Air Pressure

Air Pressure

Air Pressure

 Air is matter and is made of gasses and therefore are made up of matter.

Multiple Choice

Which gas is the most abundant in the atmospehre?

• Oxygen

• Nitrogen

• Carbon Dioxide

• Methane

Multiple Choice

What gas levels are rising due to excess use of fossil fuels

• Oxygen

• Nitrogen

• Carbon dioxide

• Sulfur dioxide

Air Pressure

 Because air has mass it pushes down on objects in all directions.

 This is called air pressure.

Why doesn't air pressure crush us?

 Because we have

air inside pushing
out.

 And this balances

the forces so we

do not feel it.

Air Pressure

 We measure air Pressure with a

barometer.

 There are two basic types of barometers
- Mercury
- Aneroid

Mercury Barometer

 This is a tube of mercury with the

open end in a dish of mercury.

 The higher the air pressure the

further the mercury rises.

Multiple Choice

What feature of Mercury makes it a good choice for barometers?

• Has a high density

• Does not leave a meniscus

• Has a high adhesion so you can see where the mark is.

• Is liquid at room temperature

 Water can also

be used to
show changes
in air
pressure.

Aneroid Barometer

 This works without liquid.
 When the air pressure increases it

squeezes the can causing needle to
move.

Multiple Choice

What type of barometer is the one you made?

• Mercury

• Aneroid

• Solenoid

Measuring Air Pressure

 Air pressure is measured by the

height of the column of mercury.

- inches of mercury or millimeters of
mercury (mmHg)

Other units for pressure that are commonly used are psi (pounds per square inch) ATM (atmospheres) or torr.

Measuring Air Pressure

 A more common measurement is

based on the average barometric
pressure.

 This unit is called a millibar and is used by the National Weather Service.

Each line represents where the pressures are the SAME!

Factors Effecting Air pressure

 Temperature:
 As air heats it expands, increases in

volume, becomes less dense and
exerts less pressure.

 As air cools it contracts, decreases in

volume, becomes more dense and
exerts more pressure.

Factors Effecting Air pressure

 WARM AIR ---- LOW PRESSURE

 COLD AIR ---- HIGH PRESSURE

Factors Effecting Air pressure

 Increasing Altitude
 As you go higher in the atmosphere

there is less air pushing down from
above.

 This causes the air pressure to

decrease with an increase in altitude.

 ESRT shows this in a graph.

 The greatest air pressure is at the

surface of earth

 As you go up in the atmosphere

pressure becomes less and less

Factors Effecting
Air pressure

Humidity and Air Pressure

 Air containing a lot of water vapor

(high relative humidity) has a lower
pressure then dry air.

Humidity and Air Pressure

 The more water vapor in air causes

the air to be expanded and take up
more volume.

 This results in air that is less dense

and exerts less pressure.

 Dry air would be more dense and

exert more pressure.

Common Trends

 Low Pressure usually happens when:
-the weather is changing (windy)
-it is raining
-it is cloudy
 High Pressure usually happens when:
-the weather is stable (not changing)
-it is clear and sunny
-it is not raining

Directions of Movement….

 Low pressure systems spin counter

clockwise, the air moves towards the
center, and the air rises.

Directions of Movement…

 High pressure systems spin clockwise,

the air sink to the bottom, and moves
away from the center of the High.

Acting Together they look like
this……