• Discuss the characteristics, movements and source regions of North American
air masses
• Define and draw a profile of a typical warm front
• Define and draw a profile of a typical cold front
• Diagram and label all parts of an idealized, mature, midlatitude cyclone
• Interpret the data presented on a surface weather map
• Prepare and analyze a simple surface weather map using standard techniques
• Use a surface weather map to forecast the weather for a city

Materials Needed:
• Lab Manual
• textbook
• Pencil
• Colored Pencils
• Laptop

SECTION 7.1 – INTRODUCTION
For many people living in the middle latitudes, weather patterns are the result of the
movements of large bodies of air and the associated interactions among the weather
elements. Of particular importance are the boundaries between contrasting bodies of
air which are often associated with precipitation follows by a change in the weather.
This lab investigates those atmospheric phenomena that most often influence our
day-to-day weather, namely air masses, fronts and traveling midlatitude cyclones.
Using the standard techniques for plotting weather station data, the exercise
concludes with the preparation and analysis of a typical December surface map. You
will also be asked to perform your own forecast for Boston, Massachusetts and verify
your accuracy.

GE101 Natural Environments: The Atmosphere Laboratory 82
SECTION 7.2 – AIR MASSES
(30 points total)
An air mass is a large body of air that has relatively uniform temperature and
moisture characteristics throughout its volume. The area where an air mass acquires
its traits is called a source region. For example, air with a source region over cool
ocean water tends to become cool and moist while air that stagnates over the
Southwestern U.S. in summer becomes hot and dry.
Air masses are set into motion by passing high and low pressure cells. When the air
mass moves out of its source region, its temperature and moisture conditions are
carried with it.
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:

GE101 Natural Environments: The Atmosphere Laboratory 83
Figure 1. Source regions of North American air masses.
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]

GE101 Natural Environments: The Atmosphere Laboratory 84
SECTION 7.3 – FRONTS
(17 points total)
A front is a surface of contact between air masses of different densities. One air
mass is often warmer, less dense and higher in moisture content than the other.
There is little mixing of air across a front and each air mass retains its basic
characteristics. A warm front occurs when warm air occupies an area formerly
covered by cooler air. A cold front forms when cold air actively advances into a
region occupied by warmer air. An occluded front develops when a cold front
overtakes a warm front and warm air is wedged above cold surface air. A stationary
front occurs between two air masses that either have not moved or are moving very
slowly. 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.
Figure 3. A typical cold front profile.

22. Label the cold front, warm front and occluded front on Figure 5. [3 pt]
23. Draw arrows showing the surface wind directions at points A, C, E, F and G.
Remember, this is a cyclone. [3 pt]
24. Label the regions most likely to experience precipitation with the word
“precipitation.” [2 pt]
25. The surface winds in the cyclone are (converging, diverging). [1 pt]
26. As the midlatitude cyclone moves eastward, the barometric pressure at point A
will be (rising, falling). [1 pt]
27. After the warm front passes, the wind at point B will be from the (south, north).
[1 pt]
28. The heaviest precipitation in the midlatitude cyclone is likely found along the
(cold, warm, occluded) front. [1 pt]
29. Examining the temperatures across the midlatitude cyclone, Point (A, C, E) has
the warmest temperature while point (A, C, E) has the coldest temperature. [2 pt]
30. According to the isobars, point (C, E, G) has the lowest pressure. [1 pt]
31. Describe the changes in wind direction and barometric pressure that will likely
occur at point D after the cold front passes. [2 pt]
32. The air in the center of the cyclone will be (subsiding, rising). What effect will
this have on the potential for condensation and precipitation? Explain your answer.
[2 pt]
33. Considering the typical air mass types and their locations in a midlatitude
cyclone, the amount of water vapor in the air will most likely (increase, decrease) at
point A after the warm front passes. [1 pt]
34. The quantity of moisture in the air at point B will most likely (increase, decrease)
after the warm front passes. [1 pt]
35. Point (C, E, G) is where you would find the warm sector. [1 pt]

GE101 Natural Environments: The Atmosphere Laboratory 88
36. Use Figure 5 to describe the current conditions at points A through E. Fill in the
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
37. Near the center of the low, a/an (warm, cold, occluded) front has formed where
the cold air mass has overtaken the warm air mass. [1 pt]
38. Using your answer to question 37, what happens to the warm, mT air mass in
this type of front? [2 pt]
39. Using your answer to question 37, with reference to the adiabatic process, why
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).
40. Describe the general weather often associated with an anticyclone. [2 pt]
As mentioned previously, midlatitude cyclones form in the belt of subpolar lows.
After you have reviewed subpolar lows, answer the following question.
41. During the (summer, winter) season the belt of subpolar lows and the polar front
are farthest south in North America and the central United States will experience a
(greater, lesser) frequency of passing midlatitude cyclones. [2 pt]

GE101 Natural Environments: The Atmosphere Laboratory 89
SECTION 7.5 – WEATHER STATION ANALYSIS
(31 points total)
In order to understand, analyze and predict weather, observers at hundreds of
weather stations throughout the United States collect and record weather data
several times a day. This information is forwarded to offices of the National Weather
Service where the surface observations and satellite data are computer processed
and mapped. Weather maps, containing data from throughout the country, are then
distributed to any interested individual or agency.
Weather Station Data
To manage the great quantity of information necessary for accurate maps,
meteorologists have developed a system for coding weather data. Figure 5 illustrates
the system and many of the symbols that are used to record data for a weather
station. Figure 6 shows you how to read a station plot. Charts of atmospheric
parameters like wind speed, wind direction, temperature and sky cover are shown in
Figure 5. Pressure corrected for sea level is included on station model plots. For
example, a pressure of 1012.6mb would be recorded as “126” on a station plot
whereas 997.3mb would be “973.” Basically, if the coded pressure on a station plot
is a value ranging between 000 to 500, add a “10” to the front of the value. If the
coded pressure is a value from 501 to 999, add a “9” to the front of the value. Also,
be sure to add a decimal point between the last two digits and the units of millibars
(mb). An example of an actual station model is shown for reference in Figure 7.
Coded Pressure = 269 Actual sea level pressure = 1026.9mb
Coded Pressure = 804 Actual sea level pressure = 980.4mb