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
thick
• 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
temperature.
Adding air
molecules
increases the
pressure in a
balloon.
Heating the air
also increases
the pressure.
Air pressure is
equal in all
directions.
Pressure = force per unit area
As
elevation
goes up
Barometric
pressure
goes
down.
This is an inverse relationship.
A Barometer
is
used
to
measure
air
pressure.
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
dish.
As
atmospheric
pressure
increases…
The mercury in
the tube rises.
The Mercury Barometer
Good:
Bad:
•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!
MILLIBARS
An aneroid barometer
uses a cell which has
had most of the air
removed.
As the air pressure
around the cell
increases, it presses
on 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.
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
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.
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
skies.
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
readings.
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.
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
pressure.
• High air density results in descending air and high
surface pressure.
Isobars in millibars, the closer the isobar the
stronger the winds
Rain
Low Pressure
High Pressure
DO NOW: What’s in a letter?
Take a moment to think about what each
letter stands for when used on a weather
map
Compare/Contrast Chart
High and Low Pressure
High Pressure
Type of phenomenon
Determined by…
Moving inward on
isobars…
Density of air
Representation on a map
Motion of air
Also known as…
Motion of air causes a
zone of…
Stability of atmosphere
Low Pressure
Weather system
Changes in air pressure
Pressure Increases
Pressure Decreases
Higher
H (typically blue)
Clockwise, air sinks
Anticyclone
Divergence
Lower
L (typically red)
Counterclockwise, air
rises
Cyclone
Convergence
Stable
Unstable
High Pressure
LOW PRESSURE
Low vs. High
Heating and Cooling of
Air