Ever since Benjamin Franklin flew his kite and nearly blew himself up with the resultant lightning strike, we have been attempting to discover exactly how lightning works. A discharge of lightning is probably one of the most impressive displays in the atmosphere, yet surprisingly, despite years of study, the exact process is still shrouded in some mystery.
Lightning results from the build up of opposite electrical charges within a cloud. Ice crystals which form in the upper reaches of a cloud are generally positively charged, and water droplets which sink to the lower part of the cloud, are usually negatively charged. It appears that the ice crystals are positively charged as a result of collisions, within the maelstrom of a thunderstorm, with warmer water droplets. When the warmer droplet collides with an ice crystal it passes on its positive charge, and consequently the water droplet is now negatively charged.
Okay, so, as the storm now tracks across the land it is positively charged in its upper reaches and negatively charged in its lower portions. Because opposites charges attract a positive charge now builds up on the surface of the land directly below the storm. However, the air is an excellent insulator and initially stops these charges from joining each other. But as the potential difference builds between the cloud and the land eventually the inevitable happens and the air can no longer hold back the negative electrons and a process known as the ‘stepped leader’ occurs.
In the ‘stepped leader’ a succession of steps are followed. The electrons do not flow in a steady and continuous manner but in a more erratic series of steps as follows:
1. Electrons surge downwards towards the ground at a speed of 14,000 mph for around 200-300 feet.
2. The electrons stop for a fifty-millionth of a second.
3. They travel on again for another 150-200 feet
4. Positive ions start to move up from the ground from any protruding object such as a tree or a golfer
5. When they meet, a huge electric current flows carrying positive ions into the cloud.
These ions are carried on a channel a few centimetres wide and as they move upwards they glow enough to be seen. So, like a giant spark, the lightning travels upwards into the cloud so fast that it is impossible to judge whether it is travelling up or down. Your typical lightning flash will consist of approximately 4 leaders and 4 return strokes indiscernible apart from the familiar flashing.
Lightning is incredibly hot, five times hotter than the surface of the Sun at 30,000 degrees Celsius.
As a consequence the air around a bolt of lightning expands in an explosion that causes a massive shock wave. This shock wave travels through the sky as the sound of thunder.
Because, relative to light, sound is very slow moving you can calculate how far away a lightning flash is by measuring the length of time between the flash and the thunder. Every 3 seconds counted is roughly half a mile away. But beware…the danger zone for a lightning strike is within 6 miles of the storm!