What layer of the atmosphere does Santa and his reindeer fly?

Answer 1

They fly through the mythosphere layer of the atmosphere. Around most of the earth this layer of the atmosphere is quite high up but, near the poles, the earth's magnetosphere causes the layer to move closer to the earth. This means that Santa and his reindeer can get into this layer almost straight away. They then rise to their cruising altitude which is high enough so that the air is thin and the reindeer have less aerodynamic drag. The reindeers' horns do break up the air flow so that there is less turbulence, but even so, flying close to ground level is a lot more effort for them. Because of the high altitude, Santa and his reindeer are not easily visible while they are cruising.