Lightning

1. Lightning ch 11 pp.307-314

2. Lightning’s affect on structures • Most strikes will result in total destruction of the structure due to fire • This house the exception • Several factors • Concrete structure • Blessing and cause • Rebar in walls attracted • Concrete prevented fire • Icining insulation • Suppressed the fire

3. Lightning’s affect on aircraft

4. Lightning’s affect on aircraft • Lightning can • Melt and deform aircraft’s skin • explosive vaporization of conductors, • sparks in the fuel system which can cause • explosion and damage in externally mounted composite material • cause interference to the electronic equipment Blackhawk helicopter: charge density

5. Lightning’s affect on aircraft Prototype of Glasair III LP

6. Langley F106B research aircraft: Where paint is missing….lightning attachment points

7. Lightning attachment hole

8. On one particular Mission, the aircraft Was struck 72 times • NASA found in 1980s • That lightning strikes • Occurred between 5oC and • -65oC • Most above -40oC

9. St. Elmo’s Fire

11. Lightning’s affect on trees • Height is the major attractor • Most trees will not explode like this one but will become weak and susceptible to disease and pests • General rule is to stay at least two tree lengths away from any trees

12. Lightning’s affect on Humans and Animals • Most common cause of death is cardiac arrest • In humans 70% of victims survive because CPR was given in time • Animals have a nearly 100% mortality rate because CPR is unable to be given to them • Most common visual injury is burns • Do not affect the body as bad as electrical burns can • Often do not have an entrance or exit point • Most lasting injury is to the nervous system

13. Who is at the highest risk of being struck • There is an equal risk of being struck whether you are at work, home or outdoors doing activities. • But when you are outdoors the chance of being struck is higher due to the lack of protection. • The activities which tend to have the highest strikes and fatalities are: • Golfing • rated at have the highest number of strikes • Water sports • have the highest number of fatalities • Camping/picnicking • has a high survivability strike rate • Hiking • Half the people struck wound up as a fatality

14. Indoor Safety Facts • Three main ways lightning enters a structure • A direct strike • Through wires or pipes that extend outside the structure • Through the ground • Lightning Safety tips for Inside the Home • Avoid contact with corded phones • Leading cause of indoor lightning injuries • Avoid contact with electrical equipment or cords • Avoid contact with plumbing • Do not take a bath, shower, or wash dishes • Stay away from windows and doors, and stay off porches

15. Key Facts to Know • Outdoors is the most dangerous place to be during a lightning storm • All thunderstorms produce lightning and are dangerous • Lightning often strikes as far as 10 miles away from rainfall • If you hear thunder you are in danger • Look for dark cloud bases and increasing winds • Blue skies and lightning: Lightning can travel up to 10 miles sideways • At least 10% of lightning occurs without visible clouds overhead in the sky

16. Lightning Can Strike Well Away From the Base of the Storm…..

17. Outdoor Safety Rules • If you are outdoors when a thunderstorm threatens go as quickly as you can to a safe shelter • What constitutes as a safe shelter • Safe buildings are buildings that are fully enclosed with a roof, walls, and floor. • They are considered safe because they have wiring and plumbing • Safe vehicles are those with hard tops. • Make sure all doors are closed and windows rolled up and do not touch any metal surfaces • Large Boats with Cabins • Best policy is if you are on the water and a storm is approaching is to get out of the water

18. Outdoor Safety Rules • The question begs to be asked: What do you do if there is no safe shelter nearby to go to. • Move away from tall trees • Keep away from metal objects and fences • If you are in a group spread out with at least 15 ft between you so that there is someone to assist the struck victim. • If you feel your hair rising or your skin feel prickly you are about to be struck so assume the crouching position shown • This position where the heels are touching encourages the lighting to return to ground without going through your whole body • Protect your ears from the resulting thunder by covering them with your hands

19. When thunder roars go indoors

20. Lightning Types • Cloud to ground • Cloud to cloud or Intracloud

21. About 80 percent of all lightning is cloud-to-cloudlightning, or sheet lightning, which occurs when the voltage gradient within a cloud, or between clouds, overcomes the electrical resistance of the air. The result is a large and powerful spark that partially equalizes the charge separation.

22. Cloud-to-ground lightning occurs when negative charges accumulate in the lower portions of the cloud. Positive charges are attracted to a relatively small area in the ground directly beneath the cloud establishing a large voltage difference between the ground and the cloud base. The positive charge at the surface is a local phenomenon; it arises because the negative charge at the base of the cloud repels electrons on the ground below. Farther away, the surface maintains its normal negative charge relative to the atmosphere.

23. All lightning requires the initial separation of positive and negative charges into different regions of a cloud. Most often the positive charges accumulate in the upper reaches of the cloud, negative charges in lower portions. Small pockets of positive charges may also gather near the cloud base.

24. The actual lightning event is preceded by the rapid andstaggered advance of a shaft of negatively charged air,called a stepped leader.

25. When the leader approaches the ground, a spark surges upward from the ground toward the leader (top). When theleader and the spark connect, they create a pathway for the flow of electrons that initiates the first in a sequence of brightly illuminated strokes, or return strokes (bottom).

26. Another leader (the dart leader) forms within about a tenth of a second, and a subsequent stroke emerges from it. This sequence of dart leaders and strokes may repeat itself four or five times. Because the individual strokes occur in such rapid succession, they appear to be a single stroke that flickers and dances about. We call the combination of strokes a lightning flash, the net effect of which is to transfer electrons from the cloud to the ground.

27. A bizarre type of electrification called ball lightning appears as a round, glowing mass of electrified air, up to the size of a basketball, that seems to roll through the air or along a surface for 15 seconds or so before either dissipating or exploding. Seen escorting bomber planes in World War II just off their wingtips. Pilots referred to it as “foo fighters” as they initially were thought to be enemy planes.

28. The tremendous increase in temperature during a lightning stroke causes the air to expand explosively and produce the familiar sound of thunder. The decrease in the density of air with height causes sound waves from lightning strokes over 20 km away be to be bent upward. As a result, the lightning seems to occur without thunder and is sometimes called heat lightning.