Download Air Masses and Fronts

Air Masses and Fronts
Chapter 11
Part 1
March 17
17, 2009
Air masses
• Air mass = large body of air that has similar temperature
and humidity properties throughout
• Properties
P
ti off air
i masses, however
h
are modified
difi d as th
they
move over areas outside where they originate
• Source regions = regions where air masses develop
their characteristics
– need large areas with similar characteristic
• Mid-latitudes not considered source regions for air
masses as temperature,
p
humidity
y vary
y considerably
y
• Rather mid-latitudes are greatly affected by cold or
warm, moist or dry air masses moving in
Cold air mass over eastern,, central US
Air Mass Classification
• Air masses have 4 major classifications
• They are Polar (P) or Tropical (T), continental (c)
or maritime ((m);
); extremelyy cold air masses are
Arctic (A)
• Winds aloft may move air masses from their
source regions
i
• Cold air mass moving over warmer surface
warmed form belowbelow instabilityinstability may have
clouds, showers
surface• Warm air mass moving over cold surface
stable- can have fog, stratiform coluds, pollution
buildup
Air Mass Classification
Continental polar (cP) and Continental
Arctic (cA) air masses
• cP and cA air masses originate in northern Canada and
Alaska and bring very cold air to much of the US at times
in winter- following passage of cold fronts associated
with
ith mid-latitude
id l tit d cyclones
l
• Long clear nights in winter allow for strong radiational
g – thus these air masses are very
y cold- also very
y
cooling
dry
• Due to lack of terrain, air easily moves south through the
Great Plains- warms a bit
bit, but still dry so few clouds
• Air mass stable- subsidence and warming aloft, cold air
below- good for pollution buildup
• Western US less influenced by cP or CA air masses as
coldest air usually stays east of continental divide
Air mass source regions
g
and paths
2 very cold arctic outbreaks- including
one setting records in Las Vegas
Lake effect snow
• Lake effect snows- cP or cA air mass moves cold air over
relatively warm Great Lakes in early to mid winter
• Air above lake warms and is saturated, cumulus clouds
f
form
and
d grow iin unstable
t bl airi snow begins
b i and
d iis
enhanced by lifting of air over hills and convergence at
surface due to slowing
g of air from increased surface
friction
• Lake effect snows greatest within first few kilometers of
l k extending
lake,
t di up tto 50 kkm iinland
l d
• Need reasonable travel distance (fetch) over water (80 km
or so) for significant lake effect snow
• Can occur on lee side of other large lakes (e.g. Great Salt
Lake) if land/lake temperature difference is large
Formation of Lake Effect Snows
• Cold, dry air crossing the lake gains
moisture and warmth from the water
• The more buoyant air now rises, forming
clouds that deposit large quantities of
snow on the lake’s leeward shores
Profiles of cP Air Masses
• cA air masses reaching
south Texas and south
Florida may cause large
damages to fruit and
vegetable
g
crops
p
• Summertime excursions
of cP air into the US are
moderately cool and dry
and give a break to aeras
i th
in
the eastern
t
US ffrom
hot, humid weather
Maritime Polar (mp) air masses
• Wi
Wintertime
t ti
cP
P and
d cA
A air
i masses over northern
th
Asia and the Arctic travels over the Pacific
Ocean by the Aleutian Low and is modified by
added warmth and moisture- changes into
maritime polar air mass (mP)
• Conditionally unstable- as it moves inland over
mountains, orographic precipitation common
• Dries out as it moves over series of mountain
ranges in western US
• Can have chinook winds develop
• Brings moderate weather to east slope of
Rockies when replacing cP or cA airmass
mP
Modification of mP airmass crossing western US
mP airmass less common in northeastern USgives cold,
cold damp drizzly weather to NE US
Hatteras lows and Nor’easters may develop
Maritime Tropical
p
((mT)) air masses
• Wintertime source for mT airmasses is subtropical
p
east Pacific- warm, humid air coming from near
Hawaiian Islands can produce very heavy rains and
fl di along
flooding
l
westt coastt (sometimes
(
ti
called
ll d
“Pineapple Express”)
• Eastern
E t
US mT
T air
i ffrom Gulf
G lf off Mexico
M i and
d
Caribbean common in summer, less so in winter
• Winter
Wi t – mT
T usually
ll confined
fi d tto G
Gulf
lf Coast,
C
t Florida
Fl id
• Briefly may move north, causing advection fog and
l
low
clouds
l d as it moves over cold
ld ground
d
• mT airmass pushed back to south from cold front
b i i iin cP
bringing
P airmass
i
mT airmass into California- heavy rain
(Pineapple Express)
mT eastern US,
US cP western US
Summer mT Effects
• mT airmass frequent over eastern US in
summer- clockwise circulation around Bermuda
High
• Summer mT airmass brings
g much moisture and
conditional instability- thunderstorms common
monsoon- warm moist air from
• Southwestern monsoon
Gulf of Mexico or Gul of Mexico
Continental Tropical Air Masses
• cT source areas in northern Mexico and
southwestern US during
g summer
• Dry, hot, usually upper level high and
subsidence
• Sometimes moves into Great Plains
cT airmass into central,, western US
Air mass summary
• Ai
Air mass is
i large
l
body
b d off air
i with
ith ffairly
i l uniform
if
properties of temperature and humidity
• Air
Ai mass source regions
i
tend
t d to
t be
b flat
fl t with
ith hi
high
h
pressure and light winds
• Continental
C ti
t l air
i masses fform over lland,
d maritime
iti
over water
• Polar
P l air
i masses ffrom polar
l regions;
i
ttropical
i l air
i
masses from warm, tropical regions
• cP=
P cold,
ld d
dry; cA=very
A
cold,
ld d
dry; cT=hot,
T h t d
dry;
mT=warm, moist; mP=cool, moist
Fronts
• Front = transition zone between 2 air masses of
different densities (temperatures) and often
humidityy
• Frontal location often identified by:
1. Sharp temperature difference over short distance;
1
2. Change in moisture content (e.g. dew point
temperature);
3. Shift in wind direction;
4 Pressure and pressure changes;
4.
5. Clouds and precipitation patterns
• Lowest pressure at frontal boundary
Example surface map showing pressure systems,
f
fronts,
air
i masses, isobars,
i b
precipitation
i i i
Note kink in
isobars and wind
shift at front
Pressure
falling before
frontal
passage,
rising after
Example cold
f t and
front
d
associated
s rface weather
surface
eather
Cold Fronts
• Cold, dry stable air replaces warm, moist
unstable air
• Blue
Bl liline with
ith bl
blue ttriangles
i
l
• Clouds of vertical development
• Thunderstorms, squall lines
Vertical structure of cold front
Cold air wedges under warm air
at front, causing warm air to rise
often
ft producing
d i showers
h
or
thunderstorms
Front is steep near the ground
due to friction typical slope
about
b t 1:50
1 50 ttypical
i l speed
d about
b t
25 knots