Weather: Flashy theories of ancient Greeks

The causes of thunder and lightning have been a matter of scientific debate for at least 2,500 years. We still do not completely understand the pattern of electrical activity within clouds.

Lightning, according to the Greek philosopher Empedocles in the 5th century BC, consists of rays of the sun that are intercepted by clouds. Anaxagoras, at about the same time, said it was the gleam of the fiery upper ether which had descended from above, and thunder was the hissing noise of its being extinguished by the cloud. Aristotle, a hundred years later, produced a remarkably perceptive objection to this theory which, he said, "involves the view that lightning is actually prior to thunder and does not merely appear to be so".

Aristotle had realised that light travels faster than sound - though the way he put it was to say that sight is quicker than hearing. "The rowing of triremes illustrates this," he wrote. "The oars are going back again before the sound of their striking the water reaches us."

He believed that thunder and lightning were part of a "dry exhalation" from clouds. "If any of the dry exhalation is caught in the process as the air cools, it is squeezed out as the clouds contract, and collides in its rapid course with the neighbouring clouds, and the sound of this collision is what we call thunder." Lightning is the burning of this same exhalation.

The discovery of electric charges more than 2,000 years later rather spoilt this elegant theory, leaving in its place a highly complex, and still incomplete picture of what really happens within a thundercloud.

Between about 70km and 3,000km above the earth's surface is the ionosphere, which is positively charged because of the action of solar ultraviolet radiation. The earth's surface, however, is negatively charged during fine weather, so cloud droplets acquire an induced positive charge on their lower sides and a negative charge on their upper sides. This undoubtedly plays a part in the electrification of a cloud, but a far greater contribution is probably made by collisions between ice crystals and warmer pellets of hail.

Generally, the higher parts of a cloud will hold positively charged ice crystals, the middle parts will hold a mixture of negatively charged hail and positively charged raindrops caught in an updrift, and the lowest part will have falling hail or raindrops, both negative. But an area of positive charge observed at the bottom of clouds is still a mystery.

Lightning should be thought of as a spark between not-quite-connected parts of an electrical circuit. It usually starts just before the storm hits the ground, as the negatively charged melting hail comes close enough to the positively charged ground for the circuit to be bridged with a spark. The first stroke comes from cloud to ground, followed almost instantaneously by a return stroke from the ground. A simple path of least resistance between cloud and ground results in sheet lightning, but when the flash takes several paths from a main channel, we see the more attractive forked lightning. The extremely rare ball lightning, which appears as a bright sphere less than a foot across, and bead lightning, which appears as a succession of small balls, have so far evaded explanation.

The difference in electrical potential bridged by a lightning flash is usually hundeds of millions of volts - and that explains why lightning flashes generate such heat. The heat causes a rapid expansion of the air around it, and the resulting compression wave produces thunder as it breaks the sound barrier. Since sound waves travel at about one- third of a kilometre per second, count the seconds between lightning (which is seen almost instantaneously) and the thunder that follows, then divide by three to determine how far away the lightning flash occurred in kilometres.

Have you tried new the Independent Digital Edition apps?
Life and Style
ebookNow available in paperback
ebooks
ebookA delicious collection of 50 meaty main courses
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

SPONSORED FEATURES

ES Rentals

    iJobs Job Widget
    iJobs General

    Recruitment Genius: Sales Executive or Senior Sales Executive - B2B Exhibitions

    £18000 - £30000 per annum: Recruitment Genius: A Sales Executive or Senior Sal...

    Recruitment Genius: Head of Support Services

    £40000 - £55000 per annum: Recruitment Genius: This is an exciting opportunity...

    Recruitment Genius: Warehouse Team Leader

    £22000 per annum: Recruitment Genius: This industry leading company produces h...

    Recruitment Genius: Business Development Manager / Sales - OTE £40,000

    £20000 - £40000 per annum: Recruitment Genius: This IT provider for the educat...

    Day In a Page

    A nap a day could save your life - and here's why

    A nap a day could save your life

    A midday nap is 'associated with reduced blood pressure'
    If men are so obsessed by sex, why do they clam up when confronted with the grisly realities?

    If men are so obsessed by sex...

    ...why do they clam up when confronted with the grisly realities?
    The comedy titans of Avalon on their attempt to save BBC3

    Jon Thoday and Richard Allen-Turner

    The comedy titans of Avalon on their attempt to save BBC3
    The bathing machine is back... but with a difference

    Rolling in the deep

    The bathing machine is back but with a difference
    Part-privatised tests, new age limits, driverless cars: Tories plot motoring revolution

    Conservatives plot a motoring revolution

    Draft report reveals biggest reform to regulations since driving test introduced in 1935
    The Silk Roads that trace civilisation: Long before the West rose to power, Asian pathways were connecting peoples and places

    The Silk Roads that trace civilisation

    Long before the West rose to power, Asian pathways were connecting peoples and places
    House of Lords: Outcry as donors, fixers and MPs caught up in expenses scandal are ennobled

    The honours that shame Britain

    Outcry as donors, fixers and MPs caught up in expenses scandal are ennobled
    When it comes to street harassment, we need to talk about race

    'When it comes to street harassment, we need to talk about race'

    Why are black men living the stereotypes and why are we letting them get away with it?
    International Tap Festival: Forget Fred Astaire and Ginger Rogers - this dancing is improvised, spontaneous and rhythmic

    International Tap Festival comes to the UK

    Forget Fred Astaire and Ginger Rogers - this dancing is improvised, spontaneous and rhythmic
    War with Isis: Is Turkey's buffer zone in Syria a matter of self-defence – or just anti-Kurd?

    Turkey's buffer zone in Syria: self-defence – or just anti-Kurd?

    Ankara accused of exacerbating racial division by allowing Turkmen minority to cross the border
    Doris Lessing: Acclaimed novelist was kept under MI5 observation for 18 years, newly released papers show

    'A subversive brothel keeper and Communist'

    Acclaimed novelist Doris Lessing was kept under MI5 observation for 18 years, newly released papers show
    Big Blue Live: BBC's Springwatch offshoot swaps back gardens for California's Monterey Bay

    BBC heads to the Californian coast

    The Big Blue Live crew is preparing for the first of three episodes on Sunday night, filming from boats, planes and an aquarium studio
    Austin Bidwell: The Victorian fraudster who shook the Bank of England with the most daring forgery the world had known

    Victorian fraudster who shook the Bank of England

    Conman Austin Bidwell. was a heartless cad who carried out the most daring forgery the world had known
    Car hacking scandal: Security designed to stop thieves hot-wiring almost every modern motor has been cracked

    Car hacking scandal

    Security designed to stop thieves hot-wiring almost every modern motor has been cracked
    10 best placemats

    Take your seat: 10 best placemats

    Protect your table and dine in style with a bold new accessory