https://vimeo.com/50050728
Bill Nye on Atmosphere
Air Pressure
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Air pressure
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What is air
pressure?
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Air Pressure is
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Differences in air pressure cause
Earth’s winds and weather to
change.
Air has pressure b/c air has
mass.
Is defined as force per unit area.
The less mass a volume of air
has the less dense the parcel.
The weight of the atmosphere per
unit area. At the surface , 1 kg
per square centimeter is the
average air pressure.
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Measuring
Air pressure
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Mercury
Barometer
At sea level
the height of
the column
normally is
76 cm.
The instrument used to measure
air pressure is the BAROMETER
There are two types of
barometers—mercury and
aneroid.
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The Aneroid
Barometer
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Barograph
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Uses a thin metal can and by
pumping most of the air out of the
can, the can is made sensitive to air
pressure,
A spring or similar device keeps the
can from collapsing inward.
As the shape of the can changes
with the changes in air pressure, a
pointer attached to the can moves
over a scale.
Is a recording aneroid barometer.
Its pointer is a pen that writes on a
chart.
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Altimeter
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Since air pressure is the weight
of the air overhead, what
happens to air pressure as you
go up in altitude??
The air pressure drops with an
increase in altitude.
Is a aneroid barometer that
reads height above sea level.
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Air Pressure
Units
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Air pressure is reported in two
different ways. The first way
gives the height of the mercury
column in the barometer. It
may be given in centimeters or
in inches.
The second way uses a metric
unit called a millibar .
1000 millibar of air pressure is
roughly equivalent to the
average air pressure at sea
level.
F. Understanding Pressure Maps
1.
Isobars- areas with equal air pressure
Sea- level = 1013.3 mb
Denver, CO = 835 mb
Meridian, ID = 975 mb
2. Isobar map rules:
a. If # is 499 or less, place 10 in front
b. if # is 500 or more, place 9 in front
c. decimal is moved before last digit.
+ Isobars always make a circle!
Ex. 950 = 995.0
ISOLINE ACTIVITY:
1. In PEN, uncode each pressure: 103= 1010.3
2. In PENCIL, label the highest pressure H and the lowest
pressure L.
3. For the H draw the following isobars: 1016, 1012, 1008
mb (each in a different color)
4. For the L draw the following isobars:
mb (each in a different color)
996, 1000, 1004
5. For each wind indicator- show wind movement.
1016 mb
H
L
1016 mb
H
1012 mb
L
1016 mb
H
1012 mb
1008 mb
L
1016 mb
H
1012 mb
1008 mb
L
996 mb
1016 mb
H
1012 mb
1008 mb
L
996 mb
1000 mb
1016 mb
H
L
1012 mb
996 mb
1000 mb
1008 mb
1004 mb
1016 mb
H
L
996 mb
1012 mb
1000 mb
1008 mb
1004 mb
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Why does air
pressure change?
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Changing Temps.
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Warm air is lighter than cold air because
the molecules are farther apart.
When cold air replaces a warm air mass
then the air pressure at the ground goes
up.
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Changing humidity.
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The more water vapor the air contains the
lighter it is.
How?
When water vapor enters the atmosphere it
pushes out and equal volume of dry air.
Dry air has more nitrogen and oxygen and
both of these elements in their gaseous
state weigh more than does water vapor.
In General scientist have found that a
falling barometer means warmer
weather, rain or snow.
Changing Pressure
A rising barometer = increasing air pressure.
This usually means: Clear Cool Dry.
20
Changing Pressure
A falling barometer = decreasing air pressure.
This usually means: warm, wet, cloudy
Tut tut it
looks like
rain
21
Air is forever trying to find…
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Equilibrium
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Air travels from high to low
pressure.
This affects wind direction and
speed.
Low & High Comparison Table
Atmospheric
condition
Pressure
Temp.
Humidity
Gasses
Result
Low
warmer
Greater
Water vapor
(low mass)
Cloudy &
precipitation,
unsettled
High
Cooler
Lesser
Oxygen &
nitrogen
(high mass)
Clear & stable
Follow up questions:
1. Describe the wind motion in the H.
2. Describe the wind motion in the L.
3. In what area is the wind speeds the greatest?
4. Winds go from ___ to ____ for pressure.
5. What type of weather is associated with high
pressure systems?
6. What type of weather is associated with low
pressure systems?
* Answer questions 17-23 pg 433 blu text.