I've jumped from 7,000ft, but never 10,000ft. However, I have been to Leadville, Colorado which is at 10,120ft and I also drove my jeep over Mosquito Pass which is 13,185ft without any issue for me or my Jeep. The Jeep was getting kinda lame on horsepower but being it was fuel injected no fuel/air adjustments were required. About the weather and temp, keep in mind that he jumped sans static-line which means he went into a free fall for X amount of seconds. So, he wasn't at high altitude for very long.
My wife & kids and I used to stroll to the top of Mt. Elgon on the Kenya-Uganda border. Elevation was over 14,000 ft. Of course we lived in a nearby town with an elevation of 6,200 ft., so we were acclimated.
Altitude was NOT one of Cooper’s problems. Bad landing was #1, cold a lesser problem.
Cooper was initially thought to have jumped near Mt St Helens. Tonight’s expected temp in Longview, west of the mountain, is forecast to be 37F with rain. Uncomfortable, but not fatal. I’ve spent the night out, wet, in those conditions. As long as he kept moving he would be OK.
I spent a week in Sana Fe, NM. Santa Fe is nestled in the foothills of the Rocky Mountains at an altitude of 7,000 feet. I had no problems at all with Oxygen etc for 3 days. Then on the 4th day evening I decided I needed to go for a jog to burn up all those calories I was taking in. I had 3 or 4 cups of coffee in the afternoon and a social drink at 5 O’clock and then went jogging. No problem during the jog.
Then at 1 am I woke up with urge to pee, and disaster strikes! My heart was totally out of rhythm, beating very erratically. Since this had never happened before, it scared the dickens out of me. So I dials 911. The ambulance soon arrived, and started it’s drive to the hospital. After a couple of miles, the ambulance engine dies out! I figured it must be my time to go? They radioed another ambulance to pick me up and put me on oxygen mask. At the hospital the doc gave me Xanax and that worked like magic, my heart beating close to normal again.
Moral of the story is avoid combining caffiene+alcohol+exercise at 7000 ft altitude. And secondly, that Xanax stuff is just a miracle drug, if and when you need it.