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Copyright © 2013 Pearson Education, Inc.
The Atmosphere:
An Introduction to
Meteorology, 12th
Lutgens • Tarbuck
Lectures by:Heather Gallacher,Cleveland State University
Chapter 6: Air Pressure and Winds
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Copyright © 2013 Pearson Education, Inc.
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:
1. pressure gradient force,
2. the Coriolis force,
3. and friction.
It Is All About the Wind
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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
applied by this air on
the unit area would be
14.7 pounds per square inch.
Air Pressure is the Weight of the Air
Meteorologists use
a metric unit for pressure
called a millibar and
the average pressure at sea level is
1013.25 millibars.
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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.
Concept Check 6.2
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Measuring Air Pressure
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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.
Concept Check 6.2
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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.
What is standard sea-level pressure, in pounds per square inch?
Standard sea level pressure is 14.7 pounds per square inch.
Concept Check 6.2
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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.
Concept Check 6.1
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Air Pressure Round the Planet Varies All the Time
Why?
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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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At sea level a column of air weighs 14.7 pounds persquare 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.
14.7 lbs/in2
7.35 lbs/in2
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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)
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
13 16
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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.
13 16
HL
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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.
Cold, dry air is more dense and therefore heavier than warm, humid air. Because it is heavier, the cold, dry air will tend to sink, creating a high-pressure center.
Because it is lighter, warm, humid air will tend to rise and leave a low-pressure center.
2 If all other factors are equal, does a dry or moist air mass exert more air pressure? Explain.
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 massand its overall density.
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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.
Summary
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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.
As it converges horizontally, the air is squeezed into a smaller space, which results in a more massive air column that exerts more pressure at the surface.
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.
HL
Horizontal Air Movement Changes Surface Pressure
convergencedivergence
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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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Factors Affecting Wind
The horizontal pressure gradient is the driving force of wind.
The magnitude of the pressure gradient force is shown by the spacingof isobars.
The direction of force is always from areas of higher pressure toward areas of lower pressure and
at right angles to the isobars.
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Concept Check 6.5
2 What force is responsible for generating wind?
Winds are only generated by horizontal pressure differences, that is, by the pressure gradient force.
3 Write a generalization relating the spacing of isobars to wind speed.
Closely spaced isobars indicate a faster wind, while widely spaced isobars indicate a slower wind.
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Factors Affecting Wind
But the wind does not blow directly from high to low pressure.
Why
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Factors Affecting Wind
Pressure gradient force
Coriolis force
Friction
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Air moves out of the regions of higher pressure and into the regions of lower pressure.
However, the wind rarely crosses the isobars at right angles, as the pressure gradient force directs.
This deviation is the result of Earth’s rotation and
has been named the Coriolis force, after the French scientist Gaspard-Gustave Coriolis, who first expressed its existence.
It is important to note that the Coriolis force
cannot generate wind;
rather, it modifies airflow.
Coriolis force
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The Coriolis force acts
to change
the direction of a moving body
to the right in
the Northern Hemisphere
and to the left
in the Southern Hemisphere.
Coriolis force
http://www.youtube.com/watch?v=i2mec3vgeaI
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This deflecting force
is always directed
at right angles to the direction of airflow.
Affects only wind direction, not wind speed.
Is affected by wind speed the stronger the wind,
the greater the deflecting force.
Is strongest at the poles and weakens equatorward,
becoming nonexistent at the equator.
Coriolis force
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Concept Check 6.5
5 Which two factors influence the magnitude of theCoriolis force?
The two factors that influence the magnitude of the Coriolis force are
wind speed (the stronger the wind, the greater the deflecting force)
and latitude (strongest deflection at the poles
and zero at the equator).
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Factors Affecting Wind
Pressure gradient force
Coriolis force
Friction
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Friction significantly
influences airflow
near Earth’s surface.
Its effect is negligible
above a height of a
few kilometers.
Friction Effect on Wind
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The only force acting on a stationary parcel of air is the pressure gradient force.
Once the air begins to accelerate, the Coriolis force deflects it to the right in the Northern Hemisphere.
Greater wind speeds result in a stronger Coriolis force (deflection) until the flow is parallel to the isobars.
At this point the pressure gradient force and Coriolis force are in balance,
and the flow is called a geostrophic wind
Geostrophic Wind
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Winds AloftCurved airflow and the gradient wind
Low-pressure systems (L)
that occur in the middle latitudes
are called midlatitude cyclones.
High-pressure systems (H)
are called anticyclones,
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Winds Aloft
Curved airflow and the gradient wind
Wind flow aloft can create ridges (high) and troughs of (low)pressure
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Concept Check 6.6
3 Describe the direction of cyclonic and anticyclonic flow in both the Northern and Southern Hemispheres. Draw diagrams to answer the question.
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Surface Winds
Surface winds travel at an angle across isobars, toward low pressure.
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How Winds Generate Vertical Air Motion
Vertical airflow is associated with cyclones and anticyclones.
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How Winds Generate Vertical Air Motion
Factors that promote vertical airflow:
Friction
Increased friction causes a drop in wind speed
resulting in a pileup of air upstream from the ocean to land.
Convergence can result in cloudy weather.
Decreased friction causes an increase in wind speed
from the land to the ocean.
Subsidence and divergence results in clearing weather.
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How Winds Generate Vertical Air Motion
Vertical airflow is associated with cyclones and anticyclones.
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Concept Check 6.8
2 What general weather conditions are to be expected when the pressure tendency is rising?
When the pressure tendency is rising, it means that a high-pressure center is approaching. Because highs are associated with descending air, adiabatic warming precludes cloud formation. Thus, clear, fair weather is foretold by a rising barometer.
When the pressure tendency is falling?
Conversely, when the pressure tendency is falling, a low-pressure center is approaching. Because lows are associated with rising air, cloud formation is likely and precipitation is possible. Thus, cloudy and possibly rainy weather is foretold by a falling barometer.
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Wind Measurement
Wind direction is always measured
from the direction it is coming.
A North wind blows north to south.
A prevailing wind consistently blows
more often than not
from one direction.
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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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