Why the refractive index of the atmosphere will be different near the sea level and at the top of the mountain?

The density of air changes with altitude. As we go higher from the earth's surface, the density of air decreases.

Due to changes in the behavior of air, the refractive index also changes. The Refractive index also decreases as we move higher up from the earth's surface.

So, the refractive index of the atmosphere will be different near the sea level and at the top of the mountain.