What is the normal cruising altitude for modern jet airliners?

Modern jet airliners?
No, we aren't talking about those old DC - 3's, we aren't talking about the de Havilland Comet either.We're talking about something like the Boeing 737 or the Airbus a320. We consider modern jet airliners as any aircraft that was made at the same time the Boeing 737 was made, or after it was made. Can't think of any? Here are some examples: Boeing 747 to 787. Note: Market failures will not be classified as modern jet airliners some might not know about them. ^Air Canada 787-9^

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Let's start
We all know that when a plane takes off, after a few seconds, at most a minute, the plane is above the sky and before you know it you can't see it.If you've gone bird watching at busy airports such as New York's JFK, then you are definitely familiar with that. Okay, so you get the point; aircraft ascend at a very quick rate during takeoff. But how long do they need to climb? (And no, I know what you are thinking. They don't ascend to their cruise altitude in a few seconds. Just because we can't see the airplane doesn't mean it has stopped climbing.) Nowadays, airliners usually cruising altitude is around 32000 ft. (Feet is used when measuring an aircraft's height. But if you want the meters, it is about 9750 m.) The highest cruising altitude for any aircraft in history is around 70000 ft, which the Concorde triumphantly holds. But now, usually, airliners highest cruise altitude is 42000 ft.

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Why do we have to be so high?
The answer is quite simple. This also goes with why is most of the airplane ride so smooth? It is all because to avoid bad weather. Let's take an example; An Hainan Airlines Boeing 787-9 from Toronto to Beijing has a total flight time of around 12 hrs and 30 min. 80% of the flight, the aircraft stay at the normal cruising altitude of 32000 ft. But the other 20% the plane ascends to 36000 ft. That occurs after 60% of the flight is completed. If you have ridden on these flights, then you must have felt that at 36000 ft, it obviously was a lot smoother. (If you haven't, don't worry)                                ^Hainan Airlines 787-9^

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Safety
Flying at such a high altitude also offers a lot of safety. For example, if both engines or all engines fail, the pilots can glide for a longer distance, which can allow the pilots to glide to a nearby airport for an emergency landing.