1. Air Masses and Fronts – I

2. Air Masses
A large (thousands of kms) body of air with more or less uniform properties in any horizontal direction at any given latitude
Temperature
Humidity
Next time: fronts – the boundary between two different air masses

3. Origin and formation of air masses
Source Region: the location where the air mass formed its properties
Large, flat areas where the air stays stagnant long enough to take on the characteristics of the underlying surface.
Ideal source regions are areas of high pressure
Types of sources.
Tropical: warm
Polar: cold
Continental: dry
Water (maritime):
humid

4. Classification of air massescP
continental polar
cold, dry, stable cA
continental arctic
very cold, very dry, stable mP
maritime polar
cool, moist, unstable mT
maritime tropical
warm, moist, usually unstable cT
continental tropical
hot, dry, unstable
surface air

5. Stability of air masses
Which is lighter?
Dry air
Moist air
Dry air:
Nitrogen
Oxygen
Argon
Moist air: all this plus
H2O

6. The weight of humid air Mean molecular mass of dry air:
Mean molecular mass of wet air.
Moist air is lighter and less dense than dry air at the same temperature.
Moist air rises more readily. Evaporation enhances convection in the atmosphere.
gas m % mass of the gas mixture
N % 28x78/100+32x21/100=28.6
O %
gas m % mass of the gas mixture
Dry air % 28.6x90/100+18x10/100=27.5
H2O 18 10%

7. Air mass source regions and their paths

8. Continental Polar (cP) and Continental Arctic (cA)-winter time
Forming conditions:
Above the northwest part of Canada and Alaska
Long clear winter nights result in radiational cooling.
Properties:
Very cold temperatures;
Little evaporation -> low water content (low specific humidity and dew point);
Temperature inversion near the ground -> stable atmosphere;
The cold subsiding air results in broad high P centers (anticyclones).
Weather in the winter:
Bitterly cold, dry weather, clear sky;
It is slightly warmer on the west and the east coast because of compressional heating of the (slightly) sinking air.
The stagnant, stable ground layer can create poor visibility and air quality -> pollutants are trapped near the ground

9. cP, cA

10. Effect of cP air masses on the weather
Upper level wind flow and surface position of anticyclones during two cold spells in 1989 and Numbers represent minimum temperatures.

11. Lake-Effect Snow Description: increased snow showers on
the downwind side of large unfrozen lakes.
It is a local phenomena.
Conditions:
The water is
warmer than the
ground (the first half
of the winter).
Cold dry air is moving
over the lake.
What happens? – The cold dry air is warmed up at the surface and picks up moisture. The boundary layer becomes unstable. Warm and moist air rises and forms cold cumulous clouds. There is a continuous supply of moisture from the lake. On the downwind side the air is additionally cooled and/or lifted up by topography -> precipitation.

12. Lake Effect Clouds – satellite image

13. Snow Effect on the Great Lakes

14. cP air masses-summer time
Forming conditions:
Above the northwest part
of Canada and Alaska
The melting ice adds
moisture to the air.
Properties:
Cool temperatures (the sun is
never directly overhead);
Increased water content and
humidity;
No temperature inversion
near the ground, surface
instability
Weather in the summer:
Cooler and comfortable temperatures, the added moisture and atmospheric instability result in fair-weather clouds;

15. cP air mass moving over the ocean
Additional moisture is added to the air.
The warm ocean surface heats up the air -> instability, cloud formation
The cP air mass loses its characteristics and is given a new designation

16. Maritime Polar (mP) Air Masses
Forming conditions:
It may start over Asia and the frozen polar pacific regions as cP.
As it moves over the Pacific (around Aleutian low) the boundary layer warms up and accumulates significant amount of moisture.
Properties:
Cold (freezing) and dry air aloft (similar to cP)
Milder temperatures at the ground F (no T inversion)
It is conditionally unstable
Weather in the winter:
This air enters the US from the NW coast; is forced over the mountains
Rain on the
upwind side
Snow high in
the mountains

17. Pacific air- cold but not VERY coldmP
Rain, snow mP
Pacific air- cold but not VERY cold
Cold damp air

18. Effect of mP air masses on the weather

19. Maritime Tropical (mT) Air Masses
Forming conditions:
Over the subtropical east Pacific Ocean-spends a long time over the warm ocean water.
Properties:
In winter it is very warm and moist
Increased water content and humidity
The atmosphere is unstable (mT is the most unstable of all air masses)
Weather in the winter:
As it moves over the colder continent it produces heavy rain.
Flooding and mudslides in California in the winter.

20. Winter fog,
low clouds
Winter Heavy Rain mT mT

21. “The Pineapple Express” –IR image

22. Continental Tropical (cT) Air Masses
Forming conditions:
In the summer over northern Mexico and SW US.
Properties:
Very dry and hot;
The atmosphere is unstable;
Weather:
Dry and hot.
Practically no precipitation.
Results in heat waves and droughts across the great planes.

23. HOT & DRY cT

24. Effect of cT air masses on the weather

25. Motion of Air Masses
The prevailing upper level winds determine the motion of the air masses.
The direction of the tropospheric jet steams impacts the change in weather
The zonal component
results in the E-W
motion of the weather
patterns
The meridional
component drags
tropical air masses to
the North and Polar
air masses to the South.
The Jet stream on Sunday

26. Sunday’s temperatures