Meteorology 1010
Fall, 2014
AJ Allred, Adjunct
Student Name ____________________
Quiz 8
Explanations
Note to Question 1. Frontal weather means storminess, such as a mid-latitude cyclone that
draws differing air masses together. An occluded front has finished lifting warmer, wetter air
and there is a temporary inversion of stability: warm air won’t go down, and cold air won’t
lift. A “dry line” condition brings potentially severe storms because dry air tends to provoke
rapid lifting in wetter air that it encounters. Air mass weather is associated with air that has
stayed over a certain region long enough to pick up the characteristics of that region. Air
mass weather tends to be stable, because nothing is really moving systematically and no
other air mass is invading for the moment.
1. When an area is experiencing several consecutive days of rather constant weather, it
is experiencing:
a.
b.
c.
d.
e.
Warm front or cold front weather
Air mass weather
An occluded front
A “dry line” condition
None of the above
2. Which of the following is not a characteristic of an air-mass source region?
a.
b.
c.
d.
e.
Must be large, even continental size
Must promote uniform atmospheric conditions
Tend to provoke mid-latitude cyclones
Produces weather conditions that are stable or even stagnant for many days
Air masses that sometimes leave their source area and “invade” other regions
Note to Questions 2 and 3. Source regions produce stable conditions when air above stays in
place long enough to adopt characteristics of the source region. When air masses go
elsewhere conflict occurs. So answers ‘a’ and ‘b’ and ‘d’ are certainly true of source regions.
Answer ‘e’ is also true, when air masses go elsewhere, not when they stay in place over their
source regions. So, answer ‘c’ is not true, because source region air is not conflictual in its
own “backyard”.
For instance, the continental United States does not really have an air mass of its own, such
as is found in Central Canada, Siberia or over oceans. Instead, because of westerly winds
and trade winds (easterlies) the central United States is characterized by conflict between
invading air masses. Without a source region air mass of its own, weather in the United
States tends to be unstable as conflicting air masses collide.
3. The United States is part of a large continent but has no particular air mass of its
own. Instead, mid-latitude cyclones are common as the central United States hosts
collisions with air masses that invade from time to time.
True ___
False ___
Note to Question 4. Westerly winds tend to push air masses to the east. So, California rarely
receives invading wind from Canada. Instead, when cP air moves southward it also tends to
get pushed eastward. The southeast flow often collides with trade wind flow moving
westward, resulting in severe weather events in the central United States, sometimes called
“tornado alley”. California, with its high mountains on the eastern side and mildness from
the Pacific Ocean, seldom participates in severe weather events.
Westerly flow that passes through California tends to bring mild conditions and sometimes
plenty of rain. Check out the “Pineapple Express” westerly flow that sometimes brings
heavy precipitation to California. Keep in mind that even these unusual events are nowhere
as violent as mid-latitude cyclones in the U.S. Midwest.
4. When a cP (Continental Polar) air mass invades North America, California will
often experience agricultural frost damage and icy roads. The "westerlies" are a
major contributing factor to these cold-air invasions of California where weather is
normally mild and sunny.
True ___
False ___
Note to Question 5: When air flows across water, evaporation tends to occur. Energy and
moisture from water bodies gets carried off to places downwind, also called “lee side”.
“Lake effect” rain and snow often occurs in the Salt Lake area when wind blows across the
Great Salt Lake, picking up moisture. On the far side of the lake (lee side) that humid air
may cool enough to drop large amounts of precipitation of places in Davis County and as far
east as Park City.
5. A “Lake Effect” storm is most associated with:
a.
b.
c.
d.
e.
The eastern side of the Oregon Cascade mountains
Locations east of the Mississippi River
The eastern side of Michigan and portions of Ohio, Pennsylvania and New York
Any dry location that needs moisture to prevent drought
All of the above are prone to mid-latitude cyclones with “lake effect” precipitation
Note to Question 6. Tropical storms will always die-out eventually, when they run out of
energy that is stored in warm air, and additional energy stored as latent heat in humidity.
However, tropical storms that move out of the tropics (called “extra-tropical”) may briefly
get stronger by colliding with air masses that are much colder and drier. The contrast
between warm/wet versus cold/dry makes warm/wet air more buoyant by contrast.
So, a fading extra-tropical storm can suddenly become more vigorous if colder/drier air helps
extract remaining energy in the storm by promoting more buoyancy. Even air that is barely
warm and wet will rise if the air nearby is colder and drier. Of course, as soon as that extra
buoyancy cools adiabatically with altitude, the storm will die out completely.
6. A tropical storm that moves into the colder mid-latitudes will always die out very
quickly when it encounters a blizzard or air that is dry, but very cold.
True ___
False ___
Note to Question 7. Continental tropical air (cT) tends to be dry, because the interior of large
continents are isolated from ocean moisture. Humid continental has some moisture but is
often cold as well. Genuine tropical air (Af climate) is found near the Equator. Florida is not
near the Equator, so Florida is not in the tropics. However, the southern part of Florida is
considered a tropical climate because the region is surrounded by warm water that rises from
tropics to the south. Trade winds also help bring moist, warm conditions to Florida, so the
air mass associated with Florida is labeled “maritime tropical” (mT).
7. While on vacation in Florida you experience several days of weather that includes
temperatures above 90°F and humidity above 60%. Which of the following air
mass types is probably the cause of these conditions?
Maritime tropical (mT)
b. Continental tropical (cT)
a.
Humid continental (Df) with year-round precipitation
d. Tropical (Af), with year-round precipitation
e. None of the above
c.
Note to Question 8: Tornadoes exhibit some of the world’s most severe and dangerous
weather events, caused by collision between air masses that differ greatly in temperature and
humidity. Air that is very warm and very humid will rise much faster when colder, drier air
“wedges” underneath.
The faster air rises, the faster condensation releases latent heat for more lifting. Cumulonimbus clouds exhibit swiftly rising air, high surface winds and often lightning, thunder and
hailstones. Downdrafts, wind shear and even tornadic winds can also occur. The U.S.
tornado season begins as early as January through March when much of the continent is still
experiencing cold weather, helping provoke more vigor by contrast between air masses.
How does the information in this note to question 13 affect your thinking about Question 6
above?
8. Tornadoes in the central United States are associated with:
a.
b.
c.
d.
e.
Recent ice and snow on the ground
mT air masses
westerly winds that dry out as they move eastward
daytime surface heating and rising air “thermals”
All of the above
Note to Question #9: A vigorous storm is often a function of total heat and latent heat in the
air. Even more severe weather can occur when there is a substantial difference between
adjacent air parcels  conflict by contrast. Colder, drier air helps warmer, wetter air to rise
faster, creating a more vigorous storm.
In the choices for Question 9 below, also consider that a parcel of air doubles its capacity to
hold water vapor with each increase of 10 degrees in temperature. So, between two parcels
with equal relative humidity a parcel that is 10 degrees warmer will contain about double the
amount of latent heat energy. Consider the note to Question 8 above also.
So, look for the option that includes substantial heat and humidity and the greatest difference
between air masses:
- answer ‘a’ is too weak for a frontal or a convective storm because neither parcel of air
holds much energy and neither one is much more buoyant than the other;
Answers ‘b’ and ‘c’ are not much stronger than answer ‘a’ because both parcels are
holding relatively little energy in heat or humidity (latent heat);
Answer ‘d’ presents a lot of heat energy and latent heat energy, plus substantial contrast
between air masses. Consider that 90°F air has four times the capacity for holding vapor.
This much larger “bucket” is also much more full than the cooler, much smaller “bucket” at
70°F/30% humidity. So, both parcels are holding a great deal of energy for storminess and
the hotter parcel is also holding vastly more energy. The combination of high energy and
high contrast is a recipe for a severe cyclonic weather event.
Answer ‘e’ presents a vast amount of overall energy, but no real contrast between air
masses. This kind of situation typically results in substantial convective storminess that
delivers a lot of rain and even some windiness. But without real contrast between air parcels
there is no reason for super-fast rising air that would create winds such as would be found in
a tornado.
Answer ‘d’ is the best combination for a tornado or an overall “squall line” or “dry line”
storm. After all, “dry line” means a boundary between air masses that differ greatly in how
much latent heat they hold in the form of humidity.
9. A collision between which two air masses is most likely to result in a squall line, 'dry
line storm' or other severe weather?
a.
b.
c.
d.
e.
50°F/50% humidity versus 55°F/55% humidity
45°F/55% humidity versus 65°F/40% humidity
40°F/50% humidity versus 50°F/50% humidity
70°F/30% humidity versus 90°F/70% humidity
90°F/65% humidity versus 95°F/55% humidity
Note to Question 10. The mid-latitudes host mid-latitude cyclones. Mid-latitudes are in
between the cold, dry north and the warm, humid south, so they host vigorous storms that
involve collisions between differing air masses. Mid-latitudes also host both jet streams,
which are fast-moving, high-altitude winds that are part of the world-wide “westerlies”.
Westerlies prevail mostly from the west, although they can also undulate somewhat north and
south as they move from west to east.
Even when westerlies curve north and south (more meridional flow) they are still going from
west to east. Westerlies never go to the west. Movement north and south is temporary and is
still dominated by west to east flow.
See also the note to Question 13 below.
10. All of the following storms or weather patterns tend to move in what direction?
(These weather patterns include tornadoes, squall lines, mid-latitude cyclones, lowpressure systems and 'westerlies'.)
a.
b.
c.
d.
e.
South to west
West to east
South to north
East to west
Winds and storms in the continental United States are prone to moving in any
direction at any time. Unpredictability is a major factor.
Note to Questions 11 and 12 below: How do these two questions inform each other?
What conditions are involved with dangerous storms like hurricanes, thunderstorms and
tornadoes?
11. Hurricanes involve high winds, high tides, high waves and high atmospheric
pressure, high rainfall.
True ___
False ___
For Questions 11 and 12, also consider:
1. Stormy weather is mostly about rising air. Rising air results in lower atmospheric
pressure, because air that is rising away from the surface leaves “vacant space” or near
the surface for other air to rush in from the sides. With low pressure in the center, air can
flow into the low-pressure center from higher pressure air around the edges. So, stormy
weather involves low atmospheric pressure.
2. Low air pressure also means less atmospheric weight or mass pressing down on the
surface. So, during a storm, sea level can rise up to several feet in areas because of low
pressure. Therefore, low air pressure typically means high sea level which causing
flooding on nearby land area.
3. Low air pressure means high winds as air rushes in from the sides.
4. High winds push ocean water into high waves that can rush inland much further than
usual, causing inundation or flooding.
5. Vigorous storms also tend to bring heavy precipitation when humid air condenses back to
liquid water. So, low pressure in a storm results in high precipitation and greater chance
of high water in streams and rivers.
12. Stormy weather (rain, snow and windiness) always involves rising air and falling air
pressure.
True ___
False ___
Note to Question 13: Zonal flow is almost straight westeast. Lines of meridian (like time
zones) run north-south, so “meridional” jet stream flow is undulating more north and south
while still moving overall west to east. See also the note to Question 9 above.
13. When jet stream flow is more zonal, mid-latitude land masses tend to experience
more good weather than when jet streams move in a more meridional direction.
True ___
False ___
Note to Question 14: A stagnant air inversion and a vigorous storm may not seem to have
much in common, but both include at least one common trait in terms of air temperature near
the surface and air temperature aloft.
As a vigorous storm develops, warm, wet air lifts over cooler, drier air. When that process is
complete (occlusion) warm air has finished rising above cold air. Cold air can’t rise and
warm air can’t sink, so there is stability, with a firm boundary between these two air masses.
The stable conditions in an occluded front are very similar to the stable conditions in a
weather inversion: warm air is aloft and cold air is below. Neither one has inclination to
move or mix.
14. Consider atmospheric conditions at the conclusion of a cold front storm in places
like Colorado and Illinois. At the end of the storm, atmospheric conditions exhibit
an important similarity to stagnant, dirty-air inversions that often occur in Salt
Lake valley. (Hint: study "Occluded Front").
True ___
False ___
Note to Question 15. Cold fronts tend to move faster and provoke more vigorous storms than
do warm fronts. Cold front air is relatively heavy and flows closer to the ground, with a
more vertical or blunt edge, so collisions with warm air tend to be abrupt and fast-moving.
Storm effects tend to occur quickly and with more vigor.
By contrast, a warm front tends to spread upward and ahead across a broad or wide area, so
energy in the storm is also dispersed over a wide area. Warm front storms develop more
slowly and last longer than fast-moving cold fronts. Rain, wind and other storm effects tend
to be milder in a warm front than a cold front.
15. Because mid-latitude cyclones and other stormy weather are based on heat energy
in water vapor, a 'warm front' storm will almost always be more vigorous than a
'cold front' storm. Warm fronts contain much more energy and produce lightning,
thunder and hailstones more often than cold fronts.
True ____
False ___