Download Unit 19

What is an air mass?
How are air masses classified?
What is the life cycle of an extratropical
cyclone?
■ How do fronts form?
■ What kinds of fronts are there, and what
are their characteristics?
■ What kinds of clouds and weather are
associated with each kind of front?
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Unit 19
Air masses, fronts
and cyclones
Air masses
cP and cA air masses
Defined as any large body of air at a
given level having similar temp and
moisture properties
■ Air masses are classified according to
source region (where they originate)
■ Classification
■
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Associated with bitterly cold wx as they
move south over the US
■ Lake-effect snows due to heating and
moisture uptake over Great Lakes
mT=maritime tropical
cT=continental tropical
Weather extremes occur when air masses
migrate away from their source regions
and at air mass boundaries
Maritime tropical (mT)
West of the Rockies, source regions over
N. Pacific
■ Brings rain or showers to west coast
■ NE mP brings cold & damp to E coast
■
Continental tropical (cT)
■
■
Source regions summer Northern
Mexico and SW US
■ Hot, dry & unstable in lower atmosphere
■ High base Cu and Cb clouds
Long winter nights, radiational cooling
Very stable (coldest near surface)
■
Maritime polar (mP) air mass
■
Formed beneath intensifying highs over
Alaska and northern Canada
–
c=continental (land), m=maritime (sea)
T=Tropical, P=polar (cold), A=arctic (coldest)
Examples
cA=continental Arctic
mP=maritime polar
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■
W of Rockies, source is subtropical Pacific
–
Can produces heavy winter rains as warm,
moisture-laden air ascends Sierras
E of Rockies, source is Gulf of Mexico
■ Often channeled ahead (east) of a slowmoving trough
■ Provides moisture source for thunderstorms
over the Great Plains
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Extratropical cyclones
A cyclone is an area of low pressure around which the winds
flow counterclockwise (NH)
A developing cyclone
is typically
accompanied by a
warm front pushing
poleward (north or
NE) and a cold front
sweeping toward the
southeast. These mark
the leading edges of
warm and cold air
masses being wrapped
around the low
Cyclone life cycle
■
Development of the cyclone usually begins
with strong horizontal temperature contrasts,
where energy for the cyclone originates
–
■ Strongest
pressure gradients because of strong northsouth temperature contrast
Disturbances in the jet stream amplify into
upper-level troughs and ridges and move W-E
■ Upper-level divergence ahead of approaching
troughs creates surface low pressure
■
Surface winds
Cyclone life cycle (cont’d)
■
Under these conditions, the wind speed increases
up to the tropopause (where jet stream is located)
Because of friction, wind converges into the
surface low pressure (or cyclone)
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Does surface convergence produce rising or
sinking motion? Why is this important?
The convergence of air from different air
masses into the cyclone produces sharp
boundaries called fronts (cold, warm)
A developing cyclone
An occluded cyclone
■
Why do we expect this cyclone to deepen (i.e.
decrease in pressure)?
■
Why do we expect this cyclone to weaken (i.e.
decrease in pressure)?
■
Where will the vertical motion be?
■
Where is the vertical motion now?
250 mb
L
surface
L
L
500 mb
surface
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Fronts
A front is defined as the warm edge of a
transition zone between two air masses of
different density (temperature). Fronts extend
not only in the horizontal direction, but in the
vertical as well. Frontal surfaces are therefore
three-dimensional.
Cold frontal structure
Cold fronts
Transition zone where a cold air mass invades a
warmer air mass. Generally move from northwest to
southeast. The air behind a cold front is noticeably
colder and drier than the air ahead of it. When a cold
front passes through, temperatures can drop more
than 15ºF within the first hour.
Cold front clouds
Colder air lifts warmer air ahead of front
■ Cold fronts narrower than warm fronts, and
produce more vigorous vertical motion
■
–
–
Deeper clouds and more intense precipitation
Convective precipitation (thunderstorms) often
located ahead of and along cold fronts
Warm fronts
A warm front is the transition zone where a warm air mass
invades a cold air mass. Warm fronts generally move
from southwest to northeast, and the air behind a warm
front is warmer and more moist than the air ahead of it.
When a warm front passes through, the air becomes
noticeably warmer and usually more humid than it was
before.
Warm front structure
The frontal zone slopes up and over the colder air
mass ahead of it. Warm air rides along the front (up
and over the cold air mass), cooling as it rises,
producing clouds and precipitation in advance of
the surface warm front. Because the lifting is very
gradual and steady, generally wide spread and
light intensity precipitation develops ahead of a
warm front.
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Warm front clouds
Cyclone life cycle (cont’d)
The cold front moves faster than the warm
front, eventually catching up with it (forming
an occluded front). An occluded front is a
boundary that separates the new cold air
mass from the older cool air mass already in
place.
■ During occlusion (when cold front catches
up with warm front), the pressure inside the
cyclone increases and it weakens
■
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Why does the cyclone weaken?
Occlusion
Dying
Cyclone
subtropical jet
The Storm of the Century (March 1993)
Fierce winds coupled with the heavy snow resulted in the
cancellation of 25% of the nation's flights on two consecutive
days, interstates were impassable, and millions were without
power at one point in time or another during the passage of the
storm. Deaths totaled 270 people and total property damage
exceeded $800 million.
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