when does the sky start ??

Question: when does the sky start ??
- Keywords:
  sky,
  start
Asked by xizzyx to Adrian, Gaia, Jim, Scott, Vicky on 10 Mar 2016.
- Adrian Buzatu answered on 10 Mar 2016:
  
  Human convention says that at 100 km. But it’s just a human convention.
- Scott Lawrie answered on 10 Mar 2016:
  
  The sky is just the atmosphere (oxygen, carbon-dioxide, nitrogen etc) around us. It’s quite a thin layer around the Earth. The further you are from the ground, the less air there is. It tails off quite a long way, so there’s no real definite ‘edge’ to it. The ‘edge of space’ we have arbitrarily defined is at 100km up. Above this height, airplanes cannot fly without going so fast they zoom off into space! So that seems a reasonable enough place to define as the boundary between the sky and space 🙂
- Gaia Andreoletti answered on 10 Mar 2016:
  
  As Scott and Adrian said human convention says that at 100 km however I think that the sky can start even after 1-2 cm from our nose as is it the atmosphere surrounding us. Its is all relative at the end. 🙂

Question: when does the sky start ??