Summer Lightning Never Strikes Twice?

As we in Britain sit under yet another damp storm cloud and wonder if the summer would ever come, spare a thought for Arnie Schwarzenegger. Two weeks ago, he woke up to the news that 25 forest fires were burning in the state of California; four days later the total was 1700. Over 820 sq miles had been burnt, with flames fanned by dry winds threatening major towns such as Big Sur and Santa Barbara.

"No more fire season"

Governor Schwarzenegger and his fire-fighting team are used to seasonal fires, but now, there is 'no more fire season', it is now a year round problem. Furthermore, neither the Governor of California or his team (or even Arnie's toughest incarnation, The Terminator) can do anything about the real culprit behind the ignition of these devastating fires; dry lightning.

Dry Lightning Fires

Dry lightning occurs during a thunderstorm when no rain actually falls to the ground. Such thunderstorms, which mostly occur in the western part of the USA, happen when the rain normally produced during a storm falls through a section of very dry air, which causes the rain to evaporate before it reaches the ground.

Lightning produced in such storms is the most common cause of natural wild fires, and the resulting flames are fanned by strong gusty winds also produced during the storm. In turn, lightning can also be formed from the resulting forest fires, when dust from the fire can create its own static charge. It's no wonder that lightning can cause fires in dry areas; the air around a lightning bolt heats to around 10,000 degrees C (18,000 degrees F).

Lightning Safety When Outdoors

If you have staff that work outdoors, make sure they know about lightning safety this summer. If they can hear thunder, then lightning is possible, so your workers should retreat inside a substantial building. The safest place to be if caught outside during a lightning storm is generally agreed to be inside a metal vehicle. You should sit well inside the vehicle with no parts of your body touching any exposed metalwork.

Sheltering from Lightning

Contrary to popular belief, there is no 100% safe outdoor place to shelter during a thunderstorm. Small structures such as canopies, smoking shelters and trees are not safe from lightning strikes. Many people believe that standing under a tree in a lightning storm is dangerous because the tree may get struck and fall on them. In reality, the dangers come from the tree being blown apart, or from the electricity from the lightening conducted through wet soggy ground outwards from the tree.

Lightning and Trees

In the UK, a tree is more likely to be blown apart than necessarily catch fire, due to the poor electrical conductivity of tree sap. The tree sap is superheated instead, which causes it to form into steam. This can cause bark to blown off, or in extreme cases, actually blow the entire tree apart.

Lightning, Trees and Buildings

However, it can be useful to have a tall tree next to a building in areas susceptible to lightning. The tree acts as a natural lightning conductor as opposed to the building, and trees with a large biomass of roots help dissipate the charge into the ground safely. If the strike happens when the ground surrounding the tree is wet, the soil is much more conductive, and the charge can travel considerable distances. For example, in 2005, sixty-eight diary cattle sheltering under a tree in Australia during a storm were all killed, not by the falling or exploding tree, but by the conducted current in the wet soil.

Lightning Hotspots

If you love lightning, the place to be is in the eastern mountains of the Democratic Republic of the Congo, where 158 bolts of lightning strike per square kilometre every year. In contrast, the 'Lightning Alley' in Florida, which stretches from Tampa to Orlando, receives only 20 strikes per sq km per year.

Lightning Never Strikes Twice?

Lightning certainly does strike more than twice, and US park ranger Roy Sullivan is living proof. He has been struck by lightning seven times in 35 years, even though the odds of a person living in the US being struck is 1 in 700,000. (As a contrast, the odds of winning the jackpot in the National Lottery are 1 in 13,983,816!)

Fire Safety and Lightning

A Lightning Protection System (LPS) on a buildings works by directing the current from the strike down into the earth and away from the building and its occupants. So, if you own a building, always check that your lightning conductor complies with the new BS EN 62305 (see below), that it is in good repair and has sufficient continuity underground. Sometimes, building works and ground excavations can accidentally break up the full length of conductivity required.

Changes in Lightning Protection Law

The current British standard BS 665 for lightning protection will be fully replaced with the new BS EN 62305 in September 2008. The new standard will change the requirements for both the design and installation of lightning protection provision on all types of buildings. The new standard also requires a wider appreciation of the risk in terms not just of damage but also potential loss of life, services to the public, and cultural and economic loss.

Fire Risk Assessment

Next time you are reviewing your Fire Risk Assessment at your business premises, don't forget to check your lightning protection too. Remember, all your fire risk assessment needs are right here at Fire Protection Online, from replacement fire extinguishers to fire hoses, intumescent fire seals to fire safety signs, fire exit security devices to fire document cabinets.