Lab 7:Air Masses, the Midlatitude Cyclone and Weather Maps
SECTION 7.1 – INTRODUCTION
Using the standard techniques for plotting weather station data, the exercise
concludes with the preparation and analysis of a typical December surface map. Perform your own forecast for Boston, Massachusetts and verify your accuracy.
SECTION 7.2 – AIR MASSES
(30 points total)
1. Air masses are classified according to their source region: land vs. water and
latitude of origin. Explain/define the meaning of each of the following air mass
classification letters. [4 pt]
c: ______________________________
m: ______________________________
P: ______________________________
T: ______________________________
Figure 1 shows the source regions and directions of movement of air masses that
play an important role in the weather of North America. Use the figure to answer
questions 2-8.
2. Label each of the following North American air masses (cP, cT, mP, and mT) on
Figure 1. In the blanks below, list a likely location of the source region for each air
mass. Be specific. Give an actual geographic location. [11 pt]
Source Region
cP:
cT:
mP:
mT:
3. What would be the typical temperature and moisture characteristics of each of the
following air masses (in general terms; i.e. cold, moist, dry, etc.)? [8 pt]
Temperature Moisture
cP:
cT:
mP:
mT:
Notice the paths of air masses indicated by the arrows on Figure 1.
4. The general movement of air masses across North America is (east to west, west
to east). [1 pt]
5. Given your answer in question 4, how does the movement of air masses across
North America correspond to the global flow of wind over the continent? [1 pt]
6. Which air masses would have the greatest influence on the weather east of the
Rocky Mountains? [1 pt]
7. A (cP, mT) air mass would supply the greatest amount of moisture east of the
Rocky Mountains. [1 pt]
8. A (cP, mP) air mass has the greatest influence on the weather along the
northwest Pacific coast. [1 pt]
9. A (cP, cT) air mass is only found over North America during the summer months.
[1 pt]
10. A typical source region of a (mP, mT) air mass is northern latitude oceans. [1 pt]
SECTION 7.3 – FRONTS
(17 points total)
Selected fronts seen on surface weather maps are shown in Figure 2.
Figure 2. Symbols used for fronts on weather maps.
(Source: http://www.srh.weather.gov/srh/jetstream/synoptic/airmass.htm)
11. On Figure 1, where would the polar front be located? Draw a line indicating the
location of the polar front. [2 pt]
12. In the central United States, east of the Rocky Mountains, a (cP, mT) air mass
will most likely be found north of a front and a (cP, mT) air mass to the south. [2 pt]
Figure 3 and Figure 4 illustrates profiles through typical cold and warm fronts.
Observe the profiles closely and then answer questions 13-21.
13. Along the (cold, warm) front, the cold air is the aggressive or “pushing” air
mass. [1 pt]
14. Along the (cold, warm) front, the warm air rises at the steepest angle. [1 pt]
15. Clouds and precipitation are commonly found ahead of (cold, warm) fronts. [1
pt]
16. Along which front are extensive areas of stratus clouds and periods of prolonged
precipitation most probable? Explain why you expect longer periods of precipitation
to be associated with this type of front. [2 pt]
17. Assume that the fronts are moving from left to right in Figure 3. A drop in
temperature is most likely to occur with the passing of a (cold, warm) front. [1 pt]
18. Winds are most likely to blow from the south or southwest after a (cold, warm)
front passes. [1 pt]
19. The air following a cold front is frequently cold, dense and subsiding. (Clear,
Cloudy) conditions are most likely to prevail after a cold front passes. Explain the
reason for your choice with reference to the adiabatic process. [3 pt]
20. Clouds of vertical development (Cb, Ns, Ci) and perhaps thunderstorms are most
likely to occur along a (cold, warm) front. [2 pt]
21. As a (cold, warm) front approaches, clouds become lower, thicker and cover
more of the sky. [1 pt]
(41 points total)
Figure 5 illustrates an idealized, mature midlatitude cyclone. Use the figure to
answer questions 22-40
Remember, this is a cyclone. [3 pt]
“precipitation.” [2 pt]
will be (rising, falling). [1 pt]
[1 pt]
(cold, warm, occluded) front. [1 pt]
the warmest temperature while point (A, C, E) has the coldest temperature. [2 pt]
occur at point D after the cold front passes. [2 pt]
this have on the potential for condensation and precipitation? Explain your answer.
[2 pt]
cyclone, the amount of water vapor in the air will most likely (increase, decrease) at
point A after the warm front passes. [1 pt]
after the warm front passes. [1 pt]
blanks with the options listed below. [10 pt]
Barometric pressure (high, moderate, low)
Temperature (hot, warm, cool, cold)
Sky cover (clear, cloudy, cloudy with precipitation)
Wind direction (any cardinal direction)
Atmospheric moisture content (moist, dry)
Pressure Temperature Sky cover Wind
direction
Moisture
content
A
B
C
D
E
the cold air mass has overtaken the warm air mass. [1 pt]
this type of front? [2 pt]
is there a good chance of precipitation with this type of front? [2 pt]
After a wave cyclones passes, pressure will rise and a new air mass will enter the
region usually under the presence of an anticyclone (high pressure).
As mentioned previously, midlatitude cyclones form in the belt of subpolar lows.
After you have reviewed subpolar lows, answer the following question.
are farthest south in North America and the central United States will experience a
(greater, lesser) frequency of passing midlatitude cyclones. [2 pt]
(31 points total)
42. Figure 8 shows a map of the Northeastern United States on January 7, 2012.
Using the figure, decode the station model for Chicago, IL and Seattle, WA and put
the data in the table below. Use an atlas to find these cities if necessary. [14 pt]
Chicago, IL Seattle, WA
Temperature (°F)
Dew-point Temperature (°F)
Wind Direction
Wind Speed (mph)
Surface pressure (mb)
Sky Cover (%)
Significant Weather
front and warm front. What did you use for guidance with your placements? [3 pt]
station plot the weather conditions in the space provided below each column. Use
Figures 6 and 7 for reference. [14 pt]
City 1 City 2
Temperature (°F) 65 31
Dew-point Temperature (°F) 65 30
Wind Direction North East
Wind Speed (mph) 5 45
Surface pressure (mb) 1011.3 1001.2
Sky Cover (%) Overcast 6/8
Significant Weather Fog/Haze Light Snow
City 1 City 2
(30 points total)
45. The table below shows actual weather data for Boston, Massachusetts for a 5-
day period in late fall. The data were recorded at 5pm at Logan Airport. Examine the
data in the table. Look for trends and patterns in the data. What could have caused
the display of values seen here? Use your knowledge from the course and lab thus
far to format a thorough analysis of these data. Type a brief summary of your
observations in the data and their potential cause. Submit the printed summary with
your lab. [5 pt]
Temp
(°F)
Dew-point
Temp (°F)
Relative
Humidity (%)
Wind
Direction
Wind Speed
(kts)
Pressure
(mb)
Current
Weather
Nov. 30 68 55 64 SW 15 1019 None
Dec. 1 66 61 83 SW 13 1007.8 Light Rain
Dec. 2 46 21 36 NW 16 1015.6 None
Dec. 3 39 19 44 NW 6 1030.5 None
Dec. 4 37 16 41 NW 19 1012.5 Light Snow
Weather forecasters look at many atmospheric features before arriving at a final
forecast. Forecasters look at temperatures, dew-point temperatures, relative
humidity, moisture availability, wind speed and direction as well as pressure at the
surface and in the upper levels. Forecasters look to see how the atmosphere is going
to steer current weather features and how the weather systems will evolve or decay
over time as it enters a new environment. This is a tricky operation and you will get
a first-hand account of the complexity in the following assignment. Figures 9a-c
show the U.S. analyzed surface weather map for April 14-16. Use these maps to
answer a few conceptual questions about the birth of a midlatitude cyclone and
complete a forecast for April 17 on Figure 10.
Figure 9b, locate the same low pressure center. What direction did it travel? Where is
the low pressure center now located (state/region)? [2 pt]
9a to Figure 9b. What could have caused this change (Hint: Think about geography
and air masses)? [3 pt]
Which front seems to be associated with the largest shield of precipitation? Is this
what you would expect? Explain. [3 pt]
the central pressure of the midlatitude cyclone changed over these two days? What
does this tell you about the strength of the cyclone? [3 pt]
Figure 9c, find the same low pressure center. What direction did it travel? Where is it
now located? [2 pt]
compare to April 15-16? Does it appear to moving faster or slower? Why? [3 pt]
would expect? Explain. [3 pt]
pressure of the midlatitude cyclone changed from April 15 to April 16? What does
this tell you about the strength of the cyclone? [3 pt]
you think the midlatitude cyclone will be found on April 17. Complete your forecast
on Figure 10. Include fronts and areas of precipitation on your map. Why did you
plot the midlatitude cyclone where you did? What is ultimately responsible for the
movement of the midlatitude cyclone over April 14-17? [5 pt]
