Air pressure

Keeping an Atmosphere
Atmosphere is kept by the world’s 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.
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
Earth’s Atmosphere
• About 10 km
• Consists mostly
of molecular
nitrogen (N2)
and oxygen (O2)
The air is made up of molecules.
Gravity pulls the air molecules
toward the earth, giving them
weight. The weight of the air
molecules all around us is
called the air pressure.
High altitudes = lower pressure
Low altitudes = higher pressure
Atmospheric Pressure
Gas pressure
depends on both
density and
Adding air
increases the
pressure in a
Heating the air
also increases
the pressure.
Air pressure is
equal in all
Pressure = force per unit area
goes up
This is an inverse relationship.
A Barometer
In 1643, Evangelista Torricelli
invented the barometer
Torricelli’s barometer
used a glass column
suspended in a bowl of
mercury. The pressure
of the air molecules
pushed the mercury up
into 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
The mercury in
the tube rises.
The Mercury Barometer
•Simple to construct
•Glass tube is fragile
•Highly accurate
•Mercury is very toxic!
The Aneroid Barometer
•No fragile tubes!
•No toxic chemicals!
•No batteries!
•Never needs winding!
An aneroid barometer
uses a cell which has
had most of the air
As the air pressure
around the cell
increases, it presses
on the cell, which
causes the needle to
Television weather forecasters usually give barometric
pressure in inches of mercury. However, meteorologists
measure atmospheric pressure in millibars.
Two types of barometric pressure measurements:
Station pressure is the actual
pressure at the recording
location. It is affected by the
local altitude.
Sea level pressure is
referenced to sea
level, so it has the
same altitude
anywhere in the
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.
Most aneroid
barometers have a
needle which can be
set to remember the
previous reading.
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
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 readings
are usually associated with storm
systems. Tornadoes and hurricanes
can produce very low barometric
Air Movement and Flow
6-1 flow from areas of
• Fluids (air and water)
high pressure to areas of low pressure.
• Change in pressure across a horizontal
distance is a pressure gradient.
• Greater the difference in pressure and the shorter the
distance between them, the steeper the pressure
gradient and the stronger the wind.
• Movement of air across a pressure gradient
parallel to Earth’s surface is called a wind
and winds are named for the direction from
which they come.
Isobars in millibars, the closer the isobar the
stronger the winds
Low Pressure
High Pressure
The Atmosphere in Motion
• Atmospheric pressure is a measure of the force
pressing down on the Earth’s surface from the
overlying air.
• Pressure is often measured in different units including:
– atmospheres (1 atmosphere is the average atmospheric
pressure at sea level),
– millibars (1 atmosphere = 1013.25 millibars),
– pounds per square inch or psi (1 atmosphere = 14.7 pounds
per square inch),
– mm or inches of mercury (1 atmosphere = 760 mm or 29.92
inches of mercury)
– torrs (1 torr = the pressure exerted by 1 cm of mercury).
• Low air density results in rising air and low surface
• High air density results in descending air and high
surface pressure.
Heating and Cooling of
The Gas Law
• Ideal Gas follows kinetic molecular theory, made up of large
number of molecules that are in rapid random motion
following 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 gas
particles and the walls of the container.
• Each time a gas particle hits the wall, it exerts a force on the
• An increase in the number of gas particles in the container
increases the frequency of collisions with the walls and therefore
the 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 the
pressure of the gas outside.
• Thus, the volume of the gas is proportional to the number of gas
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 amount
of 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
• Boyle’s Law
– the product of the pressure and volume are observed to be
nearly constant.
– The product of pressure and volume is exactly a constant for
an ideal gas.
– p * V = constant