Lightning
- What is Lightning?
- Kinds of Contact
- Avoiding being struck
- Safe Haven
- Plan B: no safe haven
- Summary
- References
Lightning is a hazard that can pose a danger to anyone but one that can be particularly significant to people conducting field research who spend a lot of time outdoors, sometimes in very remote areas. Although the annualized statistics for the nation suggest that the probability of being struck by lightning is very low, the probability increases for people actually in a storm area depending on their level of exposure.
In Canada, there are around 10 people a year who die as a result of lightning and another 90-160 people who are injured by lightning. Most of the incidents occur in southern Ontario and since 1921, over 90% of the deaths related to lightning occur in Ontario, Quebec, Manitoba, Saskatchewan and Alberta. This is related to population density and the frequency of lightning strikes in these provinces. In the USA, the ranking of casualties by state is Florida, Michigan, Pennsylvania, N. Carolina and New York.
Most of the casualties occur in the summer between Thursday and Saturday and most are related to people engaged in outdoor activities (sports, hiking, working). In Colorado, the most common sites of lightning fatalities were for people in open fields (27%), near trees (16%) and close to water (13%).
What is lightning?
Lightning is a static discharge that occurs due to the build-up of charge separation within the cloud. It is very similar to the static charge and discharge that occurs when people walk on a carpet then touch a piece of metal, however, it is much larger in magnitude. In a cloud, the particles of water and ice collide and move about through updrafts and gravity such that an area of positive charge develops at the top of the cloud and an area of negative charge develops at he bottom. This charge separation produces an enormous electrical potential amounting to millions of volts and eventually, an electrical discharge occurs between the two oppositely charged areas that produces the lightning bolt and thunder. The discharge may occur within a cloud, between different clouds or between the cloud and the earth. The latter case is referred to cloud to a ground (CG) discharge and occurs because the large negative charge on the bottom of the cloud induces a positive charge on the ground.
The lightning channel is about 2.5 cm wide and can be 4-10 km long. Typical potential differences between the cloud and ground are 10 to 100 million volts with currents in the range of 10,000 to 200,000 amps (houses may be wired with a 200 amp service and arc welders that melt steel operate at 200-400 amps). The temperature of the bolt is about 28,000C (5 x hotter than the sun) and the rapidly heated air in the vicinity of the bolt creates a shock and sound wave that move faster than the speed of sound which causes the thunder clap that may be heard over 20 km from the source.
The time between the lightning flash and when the sound is heard (often called the flash to bang time) can be used to judge the distance to the bolt. Sound travels about 1 mile in 5 seconds or 300 meter/second so a flash of lightning that is heard 30 seconds later is about 6 miles (9,000 m or 9 km away).
Kinds of contact:
1. Direct strike: bolt hits the person, often to the head if the person was standing
2. Contact voltage: lightning hits something that a person is touching (golf club, umbrella, a wired telephone) and enters the body.
3. Flashover or splash voltage: when lightning hits a nearby object (tree, pole, wire fence) and then arcs over to a person.
4. Step or ground voltage: occurs when lightning hits the ground and the current fans out in all directions. The magnitude of damage will vary with the local conditions (wet/dry), orientation of the person (standing/lying) and distance from the source. This kind of contact can affect several individuals at once.
Effects of lightning on people:
Immediate effects can be burns, cardiac arrest and cessation of breathing so the first thing to do is to call for assistance (911) and apply cardiopulmonary resuscitation (CPR) to the individual. About 90% of people survive a lighting incident but there may be long term debilitating neurological, physiological and psychological problems.
Avoiding being struck by lightning:
As with bear safety recommendations, avoidance of an encounter is the first line of defence. Monitor the local weather conditions and check weather reports. Postpone activities that may expose you to severe weather and lightning if a safe refuge is not immediately at hand.
the 30-30 Rule:
The first 30 refers to seconds and says that when the time between flash and bang is less than 30 seconds, you should already be in a safe shelter. The second 30 refers to minutes and means waiting at least 30 minutes after the last thunder sound before you leave the safe shelter and resume your activities.
Rationale: 30 seconds between the flash and bang indicates the lightning is about 6 miles or 9 km away from you (sound travels about 1 mile / 5 seconds or 300 m / second). Older data suggested that successive lightning bolts could occur about 3-4 km apart but more recent information suggests there may be 9 km (or more) distance between bolts. This means that when the flash to bang is below 30 seconds, another strike could occur right where you are.
The time of 30 minutes as the minimum time to stay in a safe location is based on the observation that thunderstorms typically move at about 35-40 km/h so after 30 minutes, the storm may be 16-20 km away from you and the chance of another strike from that storm arriving at your location are reduced.
However, it is entirely possible that lightning can occur outside the actual rain core when skies nearby are clear. This is the origin of the phrase "bolt out of the blue" [sky] used to describe an 'unexpected event'. Analysis has shown that the end of storms when the strike frequency was declining, was as deadly to people as the middle of the storm when the frequency was higher. This may be because people underestimate the risk of lighting as the storm moves away and return outside when lightning can still occur.
Safe Havens in a lightning storm:
1. inside a substantial, usually inhabited, building - this refers to an enclosed building wired for electricity and with plumbing which tend to electrically ground the structure (may conduct the lightning current safely into the earth). Open structures like bus or picnic shelters, out-houses or tents are not grounded and may, because of their height, be more likely to be struck by lighting than you would be standing outside of the shelter. Stay away from these structures. Even if you are in a substantial building, stay away from windows and do not use the shower or bathtub or any electrical appliances (telephone, power tools). It has happened that lighting struck outside and the current entered the house via metal pipes or the wiring.
2. inside a fully enclosed metal vehicle with the windows closed - a convertible or something like a golf cart or ATV does not offer any protection. Lightning tends to travel along the surface of a conductor and the vehicle acts like a shell to allow the current to go around you and jump to the ground. However, do not touch anything metallic when sitting in the vehicle. The rubber tires are irrelevant for protection; they do not act as an insulator for such high voltages. The lightning bolt has just travelled some 4-8 km through the air that is also an insulator.
Plan B: if no safe structure is available for refuge:
- avoid high ground - get to a low area, a ravine or ditch. Lightning often strikes the higher objects in the area.
- avoid wide open areas (fields or beaches) - so you are not the tallest object in the vicinity; get off the water if you are in an open boat, get out of the water if you are swimming in a lake or pool.
- avoid being close to tall objects - light pole, an individual tree or small group of trees. Tall objects are more likely to be struck by lightning than low objects. It might be safer to enter a forested area if it is relatively large and the trees are similar in height but stay away from the tallest trees in the forest.
- avoid small open buildings - a bus or rain shelter
- avoid metal fences or bleachers - a metal fence may be struck far away from you and the current may travel along the fence and jump to you.
Summary:
Getting struck by lightning is a rare event but there are specific conditions where being involved in a strike is not so rare. It happens to about 130 people a year in Canada. Know that lightning can strike beyond the area where there is rain and that lightning from a single electrical disturbance can occur over a distance of 10 km or more. You should be in a safe shelter when the time between the flash and the bang is less than 30 seconds.
In defensive driving courses, the Accident Prevention Formula is given:
- know the hazard,
- understand the defence, and
- act in time
This applies to avoiding lightning (and many other hazards).
References/Links:
When Thunder Roars, Go Indoors!
Provided by the CDC - extensive information about how lightning occurs and ways to avoid strikes.
https://www.cdc.gov/features/lightning-safety/index.html
Striking Back: An Assessment of Lightning-related Fatality and Injury Risk in Canada
analysis of incidents in Canada (pdf)
Lightning-relatedinjuriesandfatalitiesinCanada_FINAL-TECHNICAL_1-September-06.pdf
National Lightning Safety Institute
lots of valuable information on lightning
http://www.lightningsafety.com/nlsi_pls.html
Colorado Lightning Resource Center
more valuable information and pictures
https://www.weather.gov/pub/lightning
Severe Storms - What to do
https://www.canada.ca/en/environment-climate-change/services/seasonal-weather-hazards/warm-season-weather-hazards.html