Quite a photo HJ!
I'm not sure the dangers are always obvious to non-locals. One time in July (July 4th in fact) I came down from a quick, late jaunt up the Desert Museum--it was about 11 and (obviously) very hot. I ran into three young guys in the parking lot who were putting on packs, etc. Tthey said they were going to do Skyline. Of course, I told them that wasn't a good idea because of the heat. They said, "But it will get cooler as soon as we go up for an hour or so!" They had no idea that the heat would be basically just as high, or higher, almost all day. They were from the east coast--Boston-- and applied the weather/geographical models they were familiar with. They set off, and I never heard they were rescued, so I imagine they just gave up after an hour or so (when it didn't get any cooler

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I think posting a warning sign would certainly be helpful, but actually, seriously, I think posting a 'closed--deadly heat conditions" sign during the summer would be even more effective, not because anyone could actually 'close' the mountain, but in the sense that people might finally grasp that they were undertaking something totally outside the realm of the sane. At the very least, any warning sign should state in the strongest terms that summer hikes are strongly recommended against. It's not enough, I think, to state the dangers and let people draw their own conclusions--the people who get in trouble are usually the ones who aren't so good at thinking things through

(And full disclosure--there's been more than a few times when that described me.)
Q: How many therapists does it take to screw in a light bulb? A: Only one, but the light bulb has to want to change ...