1. Air masses
An air mass is a relatively homogenous large mass of air in terms of temperature and moisture characteristics.
Four air masses commonly affect our region:
cP: continental polar
cT: continental tropical
mP: maritime polar
mT: maritime tropical
What do these regions have in common? Why?

2. Air mass modification

5. Continental polar (cP)
Only occurs in the northern hemisphere at middle and high latitudes
More dominant in the winter
Can be extremely cold and dry
Generally stable (cold days, bright blue skies)
However, where cP air masses meet moist air masses, the weather conditions can become severe

6. Maritime polar (mP)
Source areas are over the Pacific and Atlantic Oceans in the mid and high latitudes
Generally produce cool and unstable conditions

7. Maritime tropical (mT)
Originates in the tropics in the Atlantic and Pacific Oceans
Pacific Ocean mT air mass contains less moisture than the Atlantic mT air mass
Also more stable

8. Air mass modification
Since air masses move, they can migrate over areas and inherit characteristics from those areas
e.g. the Great Lakes
In winter cold cP or cA air can move over the lakes and pick up moisture
The moisture is precipitated in the lee of the lakes as snow
Lake effect snow

9. A front separates two different air masses.
Types of fronts
WARM COLD
STATIONARY OCCLUDED
A front separates two different air masses.

10. Cold fronts
Occurs between advancing cP or mP air and warmer air masses
Warm air is forced to rise upwards against the colder denser air, forming cumulus and cumulonimbus clouds
Cold front
symbol

12. Cloud types with a cold front
Stratocumulus
These cloud types usually occur ahead of a cold front.
In which order would you expect them to occur as a front approaches?
Cirrus
Altocumulus
Cumulonimbus

13. Before During After Temperature Warm Sharp fall Slight fall Dew Point
Identifying a cold front’s passage
Precipitation generally falls in the vicinity of the cold front.
Before
During
After
Temperature
Warm
Sharp fall
Slight fall
Dew Point
High
Winds
SSW
Shift
W to N
Pressure
Fall
Lowest
Rise

14. Warm fronts Occurs when warm air advances over colder air
Warm front symbol
Occurs when warm air advances over colder air

15. Cloud types with a warm front
Nimbostratus
These cloud types usually occur ahead of a warm front.
In which order would you expect them to occur as a front approaches?
Cirrus
Altostratus
Cirrostratus

16. Before During After Temperature Rising Warm Dew Point High Winds
Identifying a warm front’s passage
Precipitation generally falls ahead of the warm front.
Before
During
After
Temperature
Rising
Warm
Dew Point
High
Winds
E to SE
Shift
S to SW
Pressure
Fall
Lowest
Rise

18. Cold Fronts and Warm Fronts

23. Dry Line

24. Mid-latitude cyclones
Migrating low-pressure cells
Contain converging, rising air that spirals towards the center of the system, in a counterclockwise direction
Also called wave cyclones L

25. Life cycle of a mid-latitude cyclone
Takes 3-10 days
Goes through 4 stages
Cyclogenesis
Open stage
Occluded stage
Dissolving stage

27. Cyclogenesis
Occurs where cold air masses and warmer air masses collide
Where air converges at ground level it must diverge aloft
Common at the polar front, the gulf coast, eastern areas of mountain barriers

28. Open stage
Air to the south of the cold air forces it’s way northward along a warm front
Air from the north/west forces its way southward/ eastward along a cold front

29. Occluded stage Cold air “catches up on itself”
When this happens it forces warm air aloft with only cold air below
Most occluded fronts run from the north to the south

30. Dissolving stage
Usually all warm air is now aloft and all cold air is at ground level
This will eventually dissipate causing air to mix

33. Open stage
cyclogenesis
Occluded stage