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Atmospheric Pressure
• Properties of Air
• Air consists of atoms and molecules that have mass.
• Because air has mass, it also has other properties, including density and pressure.
• Density = Mass/Volume
• If there are more molecules in a given volume of air, the density is greater.
• Pressure
• Pressure = Force/Area
• The force pushing on an area or surface is known as pressure.
• Air pressure is the result of the weight of a column of air pushing down on an area.
• The molecules of air push in all directions which is why air pressure doesn’t crush objects.
• Measuring Air Pressure
• A barometer is an instrument that measures changes in air pressure.
• There are 2 types of barometers.
• Mercury barometer:  Consists of a glass tube open at the bottom end and partially filled with mercury.
• Aneroid (without liquid) Barometer: Contains an airtight metal chamber that is sensitive to changes in air pressure. The thin walls of the chamber flex in and out as air pressure changes, and the movements are recorded on the dial.
• Aneroid means “without liquid”
• Units of Pressure
• In weather reports, air pressure usually is given in inches of mercury.
• NWS maps indicate air pressure in millibars.
• One inch = 33.87 millibars
• 30 inches =1016 mb
• Altitude / Elevation
• Altitude, or elevation, is the distance above sea level.
• Air pressure varies with altitude.
• Air pressure decreases as altitude increases.
• Air pressure increases as altitude decreases.
• Sea-level air has the weight of the whole atmosphere pressing on it, so air pressure is highest at sea level.
• Air density is low at tops of mountains, therefore there is less oxygen in each cubic meter of air.