The Positive Side of Lightning
Who doesn’t love a good thunder and lightning show? The awesome power of nature writ large across the sky in a spectacular show of sound, light and fury worthy of the gods themselves.
Thunderstorms are incredibly turbulent environments (just ask anyone who has been in an aircraft close to one); strong updrafts and downdrafts occur with regularity, transporting small liquid water droplets from the lower regions of the storm to heights of up to 70,000 feet, whilst downdrafts transport hail and ice from the frozen upper regions of the storm. When these collide, the water droplets freeze and release heat. This heat in turn keeps the surface of the hail and ice slightly warmer than its surrounding environment and forms a ‘soft hail’.
When this soft hail collides with additional water droplets and ice particles, electrons are sheared off the rising particles and collect on the falling particles. Because electrons carry a negative charge, the result is a storm cloud with a negatively charged base and a positively charged top. This is the genesis of lightning.
As the positive and negative charges separate, an electric field is generated between the top and base of the storm cloud. This field increases in strength as the charges continue to separate over the lifetime of the cloud.
The atmosphere is not very conductive, even when damp with ice, hail and rain, so the electric charge has to be in the order to 30,000 amps and 100 million volts in order to be able to discharge to ground. Most lightning that occurs within a storm will actually be cloud-to-cloud lightning (70-80%), rather than the more impressive cloud-to-ground type.
The electric field within the storm is not the only one that develops. Below the negatively charged storm base, positive charge begins to accumulate within the surface of the earth. This positive charge will follow the storm, and is responsible for cloud-to-ground lightning.
As the charges continue to increase, a ‘streamer’ of positively charge particles can begin to rise up from the ground. At the same time, a channel of negative charge (a ‘stepped ladder’) descends from the base of the cloud. This streamer travels incredibly fast and invisibly. As the stepped ladder approaches the ground, the positive streamer is attracted to it and, once connected, the transfer of power is what we call lightning — a bright flash of light as the electrical charge travels down to earth. If there is any leftover residual charge in the cloud, you can get additional lightning strikes following the same path as the original. this give some cloud-to-ground strikes that flickering effect.
The reason that taller objects appear to be struck by lightning more is simply down to the resistance of the atmosphere to carrying charge — they are closer to the base of the cloud, so the charge does not have to travel as far to discharge.
Most people have seen this type of lightning. There is however, another type: positive lightning — and it is a damn sight more terrifying.
Whereas 95% of all cloud-to-ground lightning is the traditional negative lightning from the base of the cloud, occasionally you can get positive lightning from the top of the cloud. Why is it more terrifying? Well, as mentioned above, in order for lightning to travel through the atmosphere, it has to have incredible electrical power behind it. Since the tops of storm clouds can reach 95km in height, the charge needed for lightning from the top of the cloud to reach the ground is orders of magnitude greater than for negative lightning. in fact, it has been recorded at 300,000 amps and over a billion volts. Over one billion volts. Just think about that for a minute. Not only that, but a positive lightning strike usually lasts for 5-10 times as long as a negative lightning strike. Whilst you could blink and miss a negative lightning strike, you certainly won’t miss a positive lightning strike.
That’s not the end of it though, this incredible charge has to reach the ground first and, as previously mentioned, the ground near the storm attracts a positive charge. Since positive-positive charges repel, these positive lightning bolts often have to travel much further to encounter negative charged earth. Some positive lightning discharges may happen directly beneath the cloud, but most of them have to seek out the negative charge of the earth further afield — strikes up to 10 miles from the storm are not uncommon and since a thunderstorm is usually quite localised, there could be clear, blue skies overhead and only a distant grey sky when a lightning bolt carrying almost a billion volts and 300,000 amps strikes from seemingly out of nowhere.
So you may think you’d be safe a respectable distance away from a large storm; let’s say a good 50 miles away, where the storm is just a small grey cloud near the horizon. You’d be wrong. Positive lightning strikes have been recorded a good 50 miles away from a storm.
They are often responsible, due to their higher charge and longer duration, for starting forest fires and can cause significant damage to property and electrical infrastructure. The air temperature around such a strike can be flash-heated to around 30,000ºC.
Despite this, around 90% of people hit by lightning survive, although they can be badly burnt and often with a distinctive scarring, called a ‘Lichtenberg figure’. This type of scarring usually fades over time and resembles a fractal pattern, as the blood vessels beneath the skin are superheated and explode.
It’s still better to be safe than sorry in a thunderstorm and stay indoors (or inside a car with the windows rolled up), away from conductive materials and windows if the count between flashes and thunderclaps is less than 30 seconds.
