Such is the power locked up in a thunderstorm that it was once thought of as the ultimate weapon by our ancestors. Thor, the Norse god of thunder, wielded a large and heavy hammer, which delivered the thunderbolts that fell from the sky. In general, however, a god of thunder was worshipped wherever people regularly experienced heavy thunderstorms or thunder preceded the arrival of life-giving rains after prolonged periods of drought.
What is thunder?
The sound that so impressed the ancients was nothing more than the shockwave caused when lightning heats the air around it to a temperature exceeding 30,000°C (54,000°F) – many times hotter than the surface of the Sun. The superheated air expands and then almost as quickly contracts. This rapid but brief period of expansion and contraction generates a sound wave that we hear as ‘thunder’.
Thunderstorm formation
For a thunderstorm to develop, three elements must be present – warm air, moisture and an unstable air mass. The warm air rises, either when the Sun heats the ground, when a front passes and the air is pushed aloft or when the air mass moves up the side of a mountain. The rising moist air reaches a height where it cools and water vapour condenses to form cotton-wool-like cumulus clouds. During the condensation process, latent heat is released, warming more air and feeding the process. The updraught brings in even more warm moist air causing the towering cloud – probably a dark cumulonimbus by now – to grow up through the atmosphere, even into the stratosphere 16 km (10 miles) above the ground. The extraordinary updraughts in a severe thunderstorm can exceed 160 km/h (100 mph). As precipitation falls, it cools the air forming downdraughts. A mature thunderstorm then has updraughts and downdraughts that stir up the air mass and make the storm even stronger.
How far away?
You can work out very roughly the distance of an approaching or receding thunderstorm by counting the number of seconds between the flash of the lightning and the sound of the thunderclap. This is because light travels at 299,792.458 km/sec (186,282.397 mps) and sound at 1,225 km/h (761 mph), so the light arrives at the place where you are much faster than the sound. It is said that the storm is about one mile away for every five seconds interval (one kilometre away for every three seconds interval). If the flash and thunderclap occur at the same time, the storm is directly overhead.
Windsheer
The updraughts and downdraughts inside a cumulonimbus thundercloud are so violent that changes of direction of the air mass can occur over relatively short distances, both vertically and horizontally. Windsheer is of great concern to aircraft pilots. When an aircraft is landing and relying on the uplift of the headwind to keep it aloft, a sudden change of direction downward – a microburst or downburst – will cause it to plough into the ground.
Inside a storm cloud are rapidly moving masses of air, filled with water and ice. The updraughts and downdraughts sweep the ice particles up and down, each gathering increasingly more layers of ice, like an onion, to form hail, but both the water droplets and hail pellets are torn asunder and smashed together so violently by the ferocious air movements that they are charged with static electricity. Very light, positively charged particles of ice and water tend to accumulate at the top of the cloud, while those with negative charge are at the base. The difference between the charges eventually becomes so great that they neutralise within the cloud with a violent flash of lightning, so-called ‘sheet lightning’ or ‘intracloud lightning’. Charges can travel between clouds too, creating ‘spider lightning’ that can travel across the sky for up to 145 km (90 miles). However, the ground is also positively charged, so when a cloud is very tall, the shorter route for the lightning to take is not to the top of the cloud but to the ground, producing ‘forked’ or ‘cloud-to-ground’ lightning.
FACT
Lightning strikes somewhere on the surface of the Earth about 100 times every second, and at any moment 1,800 thunderstorms occur somewhere on the planet.
Electrifying
Benjamin Franklin nearly lost his life when he tried to understand the nature of lightning. In 1752, he and his son William flew a homemade silk kite in a thunderstorm in Philadelphia, Pennsylvania. Franklin senior was holding the line when lightning struck the kite. Electricity travelled down the wire and sparks jumped from a key that was attached. Fortunately, he was none the worse for his experience and went on to describe that lightning is electricity.
Over in a flash
A bolt of lightning lasts for about 0.2 seconds. It starts when a ‘stepped leader’ zigzags down from the cloud. Each step is about 45 m (150 feet) long, so when the leader is within 45 m (150 feet) of a positively charged object, such as a flagpole, tree, church spire or any object sticking out above its surroundings, a surge of electricity known as a ‘streamer’ rises up to meet it. Leader and streamer then form a channel, and the current rising rapidly up the channel causes a flash, known as a ‘down stroke’. The bolt itself is no wider than a coin, but appears wider because of the brightness.
Lightning hot spots in the United States
The occurrence of lightning is by no means universal. In the United States, central Florida has the most air-to-ground lightning strikes, while the Pacific Northwest and Hawaii have almost none. Across the country, as many as 10,000 forest fires are triggered by lightning and, since 1940, lightning has killed 8,316 people. Despite the success of weather forecasting and the forewarning of dangerous thunderstorms, 363 people are struck in the United States each year and there are 80–90 deaths. The most dangerous month is July, and 7.30 PM is five times more dangerous than 9 AM.
Lightning never strikes twice... or does it?
On 3 January 2001, a gang of 213 inmates was walking back to the prison buildings at Kabwe, Zambia, when they were caught in a severe thunderstorm. The first bolt of lightning hit a group of 20 men, who fell to the ground, and this was closely followed by a second strike on another group. Three prisoners were burned to death, and 17 others received less serious injuries.
Who is at risk?
Covering 35 years of statistics, a 1979 study of lightning strikes in the United States found that walkers in the open and people, including children, in parks and playgrounds were the most at risk.
|
Activity |
Percentage deaths |
|
Open field, park, playground |
26.8 |
|
Under trees |
13.7 |
|
Water-related activities, such as boating and swimming |
8.1 |
|
Golf courses |
4.9 |
|
Driving tractors, farm and heavy road equipment |
3 |
|
On telephones |
2.4 |
|
Near radios, transmitters, aerials |
0.7 |
|
Other activities or unknown |
40 |
Lightning course
A golf course is one of the worst places to be in a thunderstorm, as the highest object around is often you. Golfing legend Lee Trevino knows from painful experience. On 27 June 1975 during the Western Open, near Chicago, a lightning bolt hit a lake nearby and ‘bounced’, hitting both Trevino and another player. They were both treated for burns and Trevino was also operated on for a back injury that permanently damaged the flexibility of his spine. He adjusted to the problem, however, and went on to win many more tournaments.
Channeling lightning
Benjamin Franklin developed the lightning conductor, an invention that for philanthropic reasons he did not patent. It consists simply of a pointed rod that projects above a building, such as the top of a church spire, and is joined to a large metal plate buried in the ground by a copper strip. The lightning hits the rod, travels down the strip and is earthed to the ground. The copper strip can vapourise but the building will be relatively safe. In this way a building is afforded some protection against lightning strikes.
Offer prayers
Prior to Franklin’s important discoveries about lightning and its nature, Christian churches throughout Europe were prone to lightning strikes – after all, they were the tallest buildings around. Priests attempted to limit damage by saying prayers, sanctifying bells and burning witches. The sound of the bells was to ‘temper the destruction of hail and cyclones and the force of tempests and lightning; check hostile thunders and great winds; and cast down the spirits of storms and the powers of the air.’ However, it was probably the bell ringers who needed the help not the buildings for they were frequently electrocuted or burned. In 1745, it led author Peter Ahlwardts, who wrote Reasonable and Theological Considerations about Thunder and Lightning, to suggest that the last place anyone would want to seek sanctuary during a thunderstorm was a church!
FACT
Thunder can be heard about 19 km (12 miles) away at most, even when things generally are quiet, but lightning can be seen at night up to 160 km (100 miles) away. The average lighting bolt is about 4.8 km (3 miles) long and carries a current of 100 million volts at 10,000 amps.
This is a rare phenomenon that can appear after a bolt of forked lightning has struck the ground. It creates a violet, red, orange or yellow brightly glowing ball, which can be as small as a tennis ball or as large as a basketball, and it appears to ‘float’ aimlessly above the ground. It lasts for no more than a few seconds before spluttering out or disappearing with an unexpected bang. Ball lightning is generally described as spherical, but dumbbell, rod and torpedo shapes have been known.
Kitchen incident
On 8 August 1976, according to a letter in the journal Nature, a number of houses around Smethwick, in the Midlands, England, were struck by lightning. In one house, a woman was in her kitchen when a 10-cm (4-inch) diameter sphere of bright blue to purple light surrounded by a flame-coloured halo appeared over her stove. The ball moved toward her, about 95 cm (3 feet) above the floor. She felt heat, smelled burning and heard a rattling sound. As it hit her, she brushed it away and it exploded with a bang and vanished. Her hand was red and swollen and her gold wedding ring felt as if it was burning her finger. At the point of impact there was a hole in her dress and her legs were red and numb, but not burned. The synthetic material of the dress was shrivelled around the hole but not charred.
Garden incident
On 10 November 1940, Mr E. Mats was working in his garden in Coventry, England, when suddenly he was surrounded by blackness. When he looked down he saw a pale blue-green fireball about 60 cm (2 feet) across and made of what appeared to be ‘writhing strings of lights’. It rose and moved away, just missing a poplar tree. It cleared a neighbouring house by about 6 m (20 feet) and descended about 0.4 km (0.25 miles) away, where it exploded.
Flyswatter incident
On 25 August 1965, Mr and Mrs Greenlee and a neighbour were sitting on their fibreglass-screened, roofed patio, in Dunnellon, Florida. Mrs. Greenlee had just swatted a fly, when suddenly a ball of lightning with the appearance of the flash seen in arc welding appeared in front of her. The flyswatter ‘edged in fire’ fell to the floor and the ball exploded with a report ‘like a shotgun blast’. Nobody was hurt and there were no marks on the patio floor. Turning to Mrs Greenlee, the neighbour said ‘You sure got him that time!’
Mile high incident
On 19 March 1963, Eastern Airlines flight 539 from New York to Washington flew into an electrical storm and was ‘enveloped in a sudden bright and loud electrical discharge’. A few seconds later a glowing sphere about 20 cm (8 inches) in diameter came out of the flight deck and passed down the aisle at waist height. It was blue-white in colour and glowed like a 5–10-watt light bulb
FACT
Lightning generates radio waves, ranging in frequency from audible 3 KHz (very low frequency) to 10 MHz (shortwave radio). The low-frequency waves can travel all the way around the world, whereas the shortwave signatures can only make it halfway, and on the side of the Earth that is dark. These higher frequency waves can be heard as a myriad of tiny bells ringing at the same time.
Natural fireworks
On 22 July 1926, a group of academics observed very unusual lightning in the sky above Calcutta, India. There were some misty clouds but the stars were visible, and they saw flashes every minute or so from the lower clouds. Every three minutes, however, there were a series of vivid bursts coloured blue or yellow, followed by luminous trails, like purple ribbons, that shot up at an angle into the sky and exploded like fireworks. . .they were not fireworks for the flashes were too high and no thunder was heard. Similar lightning trails emanating from the tops of cumulus clouds and shooting up into the sky were seen by crews of the MV Geesthaven in the North Atlantic in October 1967, the TSS British Bombardier off southern Italy in March 1971, a Royal Australian Air Force aircraft at Broome, Australia in February 1945 and by an observer in Fiji in June 1950. The cause is a mystery.
Red Sprites
Newly discovered ‘Red Sprites’ jump the 65 km (40-mile) gap between the tops of thunderstorms and the lower ionosphere. They resemble red-coloured jellyfish with blue tentacles. They produce powerful radio emissions and atmospheric gamma ray bursts.
Crawlers and giants
Lightning that spreads across a squall line or frontal system is known as ‘lightning crawler’ or ‘spider’. Radar has detected crawlers at high altitudes – up to 6,100 m (20,000 feet). They can travel horizontally as much as 120 km (75 miles), hopping from one cloud to the next. If they should finally strike the ground, it can be extremely dangerous. Such a strike is known as a ‘positive giant’ or ‘bolt from the blue’. The strike travels from the top of the thundercloud’s ‘anvil’ down to the ground, carrying more destructive energy than a normal lightning strike. It also means lightning can appear out of a cloudless sky.
Old crawlers
On 16 July 1873, there was an extraordinary flash of lightning at Hereford, England. The storm was some 8 km (5 miles) away, but a bolt of lightning raced across the sky, missing narrowly two church spires, but singling out a small house much lower than those surrounding it. The same phenomenon was experienced near Bloomington, Indiana, on 23 July 1926, when a storm about 5 km (3 miles) away released a bolt of lightning that appeared to the local people to come out of a clear blue sky. The stroke, according to eyewitnesses, missed several buildings and hit a small house. Two children were killed. In those days, nobody knew the cause, but today we can identify those strikes as powerful positive giants.
Photographic lightning
Benjamin Franklin described the first authenticated case of photographic lightning at the French Academy in 1786. He described how a man standing opposite a tree struck by lightning later found a facsimile of the tree on his chest. Similarly, in 1853 the New York Journal of Commerce described how a young girl was standing at an open window near a maple tree. After a flash of lightning, an image of the tree was found imprinted on her body.
Which trees are vulnerable?
In 1907, the UK’s Royal Meteorological Society published the results of a study in the forests of Lippi-Detmold from 1874 to 1890 in which a Dr Hess recorded the types of tree most struck by lightning. The forest consisted of 11 per cent oak, 70 per cent beech, 13 per cent spruce, 6 per cent Scots pine and a handful of other tree types, but it was not the beech trees that were hit most often.
The study showed that all trees are liable to be struck by lightning, but that oak trees and other species with deep roots were most often struck. It went on to speculate that the deep roots made better conductors to the moist subsoil.
