1. Chapter 13 Section 3
Air Masses and Fronts

2. Essential Questions Ch.13 S3
What are the major types of air masses in North America? How do they move?
. What are the main types of fronts?
What type of weather is associated with cyclones and anticyclones?

3. Air Masses
An air mass is an immense (huge) body of air that is characterized by similar temperatures and amounts of moisture at any given altitude (height)
Usually covers hundreds of thousands of square miles (over millions of square kilometers and can be up to 10 kilometers deep)

4. Classified according to 2 characteristics:
Temperature
Humidity
4 Major types of Air Masses that influence the weather in North America
Maritime tropical
Continental tropical
Maritime polar
Continental polar

5. The characteristics of an air mass depend on the temperatures and moisture content of the region over which the air mass forms. Temperature affects air pressure.
Cold, dense air has a higher pressure , while warm, less dense air has lower pressure.
Tropical or warm air forms in the tropics and has low pressure
Polar or cold air mass forms north of the 50 degrees latitude or South of the 50 degrees latitude and has high pressure.

6. Whether an air mass is humid (moist) or dry depends on if it forms over land or water.
Maritime air masses form over oceans and the evaporated water from the oceans makes the air humid.
Continental air masses form over land and have less exposure to moisture and therefore are dry.

7. Air Masses Weather in North America  Continental Polar Air Masses
• Continental polar air masses are uniformly cold and dry in winter and cool and dry in summer.
 Maritime Tropical Air Masses
• Maritime tropical air masses are warm, loaded with moisture, and usually unstable.
• Maritime tropical air is the source of much, if not most, of the precipitation received in the eastern two-thirds of the United States.

8. Air Masses Weather in North America  Maritime Polar Air Masses
• Maritime polar air masses begin as cP air masses in Siberia. The cold, dry continental polar air changes into relatively mild, humid, unstable maritime polar air during its long journey across the North Pacific.
• Maritime polar air masses also originate in the North Atlantic off the coast of eastern Canada.

9. Air Masses Weather in North America  Continental Tropical Air Masses
• Only occasionally do cT air masses affect the weather outside their source regions. However, when a cT air mass moves from its source region in the summer, it can cause extremely hot, drought like conditions in the Great Plains.
• Movements of cT air masses in the fall result in mild weather in the Great Lakes region, often called Indian summer.

11. How Air Masses Move
When an air mass moves into an area and interacts with other masses, it causes the weather to change.
In the continental United States, air masses are commonly moved by the prevailing westeriles and jet streams.
The prevailing westerlies, the major wind belt over the U.S., generally push air masses from west to east.

12. The jet streams are embedded in the prevailing westerlies and are bands of high speed winds, As the jet streams blow from west to east, air masses are carried along their tracks.

13. Fronts
When two air masses meet, they form a front, which is a boundary that separates two air masses. Air masses don’t easily mix.
Storms and changeable weather often develop along fronts.

14. Types of Fronts
Colliding air masses can form four types of fronts: cold fronts, warm fronts, stationary fronts, and occluded fronts.
The kind of front that develops depends on the characteristics of the air masses and how they are moving.

15. Cold front
A cold front forms when cold, dense air moves into a region occupied by warmer air.
Cold fronts tend to move quickly and can cause abrupt weather changes (including thunderstorms).
After a cold front passes through an area, colder, drier air moves in. Can bring clear skies, a shift in wind, and lower temperatures.

17. Warm Front
A warm front forms when warm air moves into an area formerly covered by cooler air.
Clouds and precipitation accompany warm fronts
Weather may be rainy or cloudy for several days
After a warm front passes through an area, the weather is likely to be warm and humid.

19. Stationary Fronts
Occasionally, the flow of air on either side of a front is neither toward the cold air mass nor toward the warm air mass, but almost parallel to the line of the front. In such cases, the surface position of the front does not move, and a stationary front forms.
Neither one can move the other --a “standoff”
If it remains stalled over an area, it may bring days of clouds and precipitation.

21. Occluded Fronts
The most complex weather situation occurs at an occluded front, where a warm air mass is caught between two cooler air masses.
The warm air mass is cut off (occluded) from the ground.
As the warm air cools and its water vapor condense, the weather may turn cloudy and rain or snow may fall.

23. Cyclones
The term cyclone, in common use, is sometimes applied to a tornado. In the science of meteorology, however, the term has a different meaning. For meteorologists, a cyclone — and its counterpart, an anticyclone — is a large-scale system of air circulation in the atmosphere in the zones between the equator and either of the poles.
Middle-latitude cyclones are large centers of low pressure that generally travel from west to east and cause stormy weather.

24. It can be considered as either producing or resulting from differences in air pressure in those zones. In a cyclone the central air pressure is lower than that of the surrounding environment, and the flow of circulation is clockwise in the Southern Hemisphere and counterclockwise in the Northern Hemisphere. Cyclones are also characterized by low-level convergence and ascending air within the system.
Cyclones and decreasing air pressure are associated with clouds, winds, and precipitation.

25. Image of a large cyclone centered over the Indian Ocean
Image of a large cyclone centered over the Indian Ocean. (National Oceanic and Atmospheric Administration/Department of Commerce)
               
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Anticyclones
An anticyclone system has characteristics opposite to that of a cyclone. That is, an anticyclone's central air pressure is higher than that of its surroundings, and the airflow is counterclockwise in the Southern Hemisphere and clockwise in the Northern Hemisphere. Anticyclones are usually characterized by low-level divergence and subsiding air.
The descending air in an anticyclone generally causes dry, clear weather.
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The term cyclone, in common use, is sometimes applied to a tornado. In the science of meteorology, however, the term has a different meaning. For meteorologists, a cyclone — and its counterpart, an anticyclone — is a large-scale system of air circulation in the atmosphere in the zones between the equator and either of the poles. It can be considered as either producing or resulting from differences in air pressure in those zones. In a cyclone the central air pressure is lower than that of the surrounding environment, and the flow of circulation is clockwise in the Southern Hemisphere and counterclockwise in the Northern Hemisphere. Cyclones are also characterized by low-level convergence and ascending air within the system.
An anticyclone system has characteristics opposite to that of a cyclone. That is, an anticyclone's central air pressure is higher than that of its surroundings, and the airflow is counterclockwise in the Southern Hemisphere and clockwise in the Northern Hemisphere. Anticyclones are usually characterized by low-level divergence and subsiding air.

29. Review
What are the major types of air masses in North America? How do they move?
Maritime tropical– moves North
Continental polar – moves south and collides with maritime tropical
Maritime polar – moves southeast
Continental tropical moves north and occasionally northeast

30. What are the main types of fronts?
Cold fronts
Warm fronts
Stationary fronts
Occluded Fronts

31. What type of weather is associated
with cyclones and anticyclones?
Cyclone—clouds, wind, and precipitation
Anticyclone – dry clear weather