Lightning is a large-scale natural spark discharge that occurs within the atmosphere or between the atmosphere and the Earth’s surface. On discharge, a highly electrically conductive plasma channel is created within the air, and when current flows within this channel, it rapidly heats the air up to about 25 000°C. The lightning channel is an example of terrestrial plasma in action.

Seeing lightning

Lightning is visible as a flash of blue-white light. The extremely high temperatures generated heat the air molecules to a state of incandescence (white hot) such that they emit a vivid white light. At the same time, nitrogen gas (the dominant gas in the atmosphere) is stimulated to luminesce, producing bright blue-white. The combination of light from luminescence and incandescence gives the bolt of lightning its characteristic colour.

Lightning’s partner

Temperatures in the narrow lightning channel reach about 25 000°C. The surrounding air is rapidly heated, causing it to expand violently at a rate faster than the speed of sound, similar to a sonic boom. At about 10 m out from the channel, it becomes an ordinary sound wave called thunder.

Thunder is effectively exploding air, and when heard close to the lightning channel, it consists of one large bang. At about 1 km away, it is heard as a rumble with several loud claps. Distant thunder has a characteristic low-pitched rumbling sound. However, beyond 16 km, thunder is seldom heard.

Conditions needed for lightning to occur

It is the formation and separation of positive and negative electric charges within the atmosphere that creates the highly intensive electric field needed to support this natural spark discharge that is lightning.

The formation of electric charges in the atmosphere is due mainly to the ionisation of air molecules by cosmic rays. Cosmic rays are high-energy particles such as protons that originate from outside the solar system. On colliding with air molecules, they produce a shower of lighter particles, some of which are charged.

Within a thundercloud, the rapid upward and downward movement of water droplets and ice crystals can separate and concentrate these charges. The negative charges accumulate at the bottom part of the cloud and the positive charges towards the top.

Lightning production

As the area of negative charge at the base of the thundercloud builds up, it induces a region of positive charge to develop on the ground below. As a result of this, a potential difference or voltage is created across the cloud-to-ground gap. Once the voltage reaches a certain strength, the air between the base of the cloud and the ground develops an electrical conductivity. At first a channel, known as a stepped leader, is formed. Although invisible to the naked eye, this allows electrons to move from the cloud to the ground.

It is called a stepped leader because it travels in 50 to 100 m sections, with a slight pause in between, to the ground. As it nears the ground, a positively charged streamer fires upwards from the ground to connect with it. Streamers are most often initiated from tall objects on the ground.

Once connected, electrons from the cloud can flow to the ground and positive charges can flow from the ground to the cloud. It is this flow of charge that is the visible lightning stroke.

After the first discharge, it is possible for another leader to form down the channel. Once again, a visible lightning stoke is seen. This can happen 3–4 times in quick succession. All of this happens in a time interval of about 200 milliseconds.

Monitoring lightning

A worldwide lightning location network (WWLLN, pronounced ‘woollen’) was founded in New Zealand in 2003. Working with the collaboration of scientists from around the world, the network plots lightning discharge locations seconds after they occur.

Around the world, there are about 45 lightning flashes per second. Apart from generating the characteristic blue-white light, radio wave pulses known as sferics are also produced. The frequent crackles heard when tuned into an AM radio station during a thunderstorm are sferics from the lightning discharges.

These sferics are registered at the 60 WWLLN receiving stations around the world and provide a near real-time information dataset. This information is made available to scientists via a high-speed internet connection provided by REANNZ (Research and Education Advanced Network New Zealand).

Red sprites

High above thunderstorm clouds at altitudes between 50–90 km, large-scale electrical discharges can occur. These are triggered by thundercloud-to-ground lightning activity. They appear as fleeting, luminous, red-orange flashes and take on a variety of shapes. Unlike ‘hot plasma’ lightning, they are cold plasma forms somewhat similar to the discharges that occur in a fluorescent tube.

It is because of their fleeting nature, lasting mostly for only milliseconds, and ghost-like appearance that the term ‘sprite’ has been used.

St Elmo’s fire

In the region between a thundercloud and the ground, a very strong electric field can be set up. There is a huge potential difference (voltage) established between the negative base of the cloud and the positive ground. When this potential difference reaches a certain value, sharp-pointed ground-based objects are seen to glow, often with a hissing sound.

Because this weather-related occurrence sometimes appeared on ships at sea during thunderstorms, it was given the name ‘St Elmo’s fire’. Saint Elmo is the patron saint of sailors, and in the past, sailors regarded such an event as an omen of bad luck and stormy weather.

St Elmo’s fire is a bright blue or violet glow due to the formation of luminous plasma. It appears like fire in some circumstances coming from sharply pointed objects such as masts, spires, lightning rods and even on aircraft wings.

Nature of science

The tale of the 100-year hunt for red sprites is a story of how science works. It is a story illustrating that science, rather than knowing all there is to know, stands barely on the threshold of many more discoveries about our complex and fascinating universe. They were given little more credence than UFO sightings until 1989, when university researchers accidentally captured a red sprite on a low-light video camera.

Useful links

NASA website on lightning with easy-to-read information and good animations.
http://scijinks.nasa.gov/lightning

A new theory of lightning formation, called runaway breakdown, hypothesises that cosmic rays trigger the process.
www.pbslearningmedia.org/resource/oer09.sci.phys.maf.newlightng/a-new-theory-of-lightning

Information on ‘transient luminous events’ produced by large thunderstorms in the upper atmosphere.
www.albany.edu/faculty/rgk/atm101/sprite.htm

The science of thunder is the focus of this information sheet produced by the National Lightning Safety Institute of America.
www.lightningsafety.com/nlsi_info/thunder2.html

Up-to-date information on worldwide lightning strikes.
http://wwlln.net/

    Published 29 April 2014