Weather and Climate Chapter 1: The Atmosphere

1
Weather and ClimateChapter 1 The Atmosphere

• Section 3
• Air Pressure

2
Objectives

• After completing the lesson, students will be
  able to . . .
• Identify some of the properties of air
• Name instruments that are used to measure air
  pressure
• Explain how increasing altitude affects air
  pressure and density.

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Properties of Air

• Even though it appears that air has no mass, air
  consists of atoms and molecules that, in fact,
  have mass.
• Because air has mass, it also has other
  properties, including density and pressure.

4
Properties of Air Density

• DensityThe amount of mass in a given volume of
  air.
• Density equals mass divided by volume.
• If there are more molecules in a given volume of
  air, the density is greater.
• If there are fewer molecules, the density
  decreases.

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Properties of Air Pressure

• PressureThe force pushing on an area or surface.
• Air PressureA force that is the result of the
  weight of a column of air pushing down on an
  area.
• Air is pressing from all directions

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Measuring Air Pressure

• The changes in air pressure affect the weather.
• Falling air pressure indicates a storm is coming,
  while rising air pressure usually means the
  weather is clearing.
• BarometerAn instrument that is used to measure
  changes in air pressure.
• Mercury Barometer
• Aneroid Barometer

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Mercury Barometer

• Mercury BarometerAn instrument that measures
  changes in air pressure, consisting of a glass
  tube partially filled with mercury , with its
  open end resting in a dish of mercury.

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Mercury Barometer

• When the air pressure increases, it presses down
  more on the surface of the mercury.
• Greater air pressure forces the column of mercury
  higher, and when air pressure decreases, the
  column will drop.

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Aneroid Barometer

• Aneroid BarometerAn instrument that measures
  changes in air pressure without using liquid.
• The word aneroid means without liquid.

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Aneroid Barometer

• An aneroid barometer has an airtight metal
  chamber that is sensitive to changes in air
  pressure.
• When air pressure increases, the walls push in,
  and when air pressure decreases, the walls bulge
  out.

• The chamber is connected to a series of springs
  and levers, and as the chamber changes shape, the
  needle moves.
• Aneroid barometers are smaller than mercury
  barometers and do not contain liquid, which
  allows them to be portable and used more
  practically.

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Units of Air Pressure

• Read this section on page 27.

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Increasing Altitude

• AltitudeElevation above sea level.
• Air pressure decreases as altitude increases.
• As air pressure decreases, so does density.

13
Altitude Affects Air Pressure

• Read this section on page 28-29

14
Altitude Affects Density

• Read this section on page 30.