Lightning
Article curated by Rowena Fletcher-Wood
We don’t know what causes lightning, not really. Which is a worrying thought, when you can’t explain where something like 100 million volts of energy comes from. We don’t know what its true effects on the earth are either, especially when it comes to chemistry, but scientists are starting to notice this, and are trying to illuminate the science of lightning.
What we do know
Lightning is the result of an electrical discharge, a charge difference built up in the atmosphere that finally equalises with a burst of powerful energy.
Most lightning comes about when negative charge builds up in clouds and positive charge builds up on earth – but it can also happen the other way around. We don’t know how, or why. This backwards “positive” lightning was actually only discovered in the 1970s because our normal detectors can’t pick it up: they’re looking for the wrong kind of charge. This also means it can be extremely dangerous, and can seriously damage or even destroy planes.
But how does a thundercloud charge up in the first place?
There’s a simple model to explain the charging, but it has gaps in it. This model talks about electrons getting stripped off clouds, like static charging when things rub together. Convection currents then carry the lighter particles upwards and the heavier particles slowly descend under gravity. If the lighter particles are positive and the heavier ones are negative (or the other way round), this creates a charge separation, and so an electric field. The earth feels the charge above it (which is usually negative) and becomes slightly charged itself in response (usually positive). When lightning discharges, a step leader of charged particles joins the cloud to the ground, and here the electricity flows.
So far so good. But the simple model gets to this point and freezes up – failing to explain why negative particles are heavier than positive ones, or why you need both water and ice in a cloud above 5 km for it to charge up and become a thundercloud.
And it gets weirder. However it charges, the electrical fields in thunderclouds simply aren’t big enough to discharge spontaneously (at least that’s what we think). They’re about ten times too small, at least. In fact, that lightning makes it kilometres of distance across the sky down to the earth is an amazing feat. It’s a long way – plus, air is a pretty good insulator. Electricity doesn’t normally flow in air... yet lightning still happens. Lots.
Theories
The standard theory for the lightning initiation problem is the ice theory, which most researchers subscribe to. This theory seeks to explain the need for ice in thunderclouds to make them “go”. In the theory, tiny ice crystals collide with each other, rubbing together and losing electrons exactly like static. These charged ice crystals are known as hydrometeors. Lightning scientists reckon that they might charge up enough to ionise the air around them and discharge a lightning bolt. The theory is supported by findings that correlate ice water content/crystal size in the upper troposphere (where the storm clouds form) with lightning[1][2].However, others think that there simply needs to be more energy input to bridge the vast distance between the charged clouds and the earth. This could come from cosmic rays from space, which “seed” the lightning bolts (and if this is true, space weather could be significantly more important than we’ve ever thought before!).
Cosmic rays are mainly energetic protons and electrons that drop from space all the time and hit particles in our atmosphere hard, causing them to break up into their fundamental components under the impact. They are created by big stellar events like collisions between stars and supernova explosions. Because they are fast-moving particles, it’s possible that they not only initiate discharges in storm clouds, but also drag charged particles down through the sky towards to earth, creating the step leaders that join storm clouds to the earth.
Cosmic rays also create electromagnetic radiation when they interact with other matter, and proponents of this theory think this may be the path to settling the question of lightning – at least until any other theories pop their heads above the surface.
75% of lightning never touches the earth, but stays in the clouds, emitted as flashes between one cloud and another, or as gamma or x-rays. Gamma ray bursts appear above storm clouds – and you can see them: short, intensely bright flashes happening just above the dark storm clouds that emit the lightning.
These are well documented, but they should also be accompanied by radio waves characteristic of the cosmic rays, which are detected using radio interferometers. Some are detected; in particular, just before a lightning storm, when we get lots of strong, short radio pulses, like aliens signalling in Morse code. On the other hand, even cosmic rays shouldn’t have enough energy to create this amount of radiation or these many lightning bolts alone, and so far we’ve not seen more lightning on days with more cosmic rays, or less on days with fewer. So either there are more cosmic rays: special, powerful cosmic rays that we don’t know about (maybe belonging to more obscure kinds of particles showering down on us?), or something else is causing the big radio pulses.
One explanation is that the radio waves look bigger than they actually are because they are being magnified by the hydrometeors (we don’t know how). This could mean cosmic rays are not lightning initiators at all.
Fossil fuels vs lightning
We’re always hearing that burning fossil fuels creates pollution – CO2 mostly, but also sulphurous and nitrous oxides. Although the sulphur and nitrogen contamination in fossil fuels is small, enough traffic or a big power station can still make a heck of a lot of it. We know how much we generate, but the amount of nitrous oxide pollutants actually out there in the atmosphere is unknown. This is mainly because we’re not the only source of it, and gases are difficult to measure – they’re free, often invisible, and move and mix in three dimensions at a speed of around 500 m/s.
Nitrous oxides are leached by the ocean, and released by animals when they digest plants, but atmospheric chemists blame lightning as a major source (although probably not as much as burning fossil fuels!), and guess that it might contribute as much as 8.6 million tonnes a year[3] – though estimations do vary: we don’t really know how to measure lightning energy.
We do know it takes a lot of energy to get nitrogen and oxygen to react though. Both form stable molecules that together make up 99% of our atmosphere. The energy to make 1 kilo of NO from oxygen and hydrogen is the same as the energy needed to prepare 23 mugs of tea. That means that 8.6 million tonnes is 197,800,000,000 mugs of tea, or nearly 3 and a half years of tea drinking across the whole of the UK (and we love our tea).
Fossil fuels may lead to the formation of nitrous oxides near the earth’s surface, but 3-8 miles higher up, in the troposphere, nitrous oxide level are primarily due to lightning. Up there, these oxides undergo chemical reactions that lead to the accumulation of the ozone layer and increasing global temperatures. This essentially means that the health problems associated with nitrogen oxides are our fault, and the contribution to climate change may be safely blamed on lightning. Because nitrogen oxides near the earth’s surface react with things on the earth (to cause health problems, say), they don’t really make it up to the troposphere.
What are Nitrous Oxides?
Nitrous oxides, or NOxs, means NO and NO2. Sure, other compounds that combine nitrogen and oxygen in different amounts exist, but it's these two that we mean when we talk about NOxs. And it is these two that contribute to acid rain, smog (which harms our respiratory health and may aggravate heart disease), numbers of insect pests, and tropospheric ozone (which is bad for our health as well as a greenhouse gas).Ozone is actually made by nitrous oxides, which react with organics in the presence of sunlight, especially higher up in the atmosphere. Although the amount of NOx out there is less than 0.1% of the amount of CO2, it is 200 or 300 times better at trapping heat and has the longest lifetime of all greenhouse gases – up to 150 years!
Lightning and climate change show a positive feedback – the hotter it gets, the more lightning, and the more lightning, the more ozone, so the hotter it gets...[4]. Lightning strikes are on the increase, just like other temperature-driven extreme weather events, like rainfall.
3
Similarly, the climate governs where lightning happens, and lightning happening acts as an indicator of unstable atmospheric conditions. Lightning hotspots are organised, and depend on the rate, extent and changeability of heating. This also means that climate change moves lightning zones. Scientific models show that radiative warming happens mostly not at the surface of the earth, but high up in the troposphere around the belly of our planet.
NOx emissions also cause global cooling[5]. When ozone and water react photochemically, hydroxide radicals are produced, which destroy loads of atmospheric compounds, including greenhouse gases like methane. This effect is so pronounced that, unlike other transport systems, ship emissions actually lead to an overall net global cooling effect. Scattering solar radiation from particles also creates cooling. Although this is less than the global warming effect, it does mean that nitrous oxides are continuously mediating atmospheric chemistry and conditions. How much, however, we don’t know: we have no sure way of measuring the pre-industrial atmosphere to compare; afterall, back then we didn’t have the technology.
The Data Problem
Lightning data is rather thin on the ground as well: we only have 20 years of it. There are few long term studies, so shorter timescales are used for predictions and theories, meaning conclusions are very approximate. So far from exciting storm-chasing, lightning scientists are actually tasked with a lot of data collection.And it’s not straightforward. Whilst 100 million lightning bolts strike the earth every year, it’s not easy to monitor all of them when we don’t have a good way of predicting them. Scientists are left just waiting for the ground temperature to peak and start to drop off (which happens when lightning activity peaks, but also happens without lightning), or looking out for bursts of radio waves. Lightning activity also depends on wind, electrical fields, the height and the material content of the earth. Still, researchers are able to monitor more and more. In the US, the National Lightning Detector Network record the electromagnetic pulse from each strike to determine their times and positions. They also have land- and space-based detectors to collect flashes and image lightning[6][7][8]. Research is currently underway to develop a Geostationary Lightning Mapper (GLM) satellite that will collect data on lightning instances in the Western hemisphere. However, this data will still be cloudy: thunderclouds block our measurements of light and their changing thicknesses make them difficult to correct for. Lightning statistics look set to remain estimations for a while.
Lightning presents an interesting challenge to science, not only because there isn’t enough data to understand and predict it, but because its effect on the atmosphere, human health and a plethora of other phenomena remains mysterious.
2
For example, ball lightning has been blamed for spontaneous human combustion. This theory still requires research, but the possibility is certainly interesting.
Lightning has even been implicated in some origin of life theories – providing the “spark” that got things moving. This is known as the electric-spark hypothesis. To delve deeper into this theory, check out our blog post on the topic!
Learn more about /origin of life#.YFht2dynw2w.
2Is lightning hiding more than we guessed, and is this under-studied phenomenon perhaps the most important of natural forces? Perhaps future research will tell...
This article was written by the Things We Don’t Know editorial team, with contributions from Johanna Blee, Rowena Fletcher-Wood, Joshua Fleming, and Holly Godwin.
This article was first published on 2021-08-21 and was last updated on 2021-08-21.
References
why don’t all references have links?
[1] Petersen, W.A., H.J. Christian and S.A. Rutledge, 2005: TRMM observations of the global relationship between ice water content and lightning, Geophys. Res. Lett.,32, L14819, doi:10.1029/2005GL023236
[2] Sherwood, S., V. T.J. Phillips and J.S. Wettlaufer, 2006: Small ice crystals and the climatology of lightning, Geophys. Res. Lett., 33, L05804, doi:10.1029/2005GL025242
[3] Joel S. Levine; Tommy R. Augustsson; Iris C. Andersont; James M. Hoell Jr. & Dana A. Brewer (1984). Tropospheric sources of NOx: Lightning and biology. Atmospheric Environment. 18 (9): 1797–1804. doi:10.1016/0004-6981(84)90355-X
[4] Romps, David M., et al. Projected increase in lightning strikes in the United States due to global warming. Science 346.6211 (2014): 851-85.
[5] Change, Intergovernmental Panel On Climate. Climate change 2007: the physical science basis. Agenda 6.07 (2007): 333. Working Group I: The Physical Science Basis.
[6] Price, C., and A. Melnikov, 2004: Diurnal, seasonal and inter-annual variations in the Schumann Resonance parameters, Journal of Atmospheric and Solar-Terrestrial Physics, 66, 1179-1185.
[7] Price, C., O. Pechony and E. Greenberg, 2007: Schumann resonances in lightning research, J. of Lightning Res., 1, 1-15
[8] Christian, H.J.R.J. Blakeslee, D.J. Boccippio, W.L. Boeck et al.: Global frequency and distribution of lightning as observed from space by the Optical Transient Detector, J. Geophys. Res., 108, 4005, doi:10.1029/2002JD002347, 2003.
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