Download HIGH PRESSURE

Chapter 6
Air Pressure and Winds
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It Is All About the Wind
Unequal heating of the Earth’s surface
creates unequal air pressure.
These pressure differences create wind,
but the direction and strength of the wind
depends upon three important factors:
- pressure gradient force
- the Coriolis force
- friction.
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Air Pressure is the Weight of the Air
Consider a "unit area"
of 1 square inch.
At sea level, the weight
of the air
above this unit area is
(on average) 14.7 pounds!
That means pressure
Meteorologists use
applied by this air on
a metric unit for pressure
the unit area would be
called a millibar.
14.7 pounds per square inch.
The average pressure at sea level is
1013.25 millibars.
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Concept Check 6.2
2. Describe the operating principles of
the mercury barometer.
A glass tube filled with mercury is
inverted into a dish of mercury.
The mercury flows out of the tube
until the pressure
at the base of the mercury column
is balanced by the pressure exerted
on the surface of the mercury by
the air above.
When air pressure rises, the height
of the column increases
and when air pressure drops, so does
the height of the column.
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Measuring Air Pressure
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Concept Check 6.2
1. What is average sea-level pressure when measured in
millibars?
Average sea level pressure is measured as 1013.25 millibars.
In inches of mercury?
29.92 inches, of mercury.
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Concept Check 6.1
1 What is wind and what is its basic cause?
Wind is the horizontal movement of air and is the result of
horizontal differences in atmospheric pressure.
2 What is standard sea-level pressure, in pounds per
square inch?
Standard sea level pressure is 14.7 pounds per square inch.
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Concept Check 6.2
2. Describe the operating principles of
the aneroid barometer.
An aneroid (without liquid) barometer
consists of evacuated metal chambers
that compress as air pressure increases,
and expand when pressure decreases.
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At sea level a column of air
weighs 14.7 pounds per
square inch and therefore
exerts that amount of
pressure.
As we ascend through the
atmosphere, we find that
the air becomes less dense
because of the continual
decrease in the amount
(weight) of air above.
Therefore there is a
corresponding decrease in
pressure with an
increase in altitude.
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7.35 lbs/in2
14.7 lbs/in2
Concept Check 6.3
1 Explain why air pressure decreases with an increase
in altitude.
Air pressure is the pressure exerted by the weight of air above.
Air pressure decreases with an increase in altitude
because there is less air above
to exert a downward force
as one moves away from Earth’s surface.
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Air Pressure Round the Planet Varies All the Time
Why?
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Air Pressure Round the Planet Varies All the Time
Why does atmospheric pressure vary daily,
and why is that important?
Recall that variations in air pressure
cause the wind to blow,
which in turn causes changes in temperature and humidity.
In short, difference in air pressure
create global winds that
become organized into the systems
that bring us our weather.
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Why Does Air Pressure Vary?
Temperature is a measure of
the average molecular motion
(kinetic energy) of a substance.
Temperature influences air pressure.
Air pressure drops more rapidly
with altitude in a column of cold (dense) air
than in a column of warm (less dense) air.
We assume that both columns of air exert
the same surface pressure,
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Why Does Air Pressure Vary?
We assume that both columns of air exert
the same surface pressure, and
(although greatly exaggerated)
13
differences in the spacing of air molecules
represent differences in density.
Looking at the line drawing halfway up
notice that there are
more air molecules above this altitude
in the warm column than in the cold column
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16
Why Does Air Pressure Vary?
At the red line
16 molecules weigh more
then 13 molecules
so the pressure at the altitude
of the red line is greater
in the warm air.
This creates a “HIGH PRESSURE” aloft
While in the cold air we have a “LOW PRESSURE” aloft.
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13
16
L
H
Horizontal Air Movement Changes Surface Pressure
The movement of air can also cause
variations in air pressure.
Where there is a net flow of air into a
region, a phenomenon called
convergence, air accumulates.
L
As it converges horizontally, the air is
squeezed
divergence
into a smaller space, which results in
a more massive air column that
exerts more pressure at the surface.
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H
convergence
Horizontal Air Movement Changes Surface Pressure
By contrast, in regions where there is
a net outflow of air,
a situation referred to as divergence,
the surface pressure drops.
In summary, the pressure at the
surface
will increase when there is a net
convergence in a region and
the surface pressure will
decrease when there is
a net divergence.
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L
divergence
H
convergence
Factors Affecting Wind
Pressure gradient force
Coriolis force
Friction
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Factors Affecting Wind
Pressure gradient force:
Isobars are lines connecting places of equal pressure.
If isobars are close together a steep gradient and high winds result.
If isobars are far apart, the result is a low gradient and lower wind
speeds.
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Factors Affecting Wind
Find a station with high winds. What is the spacing of the isobars?
Find a station with low winds. What is the spacing of the isobars?
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Concept Check 6.4
*
1. Explain why a cold, dry air mass produces a higher
surface pressure than a warm, humid air mass.
Air pressure is the pressure exerted by
the weight of air above.
Air pressure decreases with an increase in altitude
because there is less air above
to exert a downward force
as one moves away
from Earth’s surface.
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Concept Check 6.4 *
2. If all other factors are equal, does a dry or moist air mass exert more air
pressure?
A dry air mass exerts more pressure
because the molecular weight of water vapor
is less than the molecular weight of nitrogen and oxygen.
In a moist air mass,
the water vapor molecules will
displace some of the nitrogen and oxygen molecules,
lowering the molecular weight
of the air mass
Moist air
is less dense
and its overall density.
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than dry air
Water vapor affects air pressure.
The amount of water vapor reduces
the density of air.
Cold dry air has higher pressure.
Warm, dry air has higher pressure
than equally warm, moist air.
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Factors Affecting Wind
How temperature differences generate wind
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Factors Affecting Wind
Pressure gradient force
Coriolis force
Friction
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Coriolis force
The Coriolis force cannot generate wind;
it modifies airflow due to the Earth’s rotation.
In the Northern Hemisphere,
a deviation to the right
In the Southern Hemisphere,
the deviation is to the left.
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Factors Affecting Wind
The Coriolis force is a deflecting force.
directed at right angles to the direction
of airflow.
controls only wind direction, not speed.
the stronger the wind, the greater
the force.
strongest at the poles and
nonexistent at the equator.
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Coriolis force
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Coriolis force
The earth rotates away from underneath the original path.
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Coriolis force
Creates permanent wind patterns around the planet
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Factors Affecting Wind
Pressure gradient force
Coriolis force
Friction
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Wind with friction from the ground
Friction significantly influences
airflow near Earth’s surface,
but its effect is negligible
at higher altitudes.
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Wind with friction from the ground
No friction
winds blow
perpendicular
from high to low pressure
With friction from the ground
winds blow across the
isobars at an angle
high to low pressure
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Winds aloft
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Winds Aloft
Geostrophic flow
Geostrophic flow occurs when
The wind flows parallel
the pressure gradient force
to the isobars
and the Coriolis force are balanced.
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in relatively straight paths.
Buys Ballot’s Law
Buys Ballot’s Law states that if you stand with the wind at your back,
low pressure will be found on your left,
high pressure on the right.
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Winds aloft
Pressure isobars are never parallel.
Seldom have “true” geostrophic winds.
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Winds Aloft
Gradient Winds
Winds around cells of high pressure or low pressure
follow curved paths.
blow at a constant speed.
parallel to the curved isobars.
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Winds Aloft
Gradient Winds
A trough is the result of isobars curving to form elongated regions of low pressure
A ridge is the result of isobars curving to form elongated regions of high pressure.
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Winds Aloft
pressure centers
Centers of low pressure
are called cyclonic;
winds flow counter-clockwise
in the Northern Hemisphere.
Associated with stormy weather
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Winds Aloft
pressure centers
Centers of high pressure
are called anticyclonic;
winds flow clockwise
in the Northern Hemisphere.
Associated with clear weather.
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Surface winds
pressure centers
Surface winds travel
at an angle across isobars,
toward low pressure
because of friction with the ground
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How Winds Generate Vertical Air Motion
Vertical airflow is associated with cyclones and anticyclones.
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Wind Measurement
Wind direction is always measured
from the direction it is coming.
A North wind blows north to south.
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Wind Measurement
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Wind Measurement
A wind vane shows wind direction.
Wind speed is often measured with a
cup anemometer, which has a dial
that shows wind speed.
An aerovane is a combination of wind vane
and anemometer.
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End Chapter 6
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