So most of our atmosphere is concentrated in a very thin shell about the solid earth. The rapid increase is due not only to the longer path through the atmospheric shell, but also due to the increased air density near the earth's surface. These small constituents and their variations are more concentrated and have greater local fluctuations in the atmosphere very near the earth's surface. Thus we should regard our atmosphere as a thermally agitated envelope whose variational effects are greatly enhanced for lines of sight near the horizon. Some discussion of wind effects and inertia can help.
Experiments dealing with air pressure and the functioning of suction cups are helpful.