I have done the 5 digits in a balloon on a couple of occasions and my colleague and I checked on each other by asking if we felt slightly pissed which was to be our first indication that decending to below 10,000 feet would be prudent. The flame did get rather wide and "feathery" at altitude.
I have also thermalled to over 10,000 in the French Alps on a paraglider which gave me a great view of Mont Blanc and plenty of time to contemplate the sanity of trusting my life to a collection of threads and a reserve chute.