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Comment by SubiculumCode | Hacker News Reader

If you want a good idea how deep is the deepest part of the ocean (Mariana Trench), look up in the sky and find a jet flying at full altitude (37,000 feet).

That is how deep.

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What is this strange anatomical unit you measure with?

Something like 11.2km I think. For some reason I often hear altitudes given in feet even alongside other values using metric units. Might be an international standard in aeronautics?

Relevant: https://en.wikipedia.org/wiki/Flight_level

The unit doesn't change how high a jet flies.

In reverse: if you're flying and the sky below you is clear of clouds, look down. That's how far deep the ocean floor is from the surface.

How can you tell if a jet is flying at full altitude? Is it actually visible?

Good question. I've seen passenger jets get pretty small to the eye sometimes, and they don't tend to go higher than 37,000 feet.

Alternatively, if you've ever flown across the country, mid-flight, look down.

The flight I took from the US back to Europe was at 42000 feet. Not that the difference in altitude matters much.

They’re not directly correlated, but generally, when you see lengthy contrails behind an aircraft, it’s probably cruising at altitude. Again, not directly related – it’s an atmospheric phenomenon[0] – but visible contrails are a safe indicator an aircraft is at or near cruise and are quite common. (Their absence, on the other hand, is not necessarily a counterindication.) And yes, an aircraft will pretty much always be visible to the naked eye, even up to 45-50k where some business jets live now. You may have to squint, but your eyes are pretty good at finding fast motion against a static background.

[0]: https://en.m.wikipedia.org/wiki/Contrail

Flightradar24

Contrails will help you find the jet, but you should be able to see it on a clear day.

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