#### Transcript Measuring Air Pressure 19.1 Understanding Air Pressure

```Chapter
19
Air Pressure and
Wind
19.1 Understanding Air Pressure
Air Pressure Defined
 Air pressure is the pressure exerted by the
weight of air.
 Air pressure is exerted in all directions—
down, up, and sideways. The air pressure
pushing down on an object exactly
balances the air pressure pushing up on
the object.
Atmospheric Pressure - force exerted by
atmospheric gas molecules on a given area
• When air masses move around the earth due to
differential heating, this value can change.
• Elevation also changes the value of atmospheric
pressure.
• Atmospheric pressure is often called barometric
pressure, as it is measured with a barometer.
Atmospheric pressure units
• Atmospheric pressure is often
measured in millibars (1000
mb = 1 bar).
• Standard pressure at sea level
is 1.0132 bar or 1013.2 mb,
often rounded to 1000 mb.
• American pilots and TV
weatherman express
atmospheric pressure in
“inches mercury” where 29.9
inches = standard pressure at
sea level.
• Atmospheric pressure is often
called barometric pressure, and
it is measured with a
barometer.
19.1 Understanding Air Pressure
Measuring Air Pressure
 A barometer is a device used for
measuring air pressure.
 When air pressure increases, the mercury
in the tube rises. When air pressure
decreases, so does the height of the
mercury column.
Measuring atmospheric pressure – the
barometer
• If you evacuate a tube
(i.e. remove all the air)
and put it in a dish of
liquid, the liquid will fill
the tube as the air
pressure pushes on the
liquid in the dish.
• If you tried this with a
dish of water, the water
would rise up to about 33
feet in the tube!
Measuring atmospheric pressure
• Using mercury, a very heavy
liquid, we find that at normal sealevel barometric pressure, the
liquid in the tube rises to a height
of 760 mm (or 29.92 inches).
• This apparatus is the original
form of the barometer, a device
used for measuring barometric
pressure.
• American TV weather reports are
usually given in terms of inches
of mercury
• Overseas reports use millibars.
Measuring barometric pressure –
the modern way
• Mercury barometers are
dangerous and difficult to
use.
• Modern aneroid
barometers use changes
within a partially
evacuated chamber to
move the pointer to the
correct value.
Boiling water at altitude
Why does water boil at a
lower temperature at a
higher elevation?
• Less air pressure allows
the water to change state
(from liquid to gas)
without being so
“energetic”.
• Since boiling water at
altitude is not as hot,
cooking times must be
altered.
A Mercury Barometer
19.1 Understanding Air Pressure
Factors Affecting Wind
 Wind is the result of horizontal differences
in air pressure. Air flows from areas of
higher pressure to areas of lower pressure.
 The unequal heating of Earth’s surface
generates pressure differences. Solar
radiation is the ultimate energy source for
most wind.
 Three factors combine to control wind:
pressure differences, the Coriolis effect,
and friction.
Pressure and wind
• Wind – movement of air from high to low
pressure areas.
• Wind is caused by pressure differences due to
unequal heating of Earth’s atmosphere
Cold air is more
dense than warm
and exerts more
Pressure than
warm air at a
given altitude
19.1 Understanding Air Pressure
Factors Affecting Wind
 Pressure Differences
• A pressure gradient is the amount of pressure
change occurring over a given distance.
• Closely spaced isobars—lines on a map that
connect places of equal air pressure—indicate a
steep pressure gradient and high winds. Widely
spaced isobars indicate a weak pressure
Isobars
Isobars-Lines of Equal Pressure
Drawn at the
earth’s surface
(there are upperlevel charts as
well)
Corrected to “sea
level” so only
effects of
weather shown,
not elevation
Units are millibars
(mb) where 1013
mb is standard
sea-level
pressure.
Note that lows and highs appear
the same; like a bullseye. Look at
numbers (and big H or L) to
determine which is which.
Isobars-Lines of Equal Pressure
Wind flags: Winds
blow away from
the flags and
towards the dots
Winds blow from
high pressure to
low pressure
Winds blow
towards the L,
away from the H
Although
temperature
infuences
pressure, no real
correlation
Note that lows and highs appear
the same; like a bullseye. Look at
numbers (and big H or L) to
determine which is which.
Sea and Land Breezes, a local phenomenon
19.1 Understanding Air Pressure
Factors Affecting Wind
 Coriolis Effect
• The Coriolis effect describes how Earth’s
rotation affects moving objects. In the Northern
Hemisphere, all free-moving objects or fluids,
including the wind, are deflected to the right of
their path of motion. In the Southern
Hemisphere, they are deflected to the left.
The Coriolis Effect
Global Wind Patterns and the Coriolis Effect
• Wind blows from high
to low pressure
• Wind blows straight in
one direction, but earth
turns underneath
• We experience the
wind as curving, not us
moving!
• This apparent “turning
force” is the Coriolis
Effect.
Global Wind Patterns and the Coriolis Effect
• If you stand behind the
wind and watch it move
away from you in the N.
Hemisphere, it appears
to curve to the right
• If you stand behind the
wind and watch it move
away from you in the S.
Hemisphere, it appears
to curve to the left.
19.1 Understanding Air Pressure
Factors Affecting Wind
 Friction
• Friction acts to slow air movement, which
changes wind direction.
• Jet streams are fast-moving rivers of air that
travel between 120 and 240 kilometers per hour
in a west-to-east direction.
Effect of Friction
Jet Stream
• Jet streams are not just
straight across, but have a
wavy pattern.
• The jets follow the contours of
low and high pressure areas
(troughs and ridges,
respectively), which move like
waves in the atmosphere
across the earth.
Jet Stream
• In winter, the polar jet dips
into the US, the troughs and
ridges affect the kind of
weather.
• If a trough is sitting over you,
it is generally very cold and
snowy or rainy.
• If a ridge is sitting over you, it
is generally warm and dry.
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