Boats,
Believe it or not, it's possible to sight in a scope with ONE shot.
If you can hold the rifle
still through the shot (you almost HAVE to be firing from a rest of some sort), after the shot, look through the scope and adjust the crosshairs so that they center on the bullet strike.
You'll be sighted in!
This is easier to do with low-recoil rifles.
Altitude will affect the sight-in of a rifle. If you're sight at, say, sea-level, and you go hunting in Denver, you'll need to re-sight your rifle. The air at altitude is thinner and the bullet trajectory will be flatter.
Temperature will not affect your sight settings, but the
temperature of your ammunition will.
Powder in cold ammo burns slower than the powder in warm ammo. Which will affect your sight setting.
Steve