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ANSWER KEY
Part I: Air Mass Classification
1.
What are the differences between cT and mT air masses?
A continental tropical air mass is hot and dry. Maritime tropical air masses are warm and moist. The cT forms over land
and the mT forms over water.
2.
What are the differences between cP and mT air masses?
Continental polar air masses are cold and dry. Maritime tropical air masses are warm and moist. The cP air mass is
formed over land and has less moisture than the mT, which is formed over water. The cP air mass is formed at higher
latitudes than the mT air mass.
3.
(a) Five air mass types affect the United States during the course of a typical year: continental polar,
continental arctic, continental tropical, maritime polar, and maritime tropical. Locate the source regions of
each air mass type using Figure 4.
a. Air mass type for source region #1 = cA
b. Air mass type for source region #2 = cP
c. Air mass type for source region #3 = mP
d. Air mass type for source region #4 = cT
e. Air mass type for source region #5 = mT
f.
Air mass type for source region #6 = mP
g. Air mass type for source region #7 = mT
(b) Which air mass is the least significant to your location and why?
This answer is somewhat subjective, depending on one’s location and interpretation of “significant.” Two possible
answers include the following:
1. The cA air mass is the least significant because it invades the U.S. on an infrequent basis. It is a shallow air mass, and
by the time it travels southward through the central U.S. from the Arctic, it may have little influence if it has been modified
(e.g., warmed). However, infrequent freezing conditions that sometimes occur along the U.S. Gulf Coast states during cA
outbreaks can be significant for local economies, as both tender vegetation and shallow water pipes freeze.
2. The mP air masses that originate over the northern Atlantic may be the least significant because they oppose the
general westerly flows that normally are observed over the mid-latitudes. Therefore, mP conditions do not have long
durations across the eastern U.S.
Lab 6 Answer Key
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Part II: Fronts
Figures 5–7 show the evolution of the air temperature (Figure 5), dew point (Figure 6), rainfall (Figure 7), and wind
fields (Figure 7) across parts of the central U.S. during the day of 1 November 2004. On Figure 5, relatively warm
temperatures are shaded in orange and relatively cool temperatures are shaded in blue. On Figure 6, relatively
high dew points (i.e., moist air) are shaded green and relatively low dew points (i.e., dry air) are shaded brown. On
Figure 7, 3–hour rainfall totals (previous 3 hours) are shaded while wind speed and wind direction are displayed
as vectors (longer arrows represent stronger winds). A continental U.S. map is inset on each figure to provide an
overall view of the surface fields.
Examine the changes in the surface air temperature, moisture, and wind fields from 3:00 AM to 9:00 PM CST on
1 November 2004 to answer questions 4 to 11.
4.
What type of front passed through central Illinois on 1 November 2004?
Type of Front Warm front
5.
Which direction did the front move?
Direction Northward
6.
What are the ranges of temperature and dew point across central Illinois during the passage of the front?
(Range = Max value – Min value)
Air Temperature Range ~27°F (77°F – 50°F)
Dew Point Range ~18°F (66°F – 48°F)
7.
Draw the location of this front on any one image using the appropriate colors and symbols (e.g., Figure 1) to
annotate that image.
8.
What air mass types were north and south of the front at 3:00 PM CST?
Air Mass Type in Northern IL at 3:00 PM CST Continental polar
Air Mass Type in Southern IL at 3:00 PM CST Maritime tropical
9.
Can you find any other fronts on the images? If so, what type of front(s) and where is it (are they) located at
3:00 PM CST on 1 November 2004?
Yes, there is a cold front moving eastward through central Kansas and west-central Oklahoma. The inset map shows that
the cold front extends through West Texas into Mexico.
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10. Select one of the fronts that you found on the sequence of maps. For that front, note the weather conditions
on Table 3 at each stage (before, during, and after) of the passage of the front. It may be easiest to pick a
specific NWS observation location and examine the change in conditions throughout the day at that site.
Table 3 – Worksheet for Identifying Conditions Before, During, and After a Frontal Passage
Type of Front Warm Front
Weather Element
Approximate Location (e.g., southern IL) Central IL.
Before Passage
During Passage
After Passage
Temperature
59°F
61°F
73°F
Dewpoint
Temperature
54°F
59°F
70°F
Wind Speed
(calm vs. strong)
Weak
Weak to moderate
Weak to moderate
East wind
Southeast wind
South wind
0 in
~0.1 in
~0.7 in
Wind Direction
Precipitation Amount
Table 3 – Worksheet for Identifying Conditions Before, During, and After a Frontal Passage
Type of Front Cold Front
Weather Element
Approximate Location (e.g., southern IL) South-central KS
Before Passage
During Passage
After Passage
Temperature
57°F
54°F
45°F
Dewpoint
Temperature
54°F
45°F
37°F
Wind Speed
(calm vs. strong)
Calm
Moderate
Strong
Wind Direction
N/A
North wind
North wind
Precipitation Amount
0 in
<0.1 in
~0.4 in
11. Using Table 3, describe, in one paragraph, the weather conditions associated with the passage of the front
that you selected.
Answers will vary depending on the front selected.
Lab 6 Answer Key
Explorations in Meteorology
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