Question

why does density of air become lower in the upper part of the atmosphere

Answer 1

because at the upper part of atmosphere has lees weight on the air molecules which makes them compressed.

Answer 2

Because there are lesser air molecules compressing the lower layer ☟ molecules so the pressure on that layer decreases so gas expands and the molecules get more scattered than that of the lower layer due to the fact density of the air become lower in the upper part of the atmosphere

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