Ch-5, Part-6: Air Masses

AIR MASSES

What are air masses?

Sections of the atmosphere which are very large, horizontally (North-South, and East-West)
Sections of the atmosphere which have uniform properties, horizontally (N-S, E-W)
Sections of the atmosphere which often move, horizontally, from place to place, over Earth's surface

Why are air masses important?

Our day-to-day weather frequently is controlled by the air masses which visit us
Understanding about air masses, plus knowing which air masses are coming, enable weather reporters, and YOU, to "forecast" the weather
Day-to-day weather is extremely important to many aspects of human activity
- Transportation
- Agriculture
- Recreation
- Human health
- Military operations
- Can you think of very many things in which the weather is truly not important?

In which ways are air masses "uniform", horizontally?

Reminder: To be an air mass, the air must be uniform in character, horizontally
An air mass can be uniform within itself in three ways: temperatures, moisture, stability
Temperatures are very similar N-S and E-W within an air mass
- Therefore, different geographic locations experience similar temperature conditions
- Note that the similarity is horizontal, NOT vertical, because elevation above the surface does influence temperatures
Moisture conditions are similar, horizontally, throughout an air mass: Moist or dry
- Dry air means little moisture available
  - Clear weather
  - No precipitation
- Moist air
  - Required for the formation of clouds and precipitation
  - May or may not be truly "stormy"
Stability of air in an air mass means the ease or difficulty in getting air to rise
- Stable air
  - Likes to "hug" the surface
  - Does not make clouds and precipitation -- it does not "want" to rise and cool
  - Brings calm, clear weather
- Instable air
  - Has a propensity (tendency) to rise
  - Can produce clouds and precipitation
  - Can contribute to very stormy weather

Horizontal extent -- How large are air masses?

Several hundred to a thousand miles or more
The size varies from time to time and from air mass to air mass happening
The point is that air masses cover sizable portions of Earth's surface at a given time

Air mass movement

Air masses move horizontally (N-S, E-W) over Earth's Surface
Air masses move in response to high and low pressure centers (cyclones and anticyclones)
- Air masses can remain stationary, in place over a geographic area, for several days
- Ordinarily, eventually, if not soon, air masses migrate to other geographic locations
Speed of air mass movement has several possibilities
- Stationary: In place, no movement
- Slowly: A few miles per hour
- Rapidly : several tens of miles per hour

How do air masses form? How do air masses get their distinctive characteristics?

Air masses derive their temperature, moisture, stability characteristics according to the surface beneath them during formation
Air masses form by remaining stationary (non-moving) over a unform underlying surface
- To form, warm air masses must remain over a surface for several days to assume underlying characteristics
- Cold air masses may require a week or more to form because stable air does not mix easily
Air masses can be very different from one another by forming over very different surfaces
- The Sahara Desert is very hot and very dry
- The North Pacific Ocean is cool and moist

Air Mass Characteristics and Code Notation ~

Air Mass Types, Regions of Origin, and Properties

mE Marine Equatorial
- Warm oceans of the equatorial zone
- Warm and very moist
mT Marine Tropical
- Warm oceans in the tropical zones
- Warm and moist
cT Continental Tropical
- Subtropical deserts
- Hot and dry
mP Marine Polar
- Midlatitude oceans
- Cool (not "cold") and moist (winter conditions)
cP Continental Polar
- Northern continental interiors
- Cold and dry (winter conditions)
cA Continental Arctic and cAA Continental Antarctic
- Regions at/near the North and South Poles
- Very cold and very dry

Air mass Source Regions;and areas of impact, North American U.S. and Canada~ ~

Source Regions are the geographic locales where air masses form and take on their temperature and moisture characteristics
mP, Marine Polar air masses from the north Pacific Ocean
- Provide moisture to the "lower 48"
- Especially during Spring, Winter and Fall
- Bring most of annual moisture received by the Pacific Northwest and Idaho
mp, Marine Polar air from the north Atlantic
- impact eastern Canada and sometimes the NE U.S.
mT, Marine Tropical air from the eastern subtropical Pacific Ocean
- Impacts southern California and sometimes Arizona
- A few times a year
- Brings the so-called "monsoon"
mT, Marine Tropical air from Gulf of Mexico and Atlantic Ocean
- Are summer moisture sources for the eastern interior of the lower 48, and the Gulf and SE Atlantic coastal states
- Also bring hurricanes to these areas
cT, Continental Tropical air from northern Mexico
- Very hot, dry air brings heat waves and drought to west Texas, New Mexico, Arizona
cP, Continental Polar air (not really "polar")
- A common source of cold to very cold, dry winter air middle and northern U.S. and southern Canada
- Sometimes to Idaho and the Pacific Northwest
cA, Continental Arctic air
- Very dry and very cold
- Cause of Arctic Express and Arctic Outbreaks
- Can travel all the way south to the Gulf and SE U.S. Atlantic coasts
- Very damaging to "off season" fruit and vegetable crops and to citrus production

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