The National Weather Service's advice for folks threatened by lightning when there is no safe building or automobile available:
Do not seek shelter under tall isolated trees. The tree may help you stay dry but will significantly increase your risk of being struck by lightning.
Do not seek shelter under partially enclosed buildings.
Stay away from tall, isolated objects.
Know the weather patterns of the area. For example, in mountainous areas, thunderstorms typically develop in the early afternoon, so plan to hike early in the day and be down the mountain by noon.
Know the weather forecast. If there is a high chance of thunderstorms, curtail your outdoor activities.
Do not place your campsite in an open field on the top of a hill or on a ridge top. Keep your site away from tall isolated trees or other tall objects. If you are in a forest, stay near a lower stand of trees. If you are camping in an open area, set up camp in a valley, ravine, or other low area. A tent offers no protection from lighting.
Wet ropes can make excellent conductors. If you are mountain climbing and see lightning and can do so safely, remove unnecessary ropes extended or attached to you. If a rope is extended across a mountain face and lightning makes contact with it, the electrical current will likely travel along the rope, especially if it is wet.
Stay away from metal objects, such as fences, poles and backpacks. Metal is an excellent conductor. The current from a lightning flash will easily travel for long distances.
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On the Net:
Lightning safety: http://www.lightningsafety.noaa.gov/