044 / / MARCH 2017

From Harry Potter’s scar to its starring role in Back to the Future, lightning is a symbol of drama. In films and TV programmes it signals the arrival of a violent or explosive crisis point. By tradition, religion has claimed lightning as a sign of heavenly wrath – the destructive power and the beauty of a lightning fork make it easy to imagine Zeus or Thor hurling bolts down to Earth.

Most of us know that thunder and lightning are essentially one and the same. That, because light travels faster than sound, there’s a delay between seeing the lightning and hearing the thunder. The booming noise is the channel of heated air that formed around the lightning bolt closing up again.

This might sound confusing. But have you ever stopped to think how the lightning occurs in the first place? For centuries it was seen as an example of magic. In the 18th century, when Benjamin Franklin and other early scientists began experimenting to find out more about lightning, they wondered if it was a fluid.

Our knowledge has grown, but it’s a complicated process that’s still not fully understood. Over the page we look at the theory of how clouds become so highly charged with static electricity that they can treat us to a cracking display.

Lightning is one of our most theatrical weather events, with summer storms bringing electrifying displays. But how much do we know about why these flashpoints occur?

Each lightning bolt can contain up to a billion volts of electricity – and about 100 lightning bolts hit the Earth’s surface every second

frightening?

Anatomy of…Lightning

V E R Y , V E R Y

046 / / MARCH 2017

S E P A R A T E C H A R G E S

When graupel collides with additional water droplets and ice

particles, this creates enough friction to sheer off electrons from

the particles travelling upwards. These electrons collect on the particles heading downwards.

Electrons carry a negative charge and collect at the base of the

cloud, while the top of the cloud becomes positively charged.

T U R B U L E N C E

Strong updrafts and downdrafts swirl around in a storm – the

updrafts whisking small water droplets up above freezing level. Meanwhile, downdrafts carry hail

and ice back towards Earth. On the way, water droplets and hail

collide to form graupel (soft hail).

L I G H T N I N G C L O U D S

The massive, fluffy mushroom cumulonimbus clouds, nicknamed

‘thunderheads’, are the ones usually responsible for lightning and other severe weather, such

as tornadoes. Anvil-like in shape, they can reach up to

14,000m in altitude.

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Anatomy of…Lightning

S T A T I C

As the positive and negative charges separate, an electric field is generated between the top of the cloud and its base. However, this stays inside the cloud thanks

to the atmosphere, which is a very good insulator. It takes a huge charge to build up before any

lightning starts. When the charge is strong enough, the electric field

overpowers the atmosphere’s insulating properties and lightning

flashes occur. There are about 75 to 80 per cent more cloud-to-air

flashes than incidences of cloud-to-ground (CG) lightning.

G I A N T S P A R K

CG lightning happens when a cloud base’s negative charge attracts a buildup of positive charge on the

ground. As the difference in charges builds, positively charged

particles rise up tall structures such as trees. A ‘stepped leader’ of

negative charge zigzags down – as it nears the ground, a positively charged ‘streamer’ reaches up.

When the two connect, a powerful electrical current flows. A flash of lightning may consist of 20 or so

return strokes travelling at around 100,000km/second.

A lightning bolt is hotter than the surface of the sun – and, yes, it can strike twice…

Turbulence causes electrons to gather at the cloud base, attracting positive charges on the ground.

A negatively charged ‘stepped leader’ travels downwards and the positive particles rise up a tall structure like a tree.

The ‘leader’ meets the positive ‘streamer’ and a powerful current flows in the form of a lightning bolt.

H O W

L I G H T N I N G

W O R K S