- First and most impactful: as the earth curves down and away from the observer's horizon, your line of sight goes through a thicker slice of the atmosphere.
Looking straight up you might have 100km of atmosphere until space (the distance is made up here, but I'm using the Kármán line as an arbitrary ruler), but looking out towards the horizon (assuming a perfectly spherical Earth), it's much, much more than that 100km, so the light will scatter off of (and/or be filtered by, depending on angle and time of day) more particles in the atmosphere, affecting the colour of the sky.
- The compounding factor here is if there are environmental factors that boost the particle count in the air, and especially particles that'd stay in lower layers of the atmosphere. Where I am, we've been dealing with wildfire smoke of varying strengths for a few weeks. Today's gentle enough, but it's bad enough that my gradient goes from rgb(115, 160, 207) at the top of the sky to rgb(227, 230, 227) at the horizon (which is shockingly accurate).
There's two main reasons for this:
- First and most impactful: as the earth curves down and away from the observer's horizon, your line of sight goes through a thicker slice of the atmosphere.
Looking straight up you might have 100km of atmosphere until space (the distance is made up here, but I'm using the Kármán line as an arbitrary ruler), but looking out towards the horizon (assuming a perfectly spherical Earth), it's much, much more than that 100km, so the light will scatter off of (and/or be filtered by, depending on angle and time of day) more particles in the atmosphere, affecting the colour of the sky.
- The compounding factor here is if there are environmental factors that boost the particle count in the air, and especially particles that'd stay in lower layers of the atmosphere. Where I am, we've been dealing with wildfire smoke of varying strengths for a few weeks. Today's gentle enough, but it's bad enough that my gradient goes from rgb(115, 160, 207) at the top of the sky to rgb(227, 230, 227) at the horizon (which is shockingly accurate).