## Name _________________

Air Masses

Air masses are large masses of air with similar moisture and thermal characteristics and are labeled according to their source region. They develop over surfaces that transfer these characteristics to the air above the surface. Higher latitude air masses tend to be cool and lower latitude air masses tend to be warm. Air masses that form over continents are often dry, while air masses that form over water are often moist. The analysis of station models identifies and classifies air masses as maritime (m) or continental (c ) according to dew point temperatures, and tropical (T) or polar (P) according to air temperatures. It is important to note that the descriptors are relative to the surrounding conditions, and not based on absolute temperature scales. The combination of these characteristics describe the principal air masses in the United States.

 mP moist, cold mT moist, warm cP dry, cold cT dry, warm

Consider the information on air masses and answer the following question.

1. Which type of air masses would you expect to find in the following locations?

 Location Type of Air Mass Minneapolis, MN Seattle, WA Miami, FL Las Vegas, NV

Station Models

The National Weather Service manages hundreds of weather stations that are used to observe meteorological variables like air temperature, dew point temperature, and wind direction across the country. These observations are plotted as station modelsonsurface weather maps. Meteorologists use these maps to analyze and forecast the weather by identifying areas of high and low atmospheric pressure, weather fronts, and significant weather events.

Station models provide a means of presenting a lot of information in a fairly concise manner. The most common meteorological variables plotted on a station model are atmospheric pressure, air temperature, dew point temperature, wind speed and direction, cloud cover, and present weather. Many other variables can be entered, such as cloud types and cloud heights. On a weather map, station models are centered on the latitude and longitude of the city where the observations were recorded.

The key to understanding a station model is in understanding the shorthand used to draw the model. According to the charts, in the station model example (Figure 10.1), the temperature is 76° F, the dew point temperature is 55° F, and the wind direction is northeast at about 20 knots (nautical miles per hour). The atmospheric pressure is 1013.8 mb, and has increased and then decreased. The pressure is now lower by 0.3 mb than it was three hours ago. The cloud cover is completely overcast, and it is raining.

Weather Fronts

A weather front is simply a boundary between different air masses where usually the moist obvious difference between the meeting air masses is temperature. Four classes of fronts exist depending on the movement of the air mass. The front symbols show the direction that the front is advancing. In the United States, cP and mT air masses are the most influential air masses on weather and climate. Cold fronts are the boundaries between cP and mT air masses, where the cP air mass is advancing. Warm fronts are the boundaries between mT and cP air masses, where the mT air mass is advancing. An occluded front occurs when the cold front (cP air mass) catches up to the warm front (mT air mass) while both are advancing in the same direction. A stationary front is a boundary between a cP and mT air mass, where neither air mass is advancing.

Frontal boundaries consist of leaning air and are lifting mechanisms in the atmosphere. Cold fronts tend to trigger abrupt lifting in the atmosphere, causing clouds and intense, brief periods of precipitation along the front line. Warm fronts overrun the cooler air mass along a gentler slope, with cloudiness and showery, light precipitation well ahead of the front line.

Station models describe several elements of surface weather and climate. In the following exercises, the station models indicate surface temperature, wind direction, and wind speed. Typically, the air mass with higher wind speeds and winds moving perpendicular to the front are advancing.

Analyze the following maps for frontal boundaries.

1. Considering air temperature, draw a boundary line separating the two air masses in the image above.

2. Compare the two air masses and label them as WARM or COLD.

3. What is the wind direction of the warm air mass?

4. In which direction is the frontal boundary advancing?

5. Modify your boundary line with the appropriate front symbol, based on the advancing air mass.

1. Considering air temperature and dew point temperature, draw a boundary line between the two air masses in the image above.

2. Compare the air masses and label them as mT or cP.

3. What direction are the winds in the cP air mass?

4. Considering wind direction and wind speed, which air mass is advancing?

5. In which direction is the frontal boundary advancing?

6. Modify the boundary line with the appropriate front symbol.

The Mid-Latitude Cyclone Model

The following questions are based on a hypothetical weather map in the United States. The map that appears below shows labeled isobars and the positions of a cold front and a warm front (unlabeled). The top of the map is north.

Six locations are marked on the map (points A, B, C, D, E, and F). A cross-section diagram along points A, B, C, D, E, and F is shown below the map.

1. On the map above (upper image), label the following:

a. Cold front (use standard weather map symbols)

b. Warm front (use standard weather map symbols)

c. Area(s) of cool/cold air

d. Area(s) of warm air

2. On the map on the previous page (upper image), use arrows to show the wind direction in the western, southern, eastern, and northern parts of the storm.

3. In which direction is the mid-latitude cyclone as a whole moving?

a. From ___________ to ___________

4. On the cross-section diagram (lower image), label the following:

a. Cold front

b. Warm front

c. Cold air mass(es)

d. Warm air mass(es)

e. Direction of cold front movement (use arrow)

f. Direction of warm front movement (use arrow)

Using your labeled map and cross section on the previous page for reference, answer the following questions:

5. What is the most likely wind direction at Point D?

From the ________________

6. At point D, is the pressure rising or falling? _________________

a. Why?

7. Is precipitation more likely at Point D or at Point C? _________________

a. Why?

8. At Point D, what general temperature change will take place after the cold front eventually passes over this location?

a. Why?

9. What is the most likely wind direction at Point B?

From the ________________

10. At point B, is the pressure rising or falling? _________________

a. Why?

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