I saw a great one-liner, and two megalogs, but no Goldilocks-sized answer, so here’s my attempt.
As air rises, the weight of air above it (all the way to space) is less, so it’s less squashed, letting it expand.
It expands by pushing out on all the air around it, and every time an air molecule bumps a neighbouring bit of air away, but isn’t bumped back so hard (so it expands), it loses a bit of energy - i.e. heat.
So as some air goes up, it expands and loses heat; or as it sinks, it squashes and gets more heat.
This is adiabatic expansion.
Appendix:
This might beg the question of why higher air isn’t just heated by neighbouring expanding air, making up for its original loss. I think that can be answered by saying overall the top air is squashing the bottom air, so overall the top is cooler. Is that fully right? Right now I feel there’s multiple ways to think about it and I can’t write any clearly without long rambling!
I saw a great one-liner, and two megalogs, but no Goldilocks-sized answer, so here’s my attempt.
As air rises, the weight of air above it (all the way to space) is less, so it’s less squashed, letting it expand.
It expands by pushing out on all the air around it, and every time an air molecule bumps a neighbouring bit of air away, but isn’t bumped back so hard (so it expands), it loses a bit of energy - i.e. heat.
So as some air goes up, it expands and loses heat; or as it sinks, it squashes and gets more heat.
This is adiabatic expansion.
Appendix:
This might beg the question of why higher air isn’t just heated by neighbouring expanding air, making up for its original loss. I think that can be answered by saying overall the top air is squashing the bottom air, so overall the top is cooler. Is that fully right? Right now I feel there’s multiple ways to think about it and I can’t write any clearly without long rambling!