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20 Cards in this Set

  • Front
  • Back
Define: Air Mass
Large section of the troposphere that has relatively uniform temperature and moisture in the horizontal

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

List and define the terms used to describe the moisture content of air masses
Maritime - moist, formed over subtropical oceans
Continental - dry, formed over frozen Arctic and moves over frozen continent

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

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

Describe the source regions of North American air masses
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

Describe the typical trajectory of each air mass
cA - general southward movement

mA - usually south eastward/eastward across continent

mP - usually eastward/north eastward across continent

mT - generally moves northward

Describe the stability of each air mass in its source region
- cA is most stable
- mT is most unstable
- Polar air mass more stable than Tropical and more unstable than Arctic

Describe how the stability of air masses is modified
- when cooled from below, becomes more stable
- when heated from below, becomes more unstable

Describe how surface temperatures influence air masses
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

Describe the onshore and offshore effects as they affect air mass stability
In summer:
- offshore flow cools and stabilizes
- onshore warms and destabilizes

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

Given conditions, state the effect air mass movement will have on temperatures
- cold flow over warmer area gives warming
- warm flow over colder area gives cooling

Given conditions, describe the effect of temperature on air mass stability
- cold flow over warmer area gives instability
- warm flow over colder area gives stability

State the relationship between moisture content and air mass temperature
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

Describe how air mass modification occurs with respect to time
As air mass moves from one region to another with different characteristics, it is influenced by change only gradually

Describe the cloud forms and visibilities associated with stable air
- stratiform clouds
- continuous or intermittent precipitation
- fog
- reduced visibilities
- drifting snow
- little turbulence

Describe the cloud forms and visibilities associated with unstable air
- cumuliform clouds
- showery precipitation
- thunderstorms
- good visibilities except in showers
- blowing snow
- turbulence

Describe how stability of the air affects visibility and turbulence
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

Describe the effects on air masses of heating from below
Destabilizes the air , favors vertical development cloud, good visibilities, and turbulence

Describe the effects on air masses of cooling from below
Stabilizes the air, favors layer type clouds, poor visibilities, and little turbulence