Chapter 6_Air Pressure

7/29/2019 Chapter 6_Air Pressure

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AIR PRESSURE


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Keeping an Atmosphere

Atmosphere is keptby the worlds gravity

Low mass (small) worlds= low gravity

=almost no atm.

High mass (large) worlds = high gravity

= thick atm.

Gravity and pressure Air pressure depends on how much gas there

is i.e. The atmospheric thickness.


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Gravity and Atmospheric Pressure

The stronger the gravity, the more gas is held by the

world and the greater the weight of atm. on a point


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Earths Atmosphere

About 10 kmthick

Consists mostlyof molecularnitrogen (N2)

and oxygen (O2)


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The air is made up of molecules.


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Gravity pulls the air molecules

toward the earth, giving them

weight. The weight of the airmolecules all around us is

called the air pressure.


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High altitudes = lower pressure

Low altitudes = higher pressure


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Atmospheric Pressure

Gas pressuredepends on both

density and

temperature.

Adding airmolecules

increases the

pressure in a

balloon.

Heating the airalso increases

the pressure.


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Air pressure is

equal in all

directions.

Pressure = force per unit area


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As

elevation

goes up

Barometric

pressure

goes

down.

This is an inverse relationship.


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to

measure

airpressure.

A Barometer

is

used


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In 1643, Evangelista Torricelli

invented the barometer


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Torricellis barometer

used a glass column

suspended in a bowl of

mercury. The pressure

of the air molecules

pushed the mercury upinto the glass tube.

The weight of the mercury in

the tube was equal to the

weight of the air pressing

down on the mercury in the

dish.


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Asatmospheric

pressure

increases

The mercury in

the tube rises.


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The Mercury Barometer

Good: Bad:

Simple to construct

Highly accurate

Glass tube is fragile

Mercury is verytoxic!


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TheAneroidBarometer

No fragile tubes!

No toxic chemicals!

No batteries!Never needs winding!


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An aneroid barometer

uses a cell which has

had most of the airremoved.

As the air pressure

around the cell

increases, it presseson the cell, which

causes the needle to

move.

Television weather forecasters usually give barometric

pressure in inches of mercury. However, meteorologists

measure atmospheric pressure in millibars.

MILLIBARS


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Two types of barometric pressure measurements:

Station pressure is the actual

pressure at the recordinglocation. It is affected by the

local altitude.

Sea level pressure isreferenced to sea

level, so it has thesame altitude

anywhere in the

world.

Station pressure on a mountain top

will be lower than station pressure

in a valley. Scientists need a fixed

point of reference in order to

compare barometer readings in

different locations. That is why

barometer readings are sometimes

adjusted for elevation above sea

level at the station location.


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Most aneroid

barometers have a

needle which can be

set to remember the

previous reading.


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Changing Pressure

A rising barometer= increasing air pressure.

This usually means:

Rising barometer readings indicate that a

high pressure system is approaching.Higher atmospheric pressure is usually

associated with fair weather and clearing

skies.


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Changing Pressure

A falling barometer= decreasing air pressure.

This usually means:

Falling barometer readings usually

indicate the approach of an area of

low pressure. Low pressure readingsare usually associated with storm

systems. Tornadoes and hurricanes

can produce very low barometric

readings.


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Air Movement and Flow


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Fluids (air and water) flow from areas ofhigh pressure to areas of low pressure.

Change in pressure across a horizontaldistance is a pressure gradient.

Greater the difference in pressure and the shorter the

distance between them, the steeper the pressuregradient and the stronger the wind.

Movement of air across a pressure gradient

parallel to Earths surface is called a wind

and winds are named for the direction fromwhich they come.

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Isobars in millibars the closer the isobar the


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Isobars in millibars, the closer the isobar thestronger the winds

High Pressure

Low Pressure

Rain


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The Atmosphere in Motion Atmospheric pressure is a measure of the force

pressing down on the Earths surface from the

overlying air. Pressure is often measured in different units including:

atmospheres (1 atmosphere is the average atmosphericpressure at sea level),

millibars (1 atmosphere = 1013.25 millibars), pounds per square inch or psi (1 atmosphere = 14.7 poundsper square inch),

mm or inches of mercury (1 atmosphere = 760 mm or 29.92inches of mercury)

torrs (1 torr = the pressure exerted by 1 cm of mercury). Low air density results in rising air and low surface

pressure. High air density results in descending air and high

surface pressure.

C


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Heating and Cooling of

Air


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The Gas Law Ideal Gas follows kinetic molecular theory, made up of large

number of molecules that are in rapid random motionfollowing perfect elastic collitions losing no momentum How the Kinetic Molecular Theory Explains the Gas Laws

The pressure of a gas results from collisions between the gasparticles and the walls of the container.

Each time a gas particle hits the wall, it exerts a force on thewall.

An increase in the number of gas particles in the containerincreases the frequency of collisions with the walls and thereforethe pressure of the gas.

Avogadro's Hypothesis As the number of gas particles increases, the frequency of

collisions with the walls of the container must increase. This, in turn, leads to an increase in the pressure of the gas. Flexible containers, such as a balloon, will expand until the

pressure of the gas inside the balloon once again balances thepressure of the gas outside.

Thus, the volume of the gas is proportional to the number of gasparticles.


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The Gas Laws Charles Law

The volume of a gas increased with the temperature The volume of a given amount of dry ideal gas is directly

proportional to the Kelvin Temperature provided the amountof gas and the pressure remain fixed.

When we plot the Volume of a gas against the Kelvin

temperature it forms a straight line. V1 / T1 = V2 / T2

Boyles Law the product of the pressure and volume are observed to be

nearly constant.

The product of pressure and volume is exactly a constant foran ideal gas.

p * V = constant


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WATER VAPOR


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(9.8oC)

Chapter 6_Air Pressure

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