In the beginning of Harry Potter and the Deathly Hallows, Harry Potter needs to get from point A to point B as quickly as possible without being intercepted by enemies. Knowing that there is a magical trace (apparently unbreakable) always in effect on Harry Potter until he turns 17 (so running into enemies is inevitable no matter the transport method used) I would think Harry Potter would want to use the safest transport method possible while still getting from point A to point B as quickly/directly as possible. In the movie, Harry and his friends are shown as flying directly to their destination, however they are also shown staying relatively low to the ground, often breaking below cloud cover, even flying/riding at street level in some cases.
Wouldn’t it make more sense to put on a bunch of thermal insulation/protection gear, apply a magical breathing spell and/or get an oxygen tank, and fly as high up into the atmosphere as possible before travel? Straight up from point A, getting somewhere around commercial aircraft cruising altitude (well below the Armstrong Limit, avoiding the need for pressurization suits), and then over, and straight down to point B. It seems doubtful the Death Eaters/Voldemort would be prepared at a moments notice (with supplies, magic, or physically) to fly 45,000-60,000 feet up and spend an extended amount of time searching around for a tiny speck, hardly even visible from ground level.
So my question is, does there exist an actual canon upper limit of magical flight (using broomsticks, magical vehicles, creatures, etc), requiring users to stick around the cloud line or below, or did the Aurors just want to add additional risk to Harry’s life in order to save an hour or two avoiding preparation and ascending/descending a few miles into the atmosphere?