simple science

Air:
The atmosphere which envelops us at all times extends more than fifty miles above us, its height being far greater than the greatest depths of the sea. This atmosphere varies from place to place; at the sea level it is heavy, on the mountain top less heavy, and far above the earth it is so light that it does not contain enough oxygen to permit man to live. Figure 40 illustrates by a pile of pillows how the pressure of the air varies from level to level.

Sea level is a low portion of the earth's surface, hence at sea level there is a high column of air, and a heavy air pressure. As one passes from sea level to mountain top a gradual but steady decrease in the height of the air column occurs, and hence a gradual but definite lessening of the air pressure.