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We are in the great outdoors and a massive thunderstorm will be over us soon. What steps should we take to be safe from a lightning strike?

This question is not "am I safe in my tent" etc. You are in a situation where it's you and your backpack and the great outdoors. You don't have a base camp or something.

It's even not about "is it safe/dangerous to be there while a thunderstorm is present". It's absolutely about how to react (what to do).

How should one react to a thunderstorm then?

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First of all: try to think and plan ahead. Don't get caught on a mountain top during a thunder storm... Keep an eye on the weather and change your route accordingly.

Location:
Make sure you're not exposed and not the highest point in the immediate vicinity. So stay off of summits, hills, and don't stand upright in the middle of a vast field.

  • Ideal: (obviously) cars or shelters which have lightning rods. (Other stationary shelter will do okay too I guess.)
  • Good: Forests (unless in an actual storm with very high wind speeds), valley ground (keep away from streams and rivers, they might suddenly swell!)
  • Passable: near cliff bottoms (make sure you're not endangered by rocks coming loose above you during the rain), taking shelter next to boulders, in trenches or other depressions in the terrain.

  • Don't stand near solitary trees. Such trees are rather likely to get hit, and standing beneath one can be very dangerous for several reasons (electric current of the lightning will spread through the ground; wood can explode when hit by lightning, resulting in shrapnel; deafening noise)

Position:
Priority one really should be to get to a safe location. If this is not possible, or you have reached the safest (but still potentially unsafe) location nearby, keeping correct body position can help:

  • Don't stand upright, especially not if this makes you higher than other points of impact (shrubs and alike...).
  • Don't lie flat on the ground. If lightning strikes nearby the current which dissipates through the ground is more likely to hurt you if you cover a larger area of the ground. The same goes for your feet: keep them together, not apart.
  • Crouch or sit on the ground: keep as small an area of contact with the ground, while at the same time trying to minimize your height.
  • (Thanks @Benedikt Bauer) If possible try to sit on some insulating material, like your backpack or sleeping pad.
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    Two additional points: If you have, crouch or sit on some insulation material (backpack, sleeping pad...) and avoid having several separated contact points with the floor, i.e. when sitting, keep stand your feet close to your buttocks or when crouching, keep your feet together. This prevents building a bridge-like current path through your body from one floor contact point to the other. – Benedikt Bauer Dec 9 '15 at 14:53
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    Avoid sheltering in shallow caves and hollows of a bank. The lightening path once grounded can be along the surface - if the easiest path is to exit the ground, carry on though you it will take it. – user5330 Dec 10 '15 at 6:35
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    Related to body postition: This postition looks like it'll help. – Felix Dec 17 '15 at 15:56
  • The danger from lying flat on the ground isn't that you'll be touching a larger area of ground. The problem comes from the fact that there will be a greater voltage potential over larger distances of ground. High-voltage can go through the ground quite easily, but it will also go through you. Minimize the "benefit" the electricity gets by going through you instead of the ground to go a certain distance. – forest Sep 9 at 5:23
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(NB: I would have added this as a comment to @fgysin's answer, but I don't have enough rep. to comment yet.)

@fgysin's answer (and the associated comments) is very thorough, and covers most points.

However, there's one other thing worth considering (unless you're alone). If you haven't been able to get to a safe location, and are sitting out the storm as advised (crouching or sitting on the ground, keeping a small contact area and minimizing you height), you should -- if the terrain allows it -- maintain some separation between members of your party.

This is because, if the worst happens, and there's a ground strike close enough to incapacitate someone in the group, you really want there to be somebody else available to start administering first aid if required. If you're all sat huddled together (not unreasonable if it's cold, wet and windy), the current could affect everybody simultaneously, resulting in multiple injuries, and making the situation that much more serious.

How far apart you should sit (again, assuming the terrain allows it), isn't so clear. A quick Google shows respected authorities recommending a minimum separation of 20 feet, 50 feet, and 100 feet.

I was in this very situation (caught out during a storm in the high mountains, with no safe place to retreat to) in the Drakensberg mountains of South Africa some years ago. Fortunately, there were no ground strikes close enough to cause any injuries, but the group of 15 or so did spend a good 20 minutes knelt on the ground, spaced well apart, as the storm passed overhead.

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