sun slide along the contours of the magnetic field towards the poles. When they reach the upper atmosphere, they interact with gases. The particles can give the air molecules enough energy to release electrons, causing them to glow in a range of colours.
Earlier in 2015, Swenson launched a rocket into the aurora
"Auroras can have many shapes and structures, depending on what the magnetosphere is doing," says Charles Swenson of Utah State University in Logan, US. "There are arcs, westward surges, beading, all kinds of names for different visible shapes. You can imagine it like a sheet flapping in the wind, and every once in a while it will get really messed up and that's when these dramatic events happen."
Earth is not the only planet with auroras. "All you need is a solar wind blowing past a planet that has gases on it and a magnetic field," says Swenson. Jupiter and Saturn both have unique auroras, as the gases of their atmospheres are very different.
The aurora also has an invisible component, which is the subject of Swenson's studies. Charged particles from the solar wind cause electrical currents in the aurora, which are hard to study from the ground. Earlier in 2015, Swenson launched a rocket into the aurora to measure these invisible elements.
"The question is, are the invisible parts of the aurora dancing and moving as rapidly as the visible parts?" he says. "It's very early days, but we think the answer will be yes."