The vast majority of us complained about the excessive heat here in the UK last week. We are now surrounded by cooler temperatures and constant rain; we can’t win. Along with the wet conditions has come thunderstorms, some of which have been pretty damn magnificent. It got me thinking, what is thunder and lightning and could we use it?
We are not alone in our recent storms. Around the world there are more than a thousand thunderstorms at any given moment, causing 6,000 lightning flashes. A thunderstorm is caused by a rapid rise and fall of air currents. The friction created by this movement creates an electrical charge within a storm cloud. As water or ice fall, they carry the electric charge to lower levels of the cloud, building a negative charge in the area, whilst the top levels are more positively charged. Usually lightning moves between these two charged areas of the cloud without a problem. However, if the negative charge becomes great enough, it will find a path to the positively charged ground below.
The current looks for a good electrical conductor (such as metal), or a tall structure which is anchored to the ground (hence you should not stand under a tree in a lightning storm). The negative charge sends out a “scout” for a short path to the ground in the form of invisible steps, commonly called a stepped leader. This can move at ~200,000 mph. As the steps near, items at ground level respond to the electric field by reaching out to them with a positive streamer. The human body even reacts this way in a storm! Once the stepped ladder and positive streamer meet, the ionised air has completed its journey to the ground and leaves a path for the current in the cloud to flow between. The flash we observe is the discharge which occurs during a strike as the cloud tries to neutralise itself (i.e. regain equal positive and negative charge within the cloud), and moves at ~200,000,000 mph!
Interesting fact: it is not the rubber tyres on a car that keep you safe in a lightning storm, instead it is the metal frame itself! A metal frame such as this, acts as a Faraday cage. This shields the objects inside from an electromagnetic field by distributing charge around the surface. Aeroplanes have aluminium exteriors to also avoid such lightning strikes.
There are 3 types of lightning strike: (1) Cloud to ground, (2) ground to cloud, or (3) cloud to cloud; and 6 types of lightning: normal, sheet, heat, ball, red and blue. Each lightning bolt can contain up to 1 billion volts of electricity (roughly enough to power a house for a month); imagine if we could harness the power of lightning, not unlike the Greek God Zeus. On average, people in the UK each used 1,985 kWh in 2010. The idea of harnessing the power of a lightning bolt would be a great form of renewable energy, but we’d have to find a way to handle those 1 billion volts and all within the few microseconds a lightning bolt lasts. The capture, storage and distribution facilities required would be far too complicated, and the power supplied is unlikely to yield enough to justify the cost. Also, even with so many volts, most of the energy is dispersed when it hits the ground, leaving only 1/4 of a kWh; so we’d need an awful lot of lightning to make a difference.
Although it is unlikely that getting an electricity supply from lightning will be economically feasible, MIT researchers remain hopeful that lightning capture will one day be possible to aid research. Despite this, storms with thunder and lightning will always be a natural wonder, and to understand the science which happens in just milliseconds is truly fascinating.