Chapter 12
Air Masses and Fronts
Air Mass – large bodies of air
having similar thermal and moisture
characteristics.
How air masses form:
 Large bodies of air reside over
the same region long enough
to assume the characteristics of
the surface below.
 Examples
-Body of air over Caribbean
for a long period becomes
i. Warm
ii. Moist
-Body of air over central
Canada for a long period
becomes
i. Cold
ii. Dry
The longer a large body of air
spends over one region, the more
time it has to reach equilibrium
(heat, moisture) with the
underlying surface.
Source Region - Those areas on
the planet where air masses form
 Restricted to high or low
latitudes
- mid-latitude weather is too
variable
 Must be large areas
Air Mass Classification
Based on:
 Moisture characteristics of
source region
 Thermal characteristics of
source region
1. m = maritime (moist)
c = continental (dry)
2. T = tropical (warm)
P = polar (cold)
A = arctic (extremely cold)
3. w = warmer than surface beneath
k = colder than surface beneath
Ignoring the w and k subclassifications, we can form 5 air
mass types:
1. cT
3. cA
5. mP
2. cP
4. mT
What about mA?
mA is NOT POSSIBLE because:
1. A modifiesP by
Conduction, OR…
2. Maritime surface freezes
Colder air masses
I. cP
Continental Polar
 Form over large, high-latitude
land masses
-Northern Canada
-Northern Siberia
 Especially in winter
-Low solar anglesshort
dayssnow cover yields high
albedo
-Very cold surface leads to
inversionpollution
 Air is especially dry due to
-Colder temperatures
-Continentally
 In summer:
-cP stays farther north (N.H.
warmer)
-cP warmer (snow melts and
days longer)
II. cA
Continental Arctic
 Similar to cP but colder
 cP separated from cA by “Arctic
front”
III. mP
Maritime Polar
 more moderate in thermal and
moisture characteristics than cP
 examples:
-cP air from Asian interior over
N. Pacific  warms and
moistens  mP
-cold, north flow on backside
of low over New England
As with cP air masses, mP air
masses can modify – take on new
characteristics than reflect new
surface beneath.
Warmer air masses
IV. cT Continental Tropical
 hot
 low-latitude
 generally desert areas
 much insolation
 cloud-free
 unstable due to hot low levels
BUT
 cloud-free dues to dryness
 hot and dry over hot moist
(TX)  UNSTABLE!!!
V. mT
Maritime Tropical
 hot
 low-latitude
 MOIST! – maritime
 Unstable and moist
-Perfect for clouds
precipitation
 Even mT modifies
-Moves poleward  surrenders
moisture  dries out
 Example: Gulf of Mexico
Fronts
 boundaries that separate air
masses with different
characteristics (usually
temperature.)
cP
front
(cold)
mT
Frontal Types
1.Cold – when cold air advances on
warmer air ahead of it.
2. Warm – when warm air advances
on colder air ahead of it
3. Stationary – a front that lacks
substantial motion in either air
mass (like cold front)
4. Occluded – separates similar,
usually polar air masses.
Symbology
Cold
Cold
Warm
Warm
Cold
Warm
Stationary
Cold
Warm
Occluded
Where fronts form…
Typical mid-latitude cyclone
(storm)
1000 km
 Fronts are usually
ASSOCIATED with a low
pressure system
 Fronts are usually NOT
independent features.
What defines a front?
1. *****Temperature changes*****
2. Humidity changes
3. Pressure changes
4. Wind changes
5. Cloud cover changes
6. Precipitation changes
Frontogenesis
 An increase in the temperature
contrast across a front.
 Strengthens and regenerates a
frontal zone.
Frontolysis
 A decrease in the temperature
contrast across a front
 Weakening and dissipation of a
frontal zone
Cold Front – boundary
separating encroaching cold
air from retreating warm air.
 Friction accounts for slope –
steeper near surface
 Vertical scale exaggerated
 Forward motion up to 30 mph
Characteristics of cold frontal
passage
Temperature
Dew point
abrupt decrease
abrupt decrease
Winds
SW  NW
speed increases
abrupt increase
improves
towering Cu or
Cb
showery, heaviest
at frontal passage
 Hail, thunder,
lightning
Pressure
Visibility
Sky
Precipitation
Warm Front – boundary
separating encroaching warm
air from retreating cold air
 Friction accounts for slope here
 Cold front has easier time
displacing warm air
 Forward motion up to 12 mph
Characteristics of warm
frontal passage
Temperature
Dew point
gradual rise
gradual rise
SE  SW
steady after fall
deteriorating,
then slight
improvement
Sky
lowering ceiling,
Ns, improves after
passage
Precipitation rain, snow 
ending after passage
Winds
Pressure
Visibility
Stationary Front
A boundary between air masses that
has stalled or has very little forward
movement
Absolutely still?
No, but
1. Fronts are better described as
zones of transition
2. Fairly wide gaps between
surface
observing stations
**Warm air still slopes over cold air**
Occluded Fronts
A boundary that represents the
merger of two other fronts
Name implies how this process
occurs
- trailing cold front approaches
- leading, slower-moving warm
front;
- cold front "catches up to" warm
front;
- warm air is "pinched" upward away
from the surface
Warm moist air pushed upward 
clouds and rain / snow.
Therefore, occluded fronts can be
areas of active weather.
BUT…
As a front, it should, by definition,
separate air masses of different
temperatures.
Q: So is it a front?
A: Debatable. For our purposes
here 
Yes.
Cold-type occlusion
- believed to be less common
~ cP air behind cold front
modifies as it flows
southward
~ reduces contrast between
cP air behind cold front
and ahead of warm front
Warm-type occlusion
Dryline
A boundary between dry and humid
air masses
- common in southern Great
Plains states of U.S.
- places warm moist (mT) air
under warm dry air (cT)
- places less dense air under
denser air