What are the differences between "The spacecraft / airplane flew into the air" & "The spacecraft / airplane flew into the sky"?

Question

the air: the space above the ground or around things

into the air

Flames leapt into the air.

through the air

He fell 2,000 metres through the air without a parachute.

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sky /skaɪ/ ●●● S2 W2 noun (plural skies)

1 [singular, uncountable] the space above the Earth where clouds and the Sun and stars appear

The sky grew dark, and a cold rain began to fall.

A shooting star sped across the night sky.

in the sky

There wasn’t a cloud in the sky.

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So I would say "The spacecraft flew into the air" when it was off the ground a bit not too high. Whereas, "The spacecraft flew into the sky" when it was off the ground very high, maybe above the cloud.

What are the differences between "The spacecraft / airplane flew into the air" & "The spacecraft / airplane flew into the sky"?

Answer 1

Technically, "air" is a substance, while "sky" is a place. You could say, "I filled the test tube with air", but you wouldn't say, "I filled the test tube with sky." Or even in a more common sense, you could say "The air in this room smells bad", but you wouldn't say "The sky in this room smells bad." Conversely, it would be perfectly valid to say, "On the Moon there is no air in the sky."

But that said, people do sometimes use "air" to mean something like "the outdoors above the ground". Like people will say, "I saw a plane flying through the air."

As I think about how people use the words idiomatically, I suppose that "air" can mean anywhere outside, but "sky" usually means high above your head. Like you could say, "The frog jumped into the air" if he jumped 12 inches. But you wouldn't say, "The frog jumped into the sky". Not unless you were trying to make a poetic exaggeration.

Answer 2

Idiomatically, there isn't really a difference. People do say "up in the air" to refer to the sky. Technically though, there is air all around us; whereas the sky is above the ground.

Phrases like "it soared into the air" are perfectly idiomatic, but so are phrases like "go into the open air" to simply mean "go outside".

"The sky" always refers to the area up above the earth. Planes fly in the sky, and when you are on a plane you might say you are "up in the sky".

So there is a difference, but you can use either in your context, and they should be understood.