Metr 201 (502) Test # 3 November 29, 2000
Form A
· Be sure to enter the test
form letter on your form.
· 40 questions, equally
weighted, 50 minutes
1) In
the figure below an offshore wind is the same as a:
a)
A
Northeast wind (NE)
b)
A
Southwest wind (SW)
c)
A
Southeast wind (SE)
d) A Northwest wind
(NW)
An offshore wind moves from land towards the
ocean. Therefore the wind is coming
from the northwest.
2) What causes “valley breezes” to form
during the day?
a) More heating of
the hillside than of the valley
b)
More
heating of the valley than of the hillside
c)
Subsidence
inversions
d)
Prevailing
westerlies
Valley breezes move from the valley towards,
and up the slopes of, the surrounding hills.
This type of circulation occurs when heating of the surface creates a
surface low, which draws air in. So,
more heating of the hillside than the valley will create a surface low that
will push wind in the expected direction.
3) In
the figure below, a rocky landscape is covered on the left side by snow (very
reflective), and is bare (not very reflective) on the right side. During the day, which direction would you
expect the surface wind to blow? (think
about what differential heating might occur)
What I tried to show here was a flat surface
as viewed from above. Each of the wind
directions were supposed to be parallel to the surface. Some students thought directions a and b
indicated rising or falling air. Since
the figure was confusing, any answer was accepted. The correct answer was c) since the thermal low created as the
rocky side absorbs light will draw air in.
4)
What
do the Santa Ana winds of California and the Chinook winds of Colorado have in
common?
a)
They
are both cold, dry winds that occur when air residing over a snow-covered
plateau sinks as it becomes more dense.
b)
They
are both northeast winds.
c)
They
are both associated with frontal boundaries.
d) They are both
warm, dry winds that result from compressional heating of the air as it moves
down the mountain.
Both Santa Ana and Chinook
winds are warm dry winds that move down the sides of mountains and heat as they
do so due to compressional heating. The
only difference between them is where they occur. These are two examples of this type of wind, but there are many
more in other regions of the world.
5)
If
the so-called single-cell model accurately described global circulation, which
of the following would be false?
a) Air would rise
near the regions where most deserts are found.
b) Air would rise near the
equator.
c) Surface winds in the
Northern hemisphere would be easterlies.
d)
Air would sink near the poles.
Hopefully you remembered that the single cell
model predicted winds that resemble a big sea breeze with rising air on the
warm side (equator) and sinking air on the cool side (poles). If you then drew the circulation that would
result, you would find that at the surface the winds would move from the pole
towards the equator. The Coriolis force
would deflect those winds to the right (west), making them easterlies. If you had forgotten all of that, you may
have realized that sinking and rising should only occur at the poles, and not
over the mid-latitudes where deserts are found.
6) Where would you expect to find a “haboob”?
a)
Louisiana
b) Arizona
c)
Hawaii
d)
Florida
You may have remembered that haboobs are
often found in Arizona. If not, you’d
have to realize that they are dust storms created by thunderstorms. Given that, it should make sense that they
would occur in arid regions where there is dust to lift into the air. All of these states except Arizona are not
exactly arid, so hopefully you could guess correctly.
7)
The
semipermanent highs around the globe (i.e., the Pacific high and the Bermuda
high) are found near the:
a)
Boundary
between the polar cell and the Ferrel cell
b)
Boundary
between the polar cell and Hadley cell
c) Boundary between
the Ferrel cell and Hadley cell
d)
Poles
Moving from the equator towards the poles, the order of the three cells
described in the 3-cell model is: Hadley =>
Ferrel => Polar. You could
probably guess where the polar cell is, but you had to remember the order of
the other two. Hopefully you realized
that the high pressure regions would be found where the air is sinking, which
is between the Hadley and Ferrel cells.
If not, you could take a pretty good guess if you remembered that the
semipermanent highs (i.e., the Pacific and Bermuda highs) are usually found
around latitude 30o, far from either the poles or the polar cell.
8)
Which
of the following would you not expect to find at the Intertropical
convergence zone (ITCZ)?
a)
Convergence
b) Sinking air
c)
Little
mixing across the ITCZ
d)
Frequent
precipitation.
The ITCZ is usually found around the equator, and represents the
boundary (little mixing) between the northeasterlies on its northern side and
the southeasterlies on its southern side.
When these two winds meet (convergence) they have to go somewhere, and
that somewhere is up (not sinking), leading to cloud formation and
precipitation.
9)
The
figure below shows the Northern hemisphere atmospheric circulation predicted by
the 3-cell model. Would the air at the
southern edge of the Ferrel cell (indicated by the arrow) be rising or sinking?
a) 
Sinking
b)
Rising
c)
Depends
on season
d)
Depends
on time of day
Just like the single cell model, air rises at
the equator and sinks at the poles in the 3-cell model. Since air rises at the equator, it has to
sink at the northern side of that (Hadley) cell. The air at the southern edge of the next cell to the north
(Ferrel) has to move in the same direction as the air at the northern edge of
the Hadley cell, so it must be sinking.
10)
The
3-cell model does a pretty good job of predicting atmospheric circulation,
although it’s not perfect. Which of the
following predictions of the 3-cell model is generally not accurate?
a)
Surface
easterlies in the lower latitudes (~ less than 30o).
b)
Surface
westerlies in the mid-latitudes.
c) Upper air
easterlies in the mid-latitudes.
d)
Upper
air convergence near latitude 30o.
For this question, you needed to remember some general facts about the
actual circulation around the world.
Living in the mid-latitudes, you probably realize that surface winds
come from the west. You might remember
that the tropical trade winds are easterlies, and from the previous question,
hopefully you know that convergence occurs in the upper atmosphere where the
Hadley and Ferrel cells meet (~30o). If you didn’t remember all of these things, hopefully you
remembered either from lecture or the text that upper air winds move from west
to east in the mid-latitudes (westerlies).
This is due to the temperature gradient between the equator and the
poles, and is the reason storm systems move towards the east in the U.S.
11)
In
which direction does air rotate around the Bermuda High?
a)
Counterclockwise
in the winter, clockwise in the summer.
b)
Clockwise
in the winter, counterclockwise in the summer.
c)
Always
counterclockwise.
d) Always clockwise.
In the Northern hemisphere (where the Bermuda high is) air always
rotates around high pressure systems in a clockwise direction.
12)
At
about what altitude would you expect to find the polar front jet stream and the
subtropical jetstream?
a)
About
100 m
b)
About
1 km
c) About 10 km
d)
About
100 km
Both the subtropical and polar front jet streams are called tropopause
jets because they form near the tropopause.
If you remembered this, you just needed to remember that the tropopause
(division between the troposphere and stratosphere) lies at about 12 km (i.e,
about 10 km).
13)
Which
of the following is not true regarding the low-level jet that travels
northward from Texas into the Plains states?
a)
It
is an important source of moisture for the Plains states in the summer.
b)
It
is occurs most frequently when a temperature inversion isolates the winds from
the surface.
c)
It
develops due to an east-west temperature gradient.
d) It always develops
directly underneath the polar front jet stream during winter.
For this, you just had to remember the factors that contributed to the
low-level jet, which did not include anything about the polar front jet stream.
14)
If
a bottle floating at the surface of the ocean in the Northern hemisphere is
moving towards the east, what is the average direction the upper 100 meters or
so of the ocean is moving?
a)
Towards
the east
b)
Towards
the west
c) Towards the
southeast
d)
Towards
the northwest
The very upper layer of the ocean moves at an angle of about 20-45o to
the right of the wind since the Coriolis force still acts on it, while a
pressure gradient force does not (that’s why the ocean circulations around the
semipermanent highs and lows were closed loops while the atmospheric ones were
not). Each layer of the ocean below the
surface is deflected even more to the right (remember the amazing CD stack
visualization) so that the average direction of the first 100 m or so is about 90o
to the right of the wind, or about 45-60o to the right of the surface water
(southeast when the surface water is moving east).
15)
Which
of the following is true regarding El Niño?
a)
Upwelling
near the coast of Peru intensifies.
b) Winds near the
equatorial Pacific switch from easterlies to westerlies.
c)
Nothing
changes in the U.S. because the effects of El Niño can’t cross the ITCZ.
d)
The
ocean temperature off the coast of Peru decreases.
El Niño describes a shift from prevailing easterlies to prevailing
westerlies over the equatorial Pacific.
This pushes water towards Peru (usually the easterlies pull it away),
which stops the upwelling that usually occurs, leading to warmer water
temperatures (upwelling brings cold water up).
As you hopefully know, El Niño and La Niña both have significant impacts
on weather across the U.S.
16)
Where
do most of the cT air masses that reach the U.S. originate?
a) Mexico
b)
Canada
c)
Over
the Labrador current
d)
The
Gulf of Mexico
Continental tropical air masses originate over land (i.e.,
continental), and should come from the south (i.e., tropical). The only choice that makes sense is Mexico.
17)
Where
do most of the cP air masses that reach the U.S. originate?
a)
Mexico
b) Canada
c)
Over
the Labrador current
d)
The
Gulf of Mexico
Continental polar air masses originate over land (i.e., continental),
and should come from the north (i.e., polar).
The only choice that makes sense is Canada.
18)
Air
mass source regions are least likely to be found in:
a)
High
latitudes
b) Middle latitudes
c)
Low
latitudes
d)
Polar
regions
In class and in the text it was explained that the middle latitudes
represent the region where different air masses collide, not where they form.
19)
Which
of the following differences between cold fronts and warm fronts is not true?
a)
Cold
fronts generally move faster than warm fronts.
b)
The
frontal surface (actual boundary) associated with a cold front is generally
steeper than that associated with a warm front.
c)
Cold
fronts often produce cumulus and cumulonimbus clouds, while warm fronts usually
produce stratus-type clouds.
d) In general, most
precipitation occurs in front of cold fronts, and behind warm fronts.
Usually, precipitation occurs over cold fronts, although sometimes it
also occurs in front of them. However,
the precipitation that is associated with warm fronts almost always occurs
ahead of the front itself.
20)
If
you were measuring atmospheric (barometric) pressure and you noticed that it
began to decrease, you might expect that:
a)
A
cold front was approaching.
b)
A
warm front was approaching.
c) Either a warm or a
cold front was approaching.
d)
Either
a warm or a cold front had passed through.
For both cold fronts and warm fronts, the minimum pressure exists at
the front itself. Therefore, decreasing
barometric pressure indicates an approaching front.
21)
Between
which of the following air mass pairs would you expect to find the most
precipitation associated with a cold front?
a)
cP,
mP
b)
cP,
cT
c)
mP,
cT
d) mP, mT
The biggest factor controlling the amount of precipitation associated
with a front is the amount of water contained in the warm air mass. After all, it is this air mass that is
lifted over the cold air, leading to clouds and precipitation. So, really what this question comes down to
is which of the warm air masses would you expect to contain the most water. Polar air masses are cold, so they don’t contain
much moisture, and continental tropical air masses are warm, but dry. So the air mass expected to hold the most
water is the mT.
22)
If
the stationary front shown below started rotating counterclockwise around point
A, how would it be shown on a weather map?
Cold air exists to the left of the stationary front, with warmer air on
the right. So when it starts rotating
counterclockwise cold air will advance (cold front) to the right along the
lower part of the front, while warm air will advance (warm front) to the left
along the upper part. This situation is
depicted in c).
23)
How
would the frontal surface shown below be depicted on a weather map?
The frontal surface shown is of an occluded front (a cold occluded
front to be exact), which is depicted in d).
24)
When
are middle latitude cyclones usually most intense?
a) At the time when
the cold front just catches up with the warm front.
b)
When
the entire front becomes occluded.
c)
At
its onset, when a frontal wave first forms.
d)
After
the fronts have dissipated and a low pressure region remains.
You may remember from class that the cyclone is most intense when the
fronts first merge to begin forming an occluded front. Little precipitation will be present as the
cyclone just forms, or after the fronts have dissipated. Some precipitation may still fall when the
entire front is occluded, but by then cold air is just pushing up other cold
air, so precipitation is light.
25)
In
the figure below, a surface low is located to the right of a surface high. Which of the upper-air plots below would
offer the upper-air support to sustain the surface low and high pressure areas?
The surface low will lead to convergence. If air is not continually pulled out of the low, the converging
air moving into it will “fill” it up.
Divergence of the air above the surface low would provide this
pull. The opposite case is true for the
surface high (needs upper air convergence).
In a) and b), air is converging above the surface low and diverging
above the surface high, while in d) air is neither converging nor diverging
above the surface low and high.
Therefore, b) is the only case where the upper air divergence is above
the low and upper air convergence is above the high, as is needed for support.
26)
We
know that a baroclinic atmosphere tends to sustain and intensify middle
latitude cyclones. What does baroclinic
actually mean?
a)
Upper-air
isobars exhibit a kink due to a shortwave.
b)
Upper-air
convergence is occurring.
c) Wind direction is
not parallel to constant temperature lines so temperature advection is
occurring.
d)
Isotherms
are parallel to isobars.
A baroclinic atmosphere is one in which winds move from regions of warm
temperature to regions of cold temperature, or vice-versa.
27)
Where
would you expect to find precipitation associated with a middle latitude
cyclone?
a)
In
front of the warm front.
b)
Over
the cold front.
c)
Over
the low pressure region at its center.
d) All of the above.
Precipitation is often associated with both of the fronts and the low
pressure itself (where convergence occurs).
28)
The
curved lines below represent a jet stream.
At which point would you expect to find the strongest divergence?
This was described briefly within the chapter, and in more detail in a
focus section. In general, strong convergence
occurs near a) and strong divergence occurs near b). You can imagine that this situation would support a surface high
to the left of a surface low, which is found with middle-latitude cyclones.
29)
Why
do we care about atmospheric vorticity?
a)
It
can provide information about the origin of an air mass.
b) It can provide
information about divergence and convergence in the atmosphere.
c)
It
can provide information about the impact of friction on the air.
d)
It
can provide information about prevailing wind direction in a given location.
Vorticity is a useful tool for quantifying convergence and divergence.
30)
Why
are “ordinary” thunderstorms often referred to as “air mass” thunderstorms?
a)
Because
they form at the frontal boundary between two air masses.
b)
Because
they are very extensive, and often approach the size of an air mass.
c) Because they form
within (not between) air masses.
d)
Because
they only develop in cT air masses.
Air mass thunderstorms are the gentle variety that occur when the
atmosphere is conditionally unstable and rising air parcels first create fair
weather cumulus clouds, and finally cumulonimbus clouds. The air rises due to reasons other than the
presence of a frontal boundary, so these thunderstorms form within a single air
mass, and not along a front.
31)
Where
do squall lines often form?
a) In front of, and
over, cold fronts.
b)
Downwind
(leeward) of north – south mountain ranges.
c)
Behind
warm fronts.
d)
Just
offshore when there are strong sea breezes.
A squall line is a long line of thunderstorms that usually forms either
as air is pushed up over a cold front, or as gravity waves created by an
advancing cold front initiate cumulonimbus cloud formation a hundred kilometers
or more ahead of the front.
32)
What
is a mesoscale convective complex (MCC)
a)
A
large rotating column of air within a thunderstorm.
b)
An organized group of closely spaced thunderstorms.
c)
The
same as a supercell, only not with a precipitation-free region.
d)
A
line of thunderstorms that form in front of a cold front.
These
are the massive collections of thunderstorms that often form in the Plains
states as downdrafts from an initial group of thunderstorms create adjacent
storms or cells. These can cover the
better part of a state and often persist for hours.
33)
Commonly,
within supercell storms there is a region where no precipitation sized drops or
crystals are observed. Where would you
expect to find this region?
a) In regions of
strong updrafts.
b)
In
regions of strong downdrafts.
c)
Right
above where a microburst would occur.
d)
In
the anvil.
Droplets and crystals will begin growing at the cloud base. However, if they don’t have much time to
grow, they will never get large enough to be precipitated out. In supercells, updrafts can be so strong
that the droplets inside don’t have time to grow.
34)
Why
is the “dry line” that is often present in Texas not considered a front?
a)
Because
it is stationary.
b)
Because
cT air masses exist on both sides of it.
c)
Because
the precipitation associated with it is much different than that found near
cold or warm fronts.
d) Because it separates
air masses with different dew points, not different temperatures.
A front divides two air masses with different temperatures (actually
densities, although they are directly related). The dry line divides cT air coming from Mexico from mT air originating
over the Gulf of Mexico. The
temperature of both of these air masses is similar, so the boundary between
them is not considered a front.
35)
What
is a mesocyclone?
a)
A large rotating column of air within a thunderstorm.
b)
Essentially
the same thing as a tornado, just before it touches the ground.
c)
The
same thing as a middle latitude cyclone, only smaller.
d)
A
small, rapidly rotating vortex inside of a tornado.
Rising air and vertical shear create a horizontal
vortex tube within a thunderstorm. When
this vortex tube is up righted the large spinning vertical column of air is
called a mesocyclone. These
mesocyclones are often several kilometers in diameter (tornadoes are usually
less than 1 km), and rotate much more slowly than the tornadoes they often precede.
36)
In
the figure below, which direction would you expect a supercell to move if the
prevailing wind direction is from the left?
(The figure represents the view from above the storm, looking down).
This was a tricky one that few of you got. I described this in class, although I did so with an
transparency, so you may not have had a chance to write it down. It was also discussed in your text, but may
have blended in with all the other details.
Supercells do not really consist of one persistent thunderstorm cell,
but rather a series of cells, each of which tends to form adjacent to the
previous one to the right of the prevailing wind direction. So, each individual cell moves in the
direction of the wind, but successive cells from towards the right, which gives
the storm the appearance of moving diagonally in the direction indicated by d).
37)
Why
does lightning appear to flicker?
a) Because several
individual strokes transfer charge between the ground and the cloud within a
second.
b)
It’s
just an optical illusion due to water vapor in the air (similar to why stars
flicker).
c)
Because
the stepped leader moves in an irregular, jerky path.
d)
Because
rain is falling in between the lightning and you.
When you see lightning, you don’t really see the stepped leader (or the
later dart leaders), so while that answer might seem plausible, it is
incorrect. What you really see is the
return stroke that transfers charge from the ground to the cloud. And since the process repeats itself many
times in a short period of time, the repetitive flashes make it look like it is
flickering.
38)
Why
are mesocyclones so important for tornado detection?
a)
Because
they produce radio waves that can be easily detected.
b)
Because they form about 30 minutes before any potential tornado.
c)
Because
they have negative vorticity, which is easy to observe.
d)
Because
they indicate the location of the “vault” where no precipitation sized droplets
are observed.
The
rotating air within a mesocyclone is easy to detect using Doppler radar, and
since about 30% of the thunderstorms that produce mesocyclones also produce
tornadoes, they serve as an excellent warning long before (~30 minutes) a
tornado is actually observed.
39)
What
are suction vortices?
a)
Small, rapidly spinning columns of air inside a tornado.
b)
Another
name for waterspouts.
c)
Another
name for dust devils
d)
Large
rotating columns of air within a thunderstorm.
Many
strong tornadoes are actually multi-vortex tornadoes, meaning that they consist
of a number of smaller vortices, each of which often rotate even faster than
the tornado as a whole. These small
individual vortices are known as suction vortices.
40)
Sean
and Michael McQuilken (shown below) have good reason to:
a)
Smile
more
b)
Run fast
If
you came to class the day I discussed
lightning this picture should be familiar.
As positive charge moves underneath a charged cloud it flows into
whatever is around (including people).
Since like-charges repel one another, the positive charge forces
individual hairs away from the surrounding hairs, which is why their hair is
sticking up. Of course, the reason this
is occurring is that a strong electric field is developing that will probably
produce lightning very soon. If the
McQuilkens happen to be the highest thing around, they could easily be struck,
so they probably shouldn’t stick around.