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.
GE101 Natural Environments: The Atmosphere Laboratory 85
Figure 4. A typical warm front profile.
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]
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SECTION 7.4 – MIDLATITUDE CYCLONES
(41 points total)
Contrasting air masses frequently collide in the area of the subpolar lows. In this
region, often called the polar front, warm, moist air comes in contact with cool, dry
air in an area of low pressure. This is also where you would find the polar jet
stream in the upper levels on the atmosphere. These conditions present an ideal
situation for atmospheric instability, rising air, adiabatic cooling, condensation and
precipitation. These low pressure systems are called cyclones. In contrast, in areas
of high pressure, called anticyclones, the air is typically subsiding.
In the Northern Hemisphere, the westerly winds to the south of the polar front and
the easterly winds to the north cause a wave of counterclockwise (cyclonic) rotation
to form along the frontal structure. As the low-pressure system called a midlatitude
(or wave) cyclone evolves, it follows a general eastward path across the United
States, bringing a sequence of passing fronts and changing weather.
Figure 5 illustrates an idealized, mature midlatitude cyclone. Use the figure to
answer questions 22-40.
Figure 5. Idealized, mature midlatitude cyclone.
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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]
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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
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Figure 6. Specimen weather station model and standard symbols. (Source:
Daily Weather Maps, U.S. Department of Commerce)
GE101 Natural Environments: The Atmosphere Laboratory 91
Figure 7. Example of a decoded station model showing all major weather
parameters.
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
43. Using the station plots in Figure 8 as a guide, draw in the location of the cold
front and warm front. What did you use for guidance with your placements? [3 pt]
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Figure 8. Map of United States station model plots for January 7, 2012.
GE101 Natural Environments: The Atmosphere Laboratory 93
44. The table below shows coded weather data for two cities. Decode and create a
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
SECTION 7.6 – WEATHER FORECASTING AND PREDICTION
(30 points total)
Weather forecasting is a delicate balance between fundamental meteorology and
computer-generated atmospheric data. A good forecaster takes the computer data
and applies his/her knowledge of the atmosphere to the measurements. The big
question a forecaster asks themselves is, to what extent can I trust this computer-
generated forecast? Meteorologists have many models to choose from when they
make their forecasts. Sometimes, the most accurate forecast exists somewhere
between what a forecaster knows about the atmosphere and what the computer
models force out.
An accurate forecast is a hybrid of multiple computer models and forecaster
knowledge and training. Just like any job, the longer a forecaster predicts the
weather, the better they get. Remaining in one area for long periods of time is very
beneficial. Weather is an ever-changing phenomena and the ability to stay in one
location and observe the local conditions and variations over years makes you a
better forecaster.
This section of the lab will introduce you to weather forecasting and the procedures
followed to generate an actual forecast. In the first part of this section, you are given
