1. Earth’s Atmosphere

2. Atmospheric Pressure
The Earth's land surface is actually located at the bottom of a vast ocean of air.
Like the water in the ocean, the air in the atmosphere is constantly pressing on the solid or liquid surface beneath it.
Atmospheric pressure. This figure depicts atmospheric pressure as the weight of a column of air. (a) Metric system. The weight of a column of air 1 cm on a side is balanced by the weight of a mass of about 1kg. (b) English system. The weight of a column of air 1 in. on a side is balanced by a weight of about 15 pounds.

3. Air Pressure Changes With Altitude
Because atmospheric pressure decreases rapidly with altitude near the surface, a small change in elevation will often produce a significant change in air pressure.
Atmospheric pressure decreases with increasing altitude above the Earth's surface.

4. Barometric Pressure: The pressure exerted on bodies by the weight of the Earth’s atmosphere
Pressure animation…click on pic