2. Air Masses Large bodies of air
1600 kilometers (1000 miles) or more across
Perhaps several kilometers thick
Similar temperature at any given altitude
Similar moisture at any given altitude
Move and affect a large portion of a continent

3. Invasion of Frigid Air

4. Air Masses
Source region – the area where an air mass acquires its properties
Classification of an air mass
By source region
By nature of surface in source region

5. Air Masses Polar (P) Tropical (T) Continental (c) Maritime (m)
High latitudes
Cold
Tropical (T)
Low latitudes
Warm
Continental (c)
Form over land
Likely to be dry
Maritime (m)
Form over water
Humid air

6. Air Masses Four basic types of air masses Continental polar (cP)
Continental tropical (cT)
Maritime polar (mP)
Maritime tropical (mT)

7. Air Masses and Weather North America
cP and mT air masses – most important, especially east of the Rockies

8. Air Masses and Weather North America (east of Rocky Mountains)
Continental polar (cP)
Responsible for lake-effect snows
cP air mass crosses the Great Lakes
Air picks up moisture from the lakes
Snow occurs on the leeward shores of the lakes
From northern Canada and interior of Alaska
Winter – brings cold, dry air
Summer – brings cool relief

9. Lake Effect Snow

10. Air Masses and Weather North America (east of the Rocky Mountains)
Maritime tropical (mT)
From the Gulf of Mexico and the Atlantic Ocean
Warm, moist, unstable air
Brings precipitation to the eastern United States

11. Air Masses and Weather Continental tropical (cT) Southwest and Mexico
Hot, dry
Seldom important outside the source region

12. Air Masses and Weather Maritime polar (mP)
Brings precipitation to the western mountains
Occasional influence in the northeastern United States causes the “Nor’easter” in New England with its cold temperatures and snow

13. Fronts Boundaries that separate air masses of different densities
Air masses retain their identities
Warmer, less dense air forced aloft
Cooler, denser air acts as wedge

14. Fronts Warm front Warm air replaces cooler air
Shown on a map by a red line with semicircles
Clouds become lower as the front nears
Slow rate of advance
Light-to-moderate precipitation

15. Fronts Cold front Cold air replaces warm air
Shown on a map by a line with triangles
Advances faster than a warm front

16. Fronts Cold front Associated weather is more violent than a warm front
Intensity of precipitation is greater
Duration of precipitation is shorter
Weather behind the front is dominated by
Cold air mass
Subsiding air
Clearing conditions

17. Fronts Stationary front
Flow of air on both sides of the front is almost parallel to the line of the front
Surface position of the front does not move

18. Fronts Occluded front Active cold front overtakes a warm front
Cold air wedges the warm air upward
Weather is often complex
Precipitation is associated with warm air being forced aloft

19. Formation of an Occluded Front

20. Midlatitude Cyclones
Primary weather producers in the middle-latitudes

21. Midlatitude Cyclones Move eastward across the United States
First signs of their approach are in the western sky
Require two to four days to pass over a region
Largest weather contrasts occur in the spring

22. Midlatitude Cyclones
Changes in weather associated with the passage of a middle-latitude cyclone
Warm front
Clouds become lower and thicker
Light precipitation
After the passage of a warm front, winds become more southerly and temperatures warm

23. Midlatitude Cyclones
Changes in weather associated with the passage of a middle-latitude cyclone
Cold front
Wall of dark clouds
Heavy precipitation – hail and occasional tornadoes
After the passage of a cold front winds become more northerly, skies clear, and temperatures drop

24. Cloud Patterns Associated with Midlatitude Cyclone

25. Satellite View of Midlatitude Cyclone