Table of Contents
how to reduce your risks in lightning storms
see also:
- most lightning injuries occur on the outskirts of storms when there is little rain rather when they are overhead as most people do seek shelter when the rain is very heavy
- the safest place in lightning is to be indoors - and not holding a copper wired phone
- the next safest place is sitting in a car without touching the exterior which acts as a Faraday cage
- cars can get hit by lightning but you might survive eg. https://7news.com.au/news/man-struck-by-lightning-while-driving-on-highway-in-moreton-bay-storm-c-13062724
- in Australia, 10 people die each year and some 100 are injured from lightning strikes
- in hot weather conditions, lightning can start bushfires which can spread rapidly and trap hikers - avoid hiking in extreme bushfire danger periods
introduction
- sound travels about 1 mile in 5 seconds (1km in 3 secs) but is generally only heard within 32km of the lightning yet we may be able to see lightning up to 160km away
- you can sometimes see lightning almost overhead but not hear it - this is due to the lightning being at very high altitudes of around 20km
- if you hear thunder:
- secure outdoor equipment
- cease outdoor activity
- plan your shelter at a low risk site (see below) and strongly consider remaining there
- if the gap between lightning and thunder is less than 30secs
- urgently seek shelter if no suitable shelter then adopt lightning position (see below)
- then wait there 30 minutes until after last thunder
- if you feel your hairs on end, a strike is imminent within seconds, immediately adopt lightning position
some lightning facts
- lightning occurs where a large voltage buildup occurs such as cloud to ground, cloud to cloud and even cloud to the ionosphere, these voltage buildups occur as a result of updrafts in the atmosphere over land which allow charge separation, a necessary process for lightning to occur, hence most lightning occurs over land masses rather than over the ocean as ocean updrafts are significantly less frequent
- lightning is a high current discharge in air producing a column of hot plasma a few inches or less in diameter and a few km to over 80 km in length
- the voltage differential produces an initial electrical current of 10-100A which forms a conductive bi-directional channel of ionized air which becomes the “leader” with both a positive and a negative end, and it is the negative end which propagates to the ground. Each leader of approx. 45m length forms a pool then more leading tips in a tree-like branching fashion. Once a leader reaches ground an “attachment” is created and a low resistance path is formed between the cloud and ground allowing a discharge to occur which we see as lightning and this lightning bolt can carry a current of tens of thousands of amperes with a temperature of up to 50,000degF. The initial strike is followed by a positive leader return stroke which is the most luminous and the nearly instantaneous heating during the return stroke causes the air to expand explosively, producing a powerful shock wave which is heard as thunder. Then in rapid succession further 3-30 strikes separated by 40-50msecs. The rapidly changing currents tend to travel on the surface of a conductor rather than through it, in what is called the skin effect, and also create electromagnetic pulses (EMPs) that radiate outward from the ionic channel. If they pass over conductive elements such as power lines, communication lines, or metallic pipes, they may induce a surge current which travels outward to its termination.
- induced surge lightning waves travel along a cable at frequencies of several hundred kHz to 1 MHz 1)
- metallic vehicles such as aeroplanes and cars are partial Faraday cages and generally can protect occupants if they are not touching metal and the car windows are closed
- Faraday cages act to distribute an electrical charge around the cage and limit its entry inside the cage depending upon the cage material's electrical conductivity, thickness and the wavelength frequency of the electrical strike
- lightning also produces electro-magnetic waves, both optical spectrum (hence we can see them although the peak is in the far UV and tails off into IR region), and radio frequencies less than 10kHz and consist of two forms:
- sferics
- these are generally trapped within the ground-lower ionosphere wave-guide but in certain conditions same may escape into the magnetosphere to form whistlers
- peak frequency at 50km is 5-10kHz
- an approx 1second delayed “whistler” is a type of magnetosphere sferic with an electromagnetic wave with a continuous tone that begins at a high frequency (~10 kHz) and rapidly decreases in frequency ending at a low frequency (several hundred Hz) within a few seconds. There are two types of whistlers - ducted (these follow the Earth’s magnetic field lines since they are constantly refracted within a density enhancement of ionospheric plasma) and non-ducted propagation (these are not constrained to follow the Earth’s magnetic field but follow a path that is determined by their frequency and the index of refraction of the plasmasphere) 2)
lightning injuries
- most lightning injuries come from indirect effects of a nearby lightning strike:
- ground conducted step potential
- current spreads across the ground from the strike zone - on wet ground, it can spread a long way perhaps 100m or more
- the further apart your feet, the greater the voltage differential between them and the more current to your heart
- most likely occurs when lightning hits a nearby structure such as a hut or tree
- accounts for 50-70% of injuries
- flashover hazard
- as current develops in a tree hit by lightning, a voltage differential develops and if this exceeds the air breakdown value to a nearby person, that person becomes part of the current (don't be touching the tree or be within 2m of it)
- this can also occur indoors if you use or are nearby a landline phone which carries lightning current from a nearby strike
- shrapnel or falling branches
- lightning hitting a tree may cause it to explode sending dangerous shrapnel flying
Check out the blast shrapnel and falling branches (image courtesy of gizmodo)
find a lower risk site
- highest risks are:
- within 5m of a tall object such as an antenna, isolated tree, tree on the edge of a forest, pavilion, bus stop, etc
- 13% of deaths are due to people sheltering under a tree
- on a hilltop (doubles your chances of strike compared to open paddock)
- on an open paddock, golf course, recreational oval
- holding a metal or carbon fibre object such as a tent pole, wire fence, umbrella, tripod, fishing rod, golf club
- in water eg. swimming or boating
- if you are stuck in a boat, go below deck if possible
- sites where lightning strikes frequently (note past lightning strikes of trees)
- in a tent in any of the above sites
- if your tent is in a high risk position, leave the tent and find a safer location
- lower your risk by:
- get inside a car or lightning shelter
- cars are much safer than tents as they act like Faraday cages while tent poles, whether aluminium or carbon fibre act like lightning rods
- there are several ways a shelter can be protected, but if metal wire is to be used it must be at least 1/4“ steel cable and several metres higher than the shelter, and be grounded properly at each end to avoid flashover hazard
- grove of trees (reduces your strike probability to 25% compared to an open paddock)
- be within distance of 3 x height of a taller object but further than 5m away and protected from any potential shrapnel although protection is best in the zone within the height of the object
- tall trees do offer an alternative path for lightning strikes if you are within a distance 3 x height of the tree but as long as you are at least 5m away to avoid the corona of the strike, significant ground currents and shrapnel as it explodes from the strike, so ideally if you pitch your tent 10-15m away from a 30m tree theoretically, the tree may be protective - as long as it doesn't fall down in your direction or you are standing in wet ground or you get hit by shrapnel.
pitch your tent assuming there may be a storm overnight
- lightning may be the least of your problems
- avoid watercourses which may flood
- check your prospective tent site for evidence of past flooding
- avoid siting tent at the lowest point of a camp site as this is where the rain will drain to and pool
- avoid being under trees with large branches which may drop without warning
- Australian eucalypt trees often drop large branches without warning even without wind or storms as a way of surviving drought or fungal disease - these kill campers and farmers although deaths are rare
- higher risk:
- horizontal branches 10-30cm in diameter
- species susceptible to “Sudden Branch Drop (SBD)” include yellow box, maiden's blue gum, mountain ash and river red gum
- trees exuding dark sap which suggests infection by tree rot
- late afternoon or early morning in dry spells or droughts
- older trees
- trees with a history of limb drop are more likely to repeat this
- trees may be blown over in gale force winds
- avoiding camping down wind of a tree - choose a 2-3m high less dangerous bush instead for wind protection
- reduce lightning risk
- avoid hilltops and exposed ridges no matter how nice the views
- avoid open paddocks
- avoid other high risk lightning sites such as next to an isolated tree or pole - a forested area is better
- consider protection from large hail which may cause bodily harm, smash windows, damage tents and cause you to lose your protective shelter from the wind and rain
- perhaps a heavy duty tarp which is well set up to withstand the wind may be of assistance
- shelter in a cave
- tents are NOT Faraday cages and they do NOT protect you from lightning
- if possible seek shelter - a building, alpine hut, hard-topped car, cave
- if you must “sleep” in your tent during a storm:
- ensure you have chosen a low risk site as above and preferably with the ground remaining dry under you in a storm (ie your tent is on higher ground but not too high it becomes the tallest object)
- place thick rubber mats under your mattress (which should ideally be a thick foam mattress) and ensure they are dry (probably best to remove your electric blanket if you use one as you don't want to be on top of a whole lot of wires)
- do not touch anything metal
- if you sense a lightning strike, get into lightning position
- if you have a wood stove with a metal chimney in your tent, it is probably best to ensure it is NOT grounded by placing non-conductive material under each leg (you don't want to encourage lightning to come into your tent by partly grounding it!) a 2-3m tall metal chimney is unlikely to be the least resistant path for lightning if there are much taller trees nearby - all the same maybe best to take the chimney down during a storm - you probably shouldn't be using it if there are severe winds anyway
the lightning position
- be on the driest soil and if possible select a low risk site
- stay away from metal objects such as cooking appliances, backpack, wire fences, tent poles, etc
- if there is time, remove metal objects such as belt buckles, etc
- have each person about 15m apart to avoid group injuries
- hopefully you are wearing rubber soles, although these are only protective for perhaps 5000V
- don’t lie down - minimize contact with the ground and keep your body’s footprint as small as possible (don't have feet apart!)
- preferably have an insulating, dry plastic sheet or pad to crouch on to reduce impact of ground current
- crouch with ankles together to keep the current ankle to ankle rather than up the body and down back to earth
- squat down, sitting on the heels of your feet with the balls and toes of your feet touching the ground to lower your height
- tuck your head down to your knees to minimize chance of a strike on the head and give lightning that does hit you a path down your back to ground - your chances of survival are greater
- try to hold this position for the hour that the storm takes to pass through
- hands over ears to prevent ear drums blowing out
- close eyes to prevent blindness
first aid
- as there is no ongoing current after a strike, there is no danger from electrocution in attending a lightning strike victim (other than from a subsequent lightning strike), and indeed, if they are unresponsive and in cardiac arrest from current passing through their heart, their only hope of survival is first aid for management of cardiac arrest and ventricular fibrillation which involves immediate CPR and early defibrillation