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This is the third in a series of 5 (at this point) questions. It's not relevant to this question, but for anyone who's curious this was the first, this the second.

Conditions for this question are:

• Earth-normal atmospheric pressure/content. (Or close)




• I'm aware that over 38,000 feet has been achieved by exploiting thermals. This slow and steady way is not what I need.




• I need the ascent to be sudden and unexpected (for an arguably inexperienced pilot), be it; meteorological, geothermal or by some other (not fantasy) means.




• It should not kill the pilot - if you can plausably argue a volcanic erruption - then fine but the pilot must survive (unconsciousness is just fine though).

Any landscape type can be specified in supporting arguments within the answer, sea nearby, desert, mountains, geisers, volcanoes, icefields - anything that could feasably contribute to the desired sudden lift.

A hang glider, the pilot taken by surprise, there is a sudden unexpected lift to 25,000 feet altitude, what could do this?

Getting caught in a thunderstorm is the most likely cause. This can happen even to experts if they are incautious enough to take risks with the weather.

Here's an example with a paraglider. (I'll see what I can find for a hang-glider)

Ewa Wisnierska was sucked into a powerful thunderstorm while training
for the world paragliding championships in Australia. She was carried,
unconscious and wearing a layer of ice, to an altitude of more than
32,000 feet into the eye of the storm. https://youtu.be/IXLdsnB5VBw

Here's another - this is a good one because it is videoed with a live commentary from the pilot who escaped the updraft before it was too late.

Caught in Cloud Suck! - Life lesson from 3Km above sea level
https://youtu.be/FdoGtqCQ2ZY

Some science

Hang-gliders are likely to find it easier to escape a storm because their forward speed can be much greater that that of a paraglider. They should be able fly out of it provided they can keep a sense of direction.

Hang-glider Beginner wing 14 mph to 45 mph. Advanced wing - 16 mph, to over 100 mph which has been clocked in speed gliding competitions
http://www.hanggliding.org/wiki/A_Comparison_of_Hang_Gliding_to_Paragliding

Paraglider Beginner wing 13mph to 22mph. Advanced wing - 14 mph to 35mph
http://www.hanggliding.org/wiki/A_Comparison_of_Hang_Gliding_to_Paragliding

EDIT (from the same source)

See under Wind penetration where my above suggestion is supported.

http://www.hanggliding.org/wiki/A_Comparison_of_Hang_Gliding_to_Paragliding

Here is a claim that it can happen to hang-gliders as well but there is no reference to back it up.

In one reported incident, two hang gliders were caught in cloud suck
by the same storm clouds. Both gliders lost consciousness above 30,000
feet, one, happily, was released by the storm, and regained lucidity
before crashing into the ground. The other glider was not so
fortunate. His frozen body was discovered later and returned to his
family.
https://www.toysperiod.com/blog/extreme-sports/hang-gliders-in-the-sky/

Cloud suck

If you want to know how people get trapped and about emergency procedures for escape, then I suggest you Google "cloud suck".

Cloud suck is a phenomenon commonly known in paragliding, hang
gliding, and sailplane flying where pilots experience significant lift
due to a thermal under the base of cumulus clouds, especially towering
cumulus and cumulonimbus. The vertical extent of a cumulus cloud is a
good indicator of the strength of lift beneath it, and the potential
for cloud suck. Cloud suck most commonly occurs in low pressure
weather and in humid conditions.

https://en.wikipedia.org/wiki/Cloud_suck

Let me add one more...

Wind Shear

Wind shear (or windshear), sometimes referred to as wind gradient, is a difference in wind speed or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal wind shear. Vertical wind shear is a change in wind speed or direction with change in altitude. Horizontal wind shear is a change in wind speed with change in lateral position for a given altitude.[1]

Wind shear is a microscale meteorological phenomenon occurring over a very small distance, but it can be associated with mesoscale or synoptic scale weather features such as squall lines and cold fronts. It is commonly observed near microbursts and downbursts caused by thunderstorms, fronts, areas of locally higher low-level winds referred to as low level jets, near mountains, radiation inversions that occur due to clear skies and calm winds, buildings, wind turbines, and sailboats. Wind shear has significant effects on control of an aircraft, and it has been a sole or contributing cause of many aircraft accidents. (Source)

Basically, (and specifically considering vertical wind shear) wind shear is a very sudden column of high-speed air. It has the habit of flipping big airplanes on their sides. And when they say "short distance," they mean it. It can affect a single wing. They're fairly common at Denver International Airport, which was developing radar specifically for detecting wind shear when I was graduating as an EE. I almost elected to intern at DIA and use the development process for my senior thesis.

If it's strong enough to flip a 767 ninety degrees, it's strong enough to lift a hang glider.

It might actually be too strong. These gusts might tear a hang glider apart. But, what's a story without a crisis, right?