The important facts of lightning are:
– It must be respected at all times, not just during the peak periods of a thunderstorm.
– It is the result of high voltage currents.
– It is unpredictable. This makes researching natural organic strikes very hard.
– Most characteristics of lightning are intelligent estimates of typical values formed from other data.
– A properly installed lightning protection system will help to protect a property or structure from lightning damage. The Empire State Building is proof of this.
There is estimated to be around 2,000 lightning storm active around the global at one time creating over 100 strikes per second. These thunderstorms generate a potential difference of 200,000 to 500,000 volts between the Earth’s surface and the ionosphere, with a fair weather current of about 2×10-12 amperes/meter2.
Data from NASA’s space-based optical sensors revealing the uneven distribution of worldwide lightning strikes.
Units: flashes/km2/yr. Image credit: NSSTC Lightning Team.
- Approximately 300,000 lightning strikes hit the ground in Britain each year with 30 percent of reported lightning strikes causing severe damage. Each year 30 to 60 people are recorded as being struck by lightning, 3 of whom, on average, die (14 in 1984 compared to 2 in 2005). In the USA around a 100 residents are killed by lightning every year. These figures are dropping with time as working practices change and we become more aware of the dangers of lightning and how to protect ourselves to reduce its risks. Using a lightning protection system is a good way to protect buildings and structures from the damaging effects of a lightning strike. As global warming makes lightning storms more prevalent the necessity to have lightning protection will rise.
- A majority of lightning occurs in the storm cloud itself and only 10 to 20 percent of all lightning reaches the ground.
- Even though a lightning strike (lightning flash or stroke) appears continuous it is really a series of short bursts following the paths of ionised air called stepped leaders. Typically they are about 30 metres in length, however they can be anything from 10 to 100 metres in length. The initial lightning strike can also be followed by secondary strokes utilising the same ionised air channel. Up to 4 secondary strokes can be expected, though much higher counts have been recorded. Each lightning stroke typically lasts only last 10 to 50 microseconds (0.000050 sec) and carries hundreds to thousands of amps.
- The air around a lightning strike is typically superheated to about 20,000 degrees Celsius or over 3 times hotter than the surface of the sun (estimated at 5,500 C). The air temperature can range from 8,000 to 30,000 degrees Celsius.
- The rumbling sound of thunder is caused by shock waves from multiple stepped leaders that make up a lightning stroke. As the air in each leader is super heated by the immense current it rapidly expands creating a supersonic shock wave. Differently timed shock waves are created from the different stepped leaders at varying attitudes in quick succession along the path of the lightning. This causes the thunder clap from each stepped leader to arrive at your ears at differing times generating the rumbling effect.
- The estimated peak power per lightning stroke is 10^12 watts (1,000,000,000,000 watts or 1,000 Giga Watts). The total energy in a large thunderstorm is thought to be enough to power the whole of the USA for 20 minutes.
- A tall thunderstorm cloud can hold over a 100 million volts of potential. The voltage potential in a lightning bolt is proportional to its length, and varies depending on the diameter of the bolt, air density and impurities of the air (humidity, dust, ash). The electrical breakdown of air (ionisation) normally take 3,000,000 volts per metre, however with the ambient electric fields of a charged thunder cloud and impurities in the air, ionisation normally takes place at much lower voltages during a storm. Lab tests have shown a leader will advantage if the tip of the streamer is about 4.5kV (4500v) for a negative charge and 5.5kV (5500v) for a positive charge.
- An average lightning strike discharges about 30,000 amperes (20,000 amperes in the UK). The current in a lightning strike typically ranges from 5,000 to 50,000 amperes depending on the strength of storm. NASA has recorded strikes of 100,000 amperes and there are other reports of strikes over 200,000 amperes.
- The resistivity of clear, fair-weather air ranges from around 4 x 10^13 ohm meters at sea level, to around 1.3 x 10^16 at 12km elevation. Typically air is considered an electrical insulator, however with impurities (water, dust ash) its resistance is lowered and varies further. Unlike metals whose resistance increases with temperature, when the temperature of air increased it has similar characteristics to semiconductors in that its resistance lowers. So when air is ionised into a plasma state and then superheated to 25000 degree Celsius it conductors electricity very well.
- Lightning can and does strike the same place twice. On average lightning strikes the Empire State Building in New York City (USA) about 100 times every year. 49 strikes have been recorded in a single day.
- A finger of charge called a positive streamer can reach upwards from the ground 15 to 50 metres in an attempt to join stepped leaders from a storm cloud. This normally happens just before a lightning strike.
Benjamin Franklin in 1752 is believed to be the first to show that lightning was electricity using his now famous kite experiment. He is certainly the inventor of lightning rods and used them in creating lightning protection for important buildings. Following a series of experiments at his own home, lightning rods were installed to the Academy of Philadelphia and the Pennsylvania State House in 1752 giving both these buildings lightning protection.
– May 10 1752, Thomas-François Dalibard of France successfully conducted Franklin’s first experiment before him using a 12 metre tall iron rod instead of a kite, and he extracted electrical sparks from a cloud.
– June 15 1752, Franklin is said to have conducted his famous kite experiment in Philadelphia and also successfully extracted sparks from a cloud, though this was not written up until 1767 some 15 years later by Joseph Priestley.
- In the northern hemisphere lightning occurs more during the summer months, however in equatorial regions, lightning appears more often during the fall and spring.
- Arctic and Antarctic have very few thunderstorms and, therefore, almost no lightning at all.
- The open oceans do not experience as many thunderstorms and lightning strikes as land due to water’s higher heat capacity preventing the heating of low-lying air which is crucial for thunderstorm formation.
- Most cloud to ground lightning strikes are generally the transfer of a negative charge from the cloud to earth, these are identified by the downward pointing branches of lightning and are called negative flashes. Ground to cloud lightning is far less frequent, however they can occur where leaders propagate from tall earth bound objects toward the storm cloud, these are identified by upward pointing branches of lightning and are called positive flashes.
- Lightning does not travel straight down to earth due to impurities in the air like dust causing it to breakdown more easily in one direction than another. The shape of the electric field generated by the charge in the cloud also has an effect. lightning may not always strike the top of the tallest object in an area, it is possible for lightning to strike very close to the base of a tall object.
- When lightning strikes the ground radial currents spread out from the site of the strike causing damage to anything nearby.
- Storm clouds called cumulonimbus clouds, can tower 12-15 miles high (Mount Everest is 5.5 miles high.). They form at altitudes of 150 to 4000 metres and typically have peaks 6100m, though in extremes cases they can be as high as 24000m.
- Lightning does strike twice in the same place. A number of secondary lightning strokes (possibly 30 to 40) can quickly follow the ionized path of the first strike giving the impression of the lightning lasting longer.
- Research shows that proximity to water (eg. pool, lake or sea) increases the risk of being struck by lightning.
- Stepped leaders are normally 2 to 5 cm in diameter, however they can be over a metre.
- The frequency and strength of thunderstorms varies for different geographical locations. The average current passed in a lightning strike is 20,000 amperes, in Florida (USA) the average is over 45,000 amperes per lightning strike. Researchers believed this is due to the hot and wet conditions in Florida (USA) being ideal for creating tall and highly charged storm cloud formations.
- Positive lightning from the top of storm clouds is normally over 6 times stronger than negative lightning due to the longer distances it typically has overcome. Sometimes referred to as “bolts from the blue” they can occur when there is very little cloud activity compared to the thunderstorms associated with negative lightning. The unexpectedness and the greater strength make positive lightning the most dangerous type of lightning. Positive lightning makes up less than 5% of all lightning strikes.
- Lightning strikes most frequently in the Democratic Republic of the Congo.