Thunder nature!!

of caude. tree is a conductor under rain.

Thunder indicates lightning, trees are popular targets, but only during thunderstorms. Much rain falls without thunder, no danger then.

Your colleagues are correct and it applies not only to trees but exposed open areas as well. Thunder results from cloud to cloud or cloud to earth lightning. There is an old rule of thumb for counting to determine the lightning distance by timing the delay between the lightning bolt and the resulting thunder but, the plain truth is, if you hear thunder, you are in danger of being struck by lightning and should take cover; preferably indoors, inside a closed metal-bodied vehicle or other shielded location.

Another point is that thunder is not a prerequisite to potential danger from lightning. If you are in an open field or other exposed location (under certain atmospheric conditions) and suddenly feel the hair on your body stand up, taste or smell ozone, or otherwise sense that you are in an energized electric field, there is a good chance that lightning is about to strike. Immediately take cover or, if that's not possible, try and make yourself as small and low to the ground as possible by lying flat on the ground in a near by depression or low area, away from all trees, metal fences and similar structures and remain there until the danger has passed.

That sounds like knowledgeable advice. Do you know, what are the effects of the type of terrain, for risk of being struck? Obviously it's all wet in a thundershower, but is being in water or on marshy ground a greater risk?

I remember one time, we had hiked about two hours to this marshy area when suddenly a thunderstorm came up. We had a choice of being under some scrawny trees (only shelter) or on the dirt road leading back through woods, or out in this large open marsh. Major debate ensued, but luckily there was no lightning near us. We just got very wet and some persons got cranky (not me I swear!). In the end we headed back out the road.

Another time when I was ten years old or so, I was on a very small island where my cousins had built a cabin. It was just a rocky outcrop, basically, well out in the ocean. There was heavy rain that day and I went outside, I saw lightning strike the rock about 30 feet away from me. I didn't have any of those warning signs you mentioned: the rain was the only sign at all. I don't even recall if I heard any thunder.

On the water or marshy ground is no place to be in a thunderstorm, because you have no access to shelter. Trees aren't much better, because many of them attract lightning. On the road, if you sensed that a strike was eminent, you could squat down to make yourself less of a target. Or if a car came along, you could hitch a ride and the car's body would act as a Faraday cage to protect you.

As far as the incident on the rock: if the rock had a high iron content, it will more likely attract lightning. Studies of lightning strikes have shown that areas with high iron content in the soil receive more strikes.
You were probably too close to hear the thunder. You might have heard a sizzle, but it would have been brief, and the proximity of the strike probably overrode the memory of the sizzle.

Thanks for the explanation. Now that you mention it, there was something like a sizzle sound - the pouring rain was pretty noisy itself, and you're right, the sight of the lightning was the most impressive part of it, for sure.

In central Texas we had a firing range that was supplied with sunken oil drums for shelter during 'dry' lightning events. These events occured during clear conditions, no rain at all. Why these particular areas was anyones guess.

Run, or jump, and lay flat, into the nearest depression if you feel the ionization prior to a strike.

Does that area have a lot of red dirt, like there is in central Oklahoma? Red dirt has a high iron content, so it is more likely to attract lightning.

No, the dirt was light to dark tan, usually about 6" to a foot deep before running into a 1-2 foot layer of calciche, which then became a sand and pea gravel mix, and may have acted as layers of capacitance (just guessing). It made the stickiest mud when wet.