40 Degree Bags on the JMT?

By itself? For the vast majority of hikers, definitely not. Nighttime temperatures at Tuolumne Meadows can be below freezing even in August.

As part of a sleep system of bag plus insulating clothing? Sure. How much of what kind really depends on when you're going.

The issue (for me) with a bear can is not the added weight – I'll just drop a liter of water as there are plenty of water sources in the Sierras – but rather the bulk. My 25° down sleeping bag wound up on the outside of my pack,and my external pockets were always full.

I should have been more specific, early August and with some aid of a sleep system, but doing the insulating layers on the cheap is another issue I'm thinking about.

I guess what I'm really looking for is average temps for July and August on the JMT.

Thanks,
Chris

Average temps at what altitude?

Good advice I was given – climb high, sleep low.

A good rule of thumb when planning camp sites: air temperature changes 2-1/2 degrees F with every 1000 feet of elevation change.

Alright, so I am the king of asking non-specific questions.

Perhaps its time to fork over the money on one of those JMT books. I suppose I don't know which altitudes there will be good camps at, but then I don't plan on camping on top of Mt. Whitney either.

I did the PCT this year. Could you get away with a 40 degree? probably. But you'll also probably do what everyone does, which is replace your cheap gear with better gear later. You'll just end up spending more than getting decent gear to begin with. I would do one of two things. One is get a good bag–I used a WM Alpinlite 20 degree down and it was good, but 3x more expensive. I have a friend who slept cold with a 30 degree on the JMT. The other option is really go through your other crap and get rid of stuff. I used a sunshirt, windshirt, and rainjacket, with zip off pants and short underwear. That dropped a lot of weight, and I didn't carry a hiking shirt, long underwear, sleep clothes, ground tarp, or even gloves on that section and did fine without. I used my rainjacket as a layer. I think weight is better dropped in other areas. Your sleeping bag should be your shrine. And, that being said, you should spend a little money. You're going to hike a sick trail, for weeks. Spend some cash. You could spend 500 bucks in no time in society, why cheap out on the gear for a great hike?

I haven't done the JMT, but I'd expect that you'd need something warmer unless you were very careful about sleeping spot selection and clothing. REI also tends to overestimate the warmth of their bags as well.

I took a -3 (Celsius) bag on the JMT this year and was OK in it, my girlfriend (sleeps cold) was in my Rab Quantum 600 (-12 allegedly)…

We like to sleep high if possible and were usually above tree-line; it dropped to a few degrees below freezing on a couple of occasions, lower down it was fine.

This is helpful – Thanks.

I thought maybe as much. I am trying to make the "cheaper than anything else for 2 weeks" argument to pops, but this is easier said than done.

Hey Chris,
I believe a warm bag is the most essential piece of gear you need.
There can, (are) also temps below freezing on the JMT year round.
I don't understand why you wouldn't want around a 25 degree bag that weighs a half pound more? Then you can drop any extra clothing and just carry a light weight down jacket for camp.
You will never be able to beat a combination weight with a better bag with 1 short, 1 long sleeve, 1 shorts, 1 pant, and 1 down jacket for layering if you were to bring a 40 degree bag.

You would probably need a good (at least) mid weight thermal, and another, (just in case) long sleeve shirt and even a heavier jacket to aid with the 40 degree bag.

All of this would come to a lot more than a half pound and would also take a lot more room in the pack.

A guaranteed good nights sleep beats packing your insecurities any day.

Oh, by the way, I slept in a 40* bag the night my profile picture here was taken. I had all of my clothing on and absolutly froze my nads off.

Chris,
If I were you I'd cut to the chase and pick up a high quality 20-30 degree bag. That, along with your clothing, should get you through nicely. My go to company is Western Mountaineering. They have several that would fit the bill.
My personal choice in a 3 season bag for the Sierra is the WM Summerlite; It's a legitimate 30 degree bag that weighs 19 oz and costs $300. Pricey, yes, but it gets the job done and, with reasonable care, will last many years. Feathered Friends also makes very high quality bags, but they tend to run a little heavier.

You can probably get the golite ultra quilt rated to 20 degrees (probably more like 30 degree?) for less than $200 on sale.