Lightning

How does lightning form?

As tiny water droplets form inside a storm cloud, they are propelled towards the top of the cloud by strong internal winds (updraughts) where they turn to ice. Some of the pieces of ice grow into hail, but others remain very small. Some of the hail that forms becomes too heavy to be propelled by the updraughts and so begin to fall back through the cloud, bumping into smaller ice particles as they do so. During these collisions, electrons are transferred to the hail giving the hail a negative charge, while the ice particles that have lost electrons gain a positive charge.

The updraughts continue to carry the ice particles upwards, giving the top of the cloud a positive charge. The hail continues to fall through in the lower part of the cloud, giving it a negative charge. As well as being attracted to the positive charge in the top of the cloud, the surplus of electrons in the cloud base are attracted to positive charge in other clouds and on the ground. If the attraction is strong enough, the electrons will rapidly move towards the positive atoms. The path they make in doing so forms the channel we see during a flash of lightning.

As negative charge builds at the base of the cloud, the electrons near the ground's surface are repelled. This leaves the ground and the objects on it with a positive charge. As the attraction between the cloud and the ground grows stronger, electrons shoot down from the cloud cutting through the air at around 270,000 miles per hour.

Lightning flashes and lightning strikes

Although often assumed to be the same thing, there is a key distinction between lightning flashes and lighting strikes. A lightning flash is what you can see, but this is often made up of several individual lighting strokes which are pulses of current which occur separately if though only hundredths of a second apart. The term lightning 'strike' refers to cloud-to-ground lightning, where lightning 'strikes' the ground.

A bolt from the blue

Most lightning strikes originate from the negative part of a cloud, however occasionally a strike can come from the positively charged top part - this is called 'positive lightning'. When positive lighting strikes it is forced to go around the negatively charged base of the cloud, this generally results in a more powerful lighting strike which shoots out sideways and sometimes can travel a mile more away from the storm cloud before connecting with the ground. The nature of this type of lightning strike is where get the term 'a bolt from the blue'.

Detecting lightning

Being able to detect the location of thunderstorms is of great importance as it is not only the lightning strike that is dangerous, but many other factors linked to thunderstorms. These include intense rain and tornadoes.

When lightning strikes it sends out pulses of radio waves and these can be used to detect lighting strokes. The Met Office LEELA (Lightning Electromagnetic Emission Location by Arrival time difference) system detects these pulses at a frequency known as VLF (Very Low Frequency) - much lower frequency than normal radio waves.

These pulses are known as 'sferics' and are capable of travelling great distances because they are reflected between the surface of the Earth and a layer of the upper atmosphere called the ionosphere - in a similar way to light travelling within a fibre optic cable.

An individual sensor is able to detect a sferic, but in order to determine a thunderstorm's exact location, a network of sensors is required (e.g. the LEELA system network of 11 sensors positioned around Europe). When a strike occurs, the network of sensors will pick up the sferic at slightly different times and through a technique known multi-lateration these readings can be used to determine the exact location of the thunderstorm. The difference in the time taken for the sferic to reach one sensor relative to another is called the ATD (Arrival Time Difference).

Types of lightning

• Ball lightning - a rare form of lightning in which a persistent and moving luminous white or coloured sphere is seen.
• Rocket lightning - a very rare and unexplained form of lightning in which the speed of propagation of the lightning stroke is slow enough to be perceptible to the eye.
• Pearl-necklace lightning - a rare form of lightning, also termed 'chain lightning' or 'beaded lightning', in which variations of brightness along the discharge path give rise to a momentary appearance similar to pearls on a string.
• Ribbon lightning - ordinary cloud-to-ground lightning that appears to be spread horizontally into a ribbon of parallel luminous streaks when a very strong wind is blowing at right angles to the observer's line of sight.
• Forked lightning - lightning in which many luminous branches from the main discharge channel are visible.
• Sheet lightning - the popular name applied to a 'cloud discharge' form of lightning in which the emitted light appears diffuse and there is an apparent absence of a main channel because of the obscuring effect of the cloud.
• Streak lightning - a lightning discharge which has a distinct main channel, often tortuous and branching, the discharge may be from cloud to ground or from cloud to air.