Lesson 107 - Air Masses

Front 1

Define: Air Mass

Back 1

Large section of the troposphere that has relatively uniform temperature and moisture in the horizontal

107.1.1

Front 2

Describe how air masses form

Back 2

When a large section of air acquires the temperature and moisture characteristics of a vast area of the earth's surface

107.1.2

Front 3

List and define the terms used to describe the moisture content of air masses

Back 3

Maritime - moist, formed over subtropical oceans
Continental - dry, formed over frozen Arctic and moves over frozen continent

107.1.3

Front 4

List and describe the three terms used to describe the temperature of air masses

Back 4

Arctic - cold air mass, formed in the Arctic

Polar - temperate air mass, forms in temperate zones by heating of Arctic air mass or cooling of Tropical air mass

Tropical - warm air mass, formed in the Tropics

107.1.4

Front 5

Name and describe the air masses found over North America in summer and winter

Back 5

Continental Arctic (winter only) - cA, dry and very cold
Maritime Arctic - mA, humid and cold
Maritime Polar - mP, humid and mild
Maritime Tropical - mT, humid and warm

107.1.5

Front 6

Describe the source regions of North American air masses

Back 6

cA - forms over frozen Arctic in winter

mA - forms from cA during winter as it moves south over cold ocean waters, and from open waters of Arctic Archipelago or numerous northern lakes during summer

mP - forms over temperate latitude oceans, from mA as it moves south over ocean or down the continent, and from mT as it moves north over ocean or up the continent

mT - forms over tropical oceans

107.1.6

Front 7

Describe the typical trajectory of each air mass

Back 7

cA - general southward movement

mA - usually south eastward/eastward across continent

mP - usually eastward/north eastward across continent

mT - generally moves northward

107.1.7

Front 8

Describe the stability of each air mass in its source region

Back 8

- cA is most stable
- mT is most unstable
- Polar air mass more stable than Tropical and more unstable than Arctic

107.1.8

Front 9

Describe how the stability of air masses is modified

Back 9

- when cooled from below, becomes more stable
- when heated from below, becomes more unstable

107.2.1

Front 10

Describe how surface temperatures influence air masses

Back 10

By a gradual change in temperatures in the low levels

Factors include:
- latitude change
- season change
- movement over surfaces with different temperatures
- daily variations
- topography

107.2.2

Front 11

Describe the onshore and offshore effects as they affect air mass stability

Back 11

In summer:
- offshore flow cools and stabilizes
- onshore warms and destabilizes

In winter:
- offshore warms and destabilizes
- onshore cools and stabilizes

107.2.3

Front 12

Given conditions, state the effect air mass movement will have on temperatures

Back 12

- cold flow over warmer area gives warming
- warm flow over colder area gives cooling

107.2.4

Front 13

Given conditions, describe the effect of temperature on air mass stability

Back 13

- cold flow over warmer area gives instability
- warm flow over colder area gives stability

107.2.5

Front 14

State the relationship between moisture content and air mass temperature

Back 14

Warmer air masses can hold more water vapour

Moisture content increases with:
- movement over water
- melting/evaporation
- movement over dense vegetation

Moisture content decreases with:
- condensation/precipitation

107.2.6

Front 15

Describe how air mass modification occurs with respect to time

Back 15

As air mass moves from one region to another with different characteristics, it is influenced by change only gradually

107.2.7

Front 16

Describe the cloud forms and visibilities associated with stable air

Back 16

- stratiform clouds
- continuous or intermittent precipitation
- fog
- reduced visibilities
- drifting snow
- little turbulence

107.3.1

Front 17

Describe the cloud forms and visibilities associated with unstable air

Back 17

- cumuliform clouds
- showery precipitation
- thunderstorms
- good visibilities except in showers
- blowing snow
- turbulence

107.3.1

Front 18

Describe how stability of the air affects visibility and turbulence

Back 18

Heating from below (unstable):
- upward movement
- turbulent
- suspended particles lifted
- visibility tends to be good

Cooling from below (stable):
- inversion
- calm
- suspended particle near surface
- visibility tends to be poor

107.3.2

Front 19

Describe the effects on air masses of heating from below

Back 19

Destabilizes the air , favors vertical development cloud, good visibilities, and turbulence

107.3.3

Front 20

Describe the effects on air masses of cooling from below

Back 20

Stabilizes the air, favors layer type clouds, poor visibilities, and little turbulence

107.3.3