Air Pressure

Pages 10-14

Weather Textbook

Air has mass therefore it also has:

• Density: The amount of mass in a given volume is density
• Pressure: The force pushing down on a surface is pressure

Air Pressure: The weight of a column of air pressing down on an area of Earth.

• The atmosphere is heavy, as heavy as a school bus!
• The reason it doesn’t crush you is that it is spread out and pushes equally in all directions
• Denser substances have more mass so they add more pressure, therefore
• Denser air exerts more pressure than less dense air.

Barometer: An instrument that measures air pressure.

• Mercury Barometer: Mercury is filled in a glass dish and in a long columnar tube.  When air pressure increases it pushes down on the mercury in the dish, forcing it up into the tube.
• Aneroid Barometer: Aneroid means without liquid. It has an airtight chamber.  When air pressure increases the chamber walls are pushed in.  The needle in the dial indicates the change in pressure.
• Normal Air pressure at sea level is 76 cm or about 30 inches
• Units of Air pressure:
• Inches or centimeters
• Millibars – 1 inch = 30 millibars so sea level would be about 2280 millibars

Altitude and its effect on air pressure and density:

·        Air pressure decreases as altitude increases

The higher up you go the less air is pushing down on you.

·        As air pressure decreases, density decreases.

The higher up you go the less dense the air is.

The molecules of air are further apart.  This means that the amount of oxygen in a given space is less, so you would have trouble breathing.