Thank you, Jimshaw, Pika and Midnight Sun, for excellent replies. We've now heard from a number of highly experienced backpackers in the Pacific Northwest and a Search and Rescue professional from Alaska. I'm quite experienced, too, but not quite on that level! However, my experience has been in the Rockies and the Pacific Northwest, not in Pennsylvania summers, which is hardly in the same category weather-wise. (Actually, I did have 6 weeks' backpacking experience in Pennsylvania, but I was only 6 at the time, so I don't remember a lot.)

I checked the American Alpine Institute website that Johndavid cited and found that he summarized it far too much. Here's what it _really_ recommended. I left out the warm weather stuff like shorts and T-shirts.

Base Layer (top and bottom)--lightweight polypro or polyester

2nd layer (top)--expedition weight long underwear top, lightweight fleece, 100 wt. powerstretch, or possibly Marmot DriClime

2nd layer (bottom)--Schoeller or nylon fabric, should be somewhat wind/water resistant

3rd layer (top)--jacket, similar to bottom 2nd layer

Insulated Jacket--not needed July-Sept. 15. Probably correct, since the 2nd and 3rd top layers together are basically equal to the insulated jacket (at least to my Montbell UL Thermawrap) and, being separate, allow more versatility.

Shell layer (top)--waterproof, breathable, durable, with hood, sized to fit over clothing

Shell layer (bottom)--waterproof, breathable, durable, preferably with full-zip legs

Liner gloves (polypro or polyester)

Modular gloves or mittens

Warm hat

Sleeping bag rated to 15* F.

Even without the insulated jacket, all these layers together, four on top and three on the bottom, should be plenty warm enough! If not, there's the 15* (not 30-40*) sleeping bag as backup. Of course these folks are being guided, and I'll bet the guides have plenty of extra stuff along just in case.

I've seen too many below-freezing nights and too much inclement weather, even in July and August, in the Cascades (at 5,000-7,500 ft.) over the years to be comfortable with any less than what is _actually_ recommended on the AAI site (4 layers top, 3 layers bottom). I have a 20* bag, but a heavier base layer, so that balances out.

I would never want to debunk the dangers of hypothermia, especially on a public forum that will be read by beginners. I've had just enough experience with it to have a pretty good idea of how dangerous and insidious it is. Even so, MNS' account above was quite an eye-opener. I just hope, Johndavid, that we won't be reading about you as another casualty in the future.