Is there an upper height limit to magical flight in Harry Potter?

Question

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?

Answer 1

Yes, there is a limit to the height that brooms can achieve.

Even the best brooms have a certain height limit, though the limit has increased with developments in broom technology.

“The Moontrimmer, which was first created by Gladys Boothby in 1901, represented a leap forward in broom construction, and for a while these slender, ash-handled brooms were in great demand as Quidditch brooms. The Moontrimmer’s principal advantage over other brooms was its ability to achieve greater heights than ever before (and remain controllable at such altitudes).”
_{^{- Quidditch Through the Ages}}

It is never specified exactly what the current best achievable altitude limit is, but it’s clear that one does exist, since the broom referenced in the paragraph above is described as reaching heights never reached before. In addition, it implies that brooms become harder to control the higher they are flown, as it is considered important that this broom can still be controlled at these higher altitudes.

Their modes of transport likely were not all capable of flying at the highest altitude of magical flight achievable by current broom technology.

The modes of transport used to move the multiple Potters were brooms, Thestrals and Hagrid’s motorbike.

“We’re going to use the only means of transport left to us, the only ones the Trace can’t detect, because we don’t need to cast spells to use them: brooms, Thestrals and Hagrid’s motorbike.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 4 (The Battle of the Seven Potters)}}

Hagrid’s motorcycle in particular was chosen for its ability to carry his weight, not its ability to reach particular altitudes.

“An’ you’re with me, Harry. That all righ’?’ said Hagrid, looking a little anxious. ‘We’ll be on the bike, brooms an’ Thestrals can’t take me weight, see. ’Not a lot o’ room on the seat with me on it, though, so you’ll be in the sidecar.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 4 (The Battle of the Seven Potters)}}

Presumably Hagrid’s motorcycle is not as capable of reaching particularly high altitudes, and it is not mentioned how high Thestrals would be capable of flying.

Answer 2

From Chapter Nine of Philosopher's Stone:

Harry had heard Fred and George Weasley complain about the school brooms, saying that some of them started to vibrate if you flew too high, or always flew slightly to the left.

While the school brooms being referred to were probably at the lower end of the broom spectrum, this passage would seem to indicate that more or better magic is necessary the higher you fly. It stands to reason, then, that at some point even the best brooms would reach the limit, albeit a higher limit than the school brooms.