Notes on 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.