Selected Answers for Quiz #4 Review Questions
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atmospheric pressure
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How does atmospheric pressure change with altitude?
A: Atmospheric pressure decreases rapidly with altitude at first
and more slowly with altitude.
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What is the difference between station pressure and sea level pressure?
A: Station pressure is the actual measured pressure by a barometer
and sea level pressure is the pressure corrected for the altitude above
sea level of the location of the measurement.
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What is a typical station pressure for Tucson?
A: A typical station pressure for Tucson is 925 mb.
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Typically, how much does this vary over the course of a day.
A: The atmospheric pressure varies by about 5-10 mb over the
course of a day. See http://www.atmo.arizona.edu/wxgraph.html
for an example.
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What is an isobar?
A: An isobar is a line along which the pressure is equal to
the same value. It is used to separate regions of lower and higher pressure.
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Ideal gas law
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If the pressure of the gas in a balloon is held constant, then how will
the density change with the temperature of the gas?
A: If the pressure of the gas is held constant, then the density
is inversely related to the temperature, i.e. if the temperature increases,
then the density decreases.
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Pressure gradient
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If the distance from Denver to Minneapolis is about 1000 km, using the
sea level pressure map http://www.atmo.arizona.edu/spring00/ATMO171-flittner/130999_sfcP.gif
what is the horizontal pressure gradient between Denver and Minneapolis?
A: I estimate the pressure at Minneapolis to be about 1012 mb
and the pressure at Denver to be ~1025mb or even 1026mb. So, the
Pressure gradient = (1026mb - 1012 mb)/1000 km = 0.014 mb/km
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Surface pressure maps and pressure surface maps
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What is an isobar?
A: An isobar is a line of constant pressure, such that points
all along the line have the same value of pressure.
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Look at the 500 mb map at http://www.atmo.arizona.edu/spring00/ATMO171-flittner/130999_500H.gif
(which is the same as the hand-out) and tell me what is the 500 mb height
over Tucson (you'll have to estimate it). Explain what this number
means (This is the more important part of the question).
A: The 5880 m contour that is in the northern part of Arizona
separates lower heights to the north of this line from higher heights to
the south of this line. There is also a 5880 m contour that is in Mexico
separates lower heights to the south of this line from higher height to
the north of this line. Since Tucson is surrounded by the 5880 m contour,
the height of the 500 mb surface over Tucson is greater than 5880 m, but
less than the next contour level (5880m+60m=5940m). I'd estimate 5890 m,
but any number greater than 5880m and less than 5940m would be acceptable.
This is the height above sea level at which you'd have to be above Tucson
so that a barometer would measure a pressure of 500 mb.
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What is the pressure at a point on the 500 mb map directly over Chicago?
A: Everywhere on this map the pressure is equal to 500 mb. What
does change is the altitude above sea level of the level in the atmosphere
where the pressure is 500 mb.
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General characteristics of High and Low pressure regions
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If the forecast for the next several days is for Tucson to be under the
influence of high pressure, what sort of weather would you expect?
A: Generally, regions of high pressure are associated with clear
skies and fair weather. This is because the air in a region of high pressure
is slowly sinking or descending. This will inhibit the formation of clouds
(which we'll learn more about later).
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If the air over the state of Oklahoma is gradually rising, would you expect
to find high pressure or low pressure at the surface?
A: Again, the general characteristics of a region of low pressure
is that the air is gradually rising and clouds could form.
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Forces that affect the movement of air
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Generally, the vertical pressure gradient is balanced by what force?
When these 2 forces are equal in size, but in opposite directions, meteorologist
call it _____ equilibrium.
A: the downward directed force of gravity; hydrostatic
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On the 500 mb map mentioned above, identify regions where the pressure
gradient force (PGF) is larger and where it is smaller. Pick a few
points on the map, say in midwest, in northern Georgia on the 5880 contour,
and south of Florida on the 5880 contour, and draw an arrow in the direction
of the PGF.
A: The size or magnitude of the PGF on any upper level map is
greater where the height contour lines are closer together. Likewise, the
size of the PGF would be smaller on the same map where the height contour
lines are further apart. For a point in northern Georgia on the 5880 m
contour, the PGF would be pointing from higher heights to lower heights,
90 degrees to the contour line toward the north, north-west. For the point
on the 5880 m contour south of Florida, the PGF would be pointing toward
the south-east.
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Know the characteristics of the Coriolis Force (See the outline
on forces )
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How does the frictional force change with altitude? Then how should the
wind change with altitude? Would you expect there to be shear?
A: The frictional force decreases with altitude. Thus the wind
speed should increase with altitude since the frictional force is in a
direction directly opposite the wind and acts to slow-down or retard the
movement of the air. Wind shear is either a change in the wind speed with
altitude or a change in the wind direction with altitude. In this case
since the wind speed should increase with altitude, there would be wind
shear.
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Winds on an upper level map and a sea level pressure map (Some of these
are the same question, just different was of asking the same question.)
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For the points used in question #2 of the Forces section above, draw the
direction of the wind, and the direction of the Coriolis force.
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On our well used 500 mb map, identify regions where the winds would be
greatest and where they would be the least.
A: The speed of the wind on the 500 mb map is dictated only
by the size of the PGF. The closer the height contours, the stronger the
PGF and thus the greater the wind speed. Likewise, the lowest winds are
in regions where the PGF is the smallest (the contours are furthest apart).
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Again, using the 500 mb map, are there any regions that where the PGF would
be equal to the CF? For a hint, remember what was said about geostrophic
balance. This will clue you into looking for regions where the contours
are straight (no curvature).
A: As the hint says, geostrophic flow or balance occurs where
the contour lines are straight. I'd say the region in upper Alberta, Canada
and in the upper right hand portion of the map would most resemble with
case of straight contour lines.
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Use the 500 mb map one last time to now estimate what would be the direction
of the wind at the surface at the points used in #2 of the Forces section
and #1 of this section.
A: The frictional force reduces the wind speed, which in turn
reduces the Coriolis force (CF). The CF alone nows doesn't balance the
PGF, and the wind blows across the isobars from higher pressure toward
lower pressure. In this case the PGF is balanced by both the frictional
force and the CF. A typical relation between the upper level wind direction
and the surface wind direction is that in the N.H. the surface wind blows
to the left of the upper level wind by about 30 degrees, from higher pressure
to lower pressure. Figure 6.18 of the text has an example of this.
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To really make sure you have an understanding of the relation between forces
and winds do the following for upper level winds;
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Draw a closed low pressure (concentric circles with the lowest pressure
at the middle) and draw the direction of the wind, PGF, & CF at two
points (i.e. the top, left side of the circle) in the Northern Hemisphere.
A: See figure 6.17 for an example.
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Now do the same for a high pressure in the Northern Hemisphere.
A: See figure 6.17 for an example.
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Now the same for a low pressure in the Southern Hemisphere. Remember
the thought experiment we did in class with the balloon initially at rest
and then releasing it to move under the influence of the PGF & CF.
Answer: The PGF initial moves the balloon in toward the center
of the low, and as the balloon starts to move the CF causes it to be deflected
to the "left" (Southern Hemisphere). The final result is that the
winds blow clockwise around a low in the Southern Hemisphere.
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Relationship between observed wind and atmospheric pressure
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The Dutchman, Buys-Ballot, noted the following relationship in the Northern
Hemisphere between the direction of the wind and the atmospheric pressure:
When the wind is at your back, then lower pressure is ___ of you.
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to the left
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to the right
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directly behind
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directly in front
A: to the left
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For tonight the forecast is for a low pressure region to move to the south
of Tucson, at which time you would expect the wind to be from the ___.
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south
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north
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east
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west
A: east
