The science of thunderstorms

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Here in the UK, a completely un-British heatwave finally came to a thundery end last weekend. Having already looked at the chemistry behind the smell of rain, here’s a look at some of the science behind thunderstorms. How does lightning happen, what gives it its blue-violet tinge, and what does it have to do with plant growth?

Let’s take a look at how lightning happens to begin with. Scientists’ understanding of the full process is still patchy in places, but we do have a relatively good general idea. Inside a thundercloud, cold temperatures and air updrafts provide perfect conditions for lightning genesis. Small, super-cooled droplets of water get carried upwards in the cloud by the updrafts, along with small ice crystals. Blobs of denser soft hail (called graupel) are comparatively unmoved or move downwards.
 
The opposite movement of ice crystals and graupel in the cloud inevitably results in collisions. During these, electrons transfer between the two. As a result, the ice crystals and super-cooled water droplets becoming positively charged, and the graupel negatively charged. With the ice crystals moving up as the graupel moves down, pretty soon there’s a charge difference between the top and bottom of the cloud. The cloud’s top becomes positively charged, while the centre and base become negatively charged.
 
It’s not just the cloud affected by this process. The accumulation of negative charge in the base of the cloud causes repulsion of electrons below it on the ground. Eventually, the attraction between the negatively charged base and the positively charged ground is large enough for a stream of electrons to jump down from the cloud at approximately 270,000 miles per hour – a lightning strike.
 
Cloud-to-ground lightning, where lightning strikes the ground, is just one possible result of the charge imbalance in storm clouds. Lightning can also jump between the differently charged regions in clouds without reaching down to the ground at all, or even between separate clouds.
 
As well as being fast, lightning can heat the surrounding air to an incredibly high temperature. It’s estimated that the temperature of the air channel through which lightning passes can reach up to 30,000˚C – significantly hotter than the surface of the sun. It’s this high temperature that causes the thunder that accompanies lightning strikes. The heating of nearby air causes it to expand rapidly; it then cools and contracts. This creates the sonic shock wave we refer to as thunder.
 
Because the sound of thunder travels at a much slower speed than the flash of lightning, you can use it to estimate the distance you are from a lightning strike. Sound travels at approximately 343 metres per second in air, so the sound of thunder travels about 1 kilometre in 3 seconds. If you can see the lightning, you can work out how far away the storm is!
 
Our discussion has focused on physics so far, but there’s also some interesting chemistry in thunderstorms. If you’ve ever been convinced you can smell a storm coming, you’re not wrong. The oddly sweet, pungent smell that sometimes precedes a storm is that of ozone. Lightning strikes split diatomic oxygen molecules in the atmosphere into individual oxygen atoms. These can then combine with other oxygen molecules to form ozone.
 
The blue-violet tinge that lightning sometimes takes on is a consequence of it ionising molecules in the air. In particular emissions from excited nitrogen atoms and hydrogen atoms (the latter from water vapour in the air) result in these colours.
 
Nitrogen is also involved in further lightning chemistry. At the high temperatures lightning generates, there is enough energy for nitrogen and oxygen in the air to combine, forming nitrogen oxides. In turn, these nitrogen oxides can dissolve in rainwater and form nitrates, which are important for plant growth. The process of usually unreactive nitrogen being turned into nitrates that plants can use is called ‘nitrogen fixing’. Lightning accounts for up to 3-10 teragrams of nitrogen fixed per year (compared to 100-300 teragrams fixed by bacteria).
 
That’s not the end of the processes lightning can trigger. Recent research has shown that the gamma rays released by lightning can even set off small-scale nuclear reactions in the atmosphere. This natural process can generate different isotopes of nitrogen, oxygen, and carbon.