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Warm Up 4/1/08
1) Which type of air mass originates in northern
Canada?
a. mT
c. mP
b. cP
d. cT
2) Which air masses have the greatest effect on
weather conditions in much of the United States?
a. cP and mTc. mP and cT
b. mP and cP
d. mT and cT
3) Which air mass often originates as a different air
mass in Siberia and is subsequently altered?
a. mT
c. mP
b. cP
d. cT
Answers: 1) b. 2) a. 3) c.
Fronts
Chapter 20, Section 2
Formation of Fronts
When two air masses meet, they form a front,
which is a boundary that separates two air masses
Fronts can form between any two contrasting air
masses
In contrast to the vast sizes of air masses, fronts
are narrow (~15-200 km wide)
In the ideal case, the front would move in the same
direction with warmer air overlying cooler air
However, the distribution of pressure across a front
causes one air mass to move faster than the other,
causing it to advance into the slow air mass
Warm Fronts
A warm front forms when warm air moves into
an area formerly covered by cooler air
The surface position of a warm front is shown by
a red line with semi-circles that point toward the
cooler air
The first sign of the approaching warm front is
the appearance of cirrus clouds
Because of their slow rate of movement and
very low slope, warm fronts usually produce
light-to-moderate precipitation over a large area
for an extended period
A gradual increase in temperature occurs with
the passage of a warm front, as does a wind
shift from the east to the southeast
Warm Front
Concept Check
What causes a warm front to form?
When warm air moves into an area
formerly covered by cooler air, a warm frot
forms.
Cold Fronts
A cold front forms when cold, dense air moves into
a region occupied by warmer air
On a weather map, the surface position of a cold
front is shown by a blue line edged with blue
triangles that point toward the warmer air mass
As a cold front moves in, it becomes steeper and
advance more rapidly than warm fronts
The forceful lifting of air along a cold front can lead
to heavy downpours and gusty winds
As a cold front approaches, towering clouds can
often be seen in the distance
The weather behind a cold front is dominated by a
cold air mass, so weather clears shortly after the
front has passed
Cold Front
Concept Check
How are cold fronts different from warm
fronts?
Cold fronts are much steeper than warm
fronts and advance more rapidly than
warm fronts do, causing more violent
weather to form.
Stationary Fronts
Occasionally, the flow of air on either side of a
front is neither toward the cold air mass nor
toward the warm air mass, but almost parallel to
the line of the front
The surface position of the front does not move,
and a stationary front forms
On a weather map, stationary fronts are shown
by blue triangles on one side of the front and red
semicircles on the other
Sometimes, gentle to moderate precipitation
occurs along a stationary front
Occluded Fronts
When an active cold front overtakes a
warm front, an occluded front forms
The weather associated with an occluded
front is complex: most precipitation is
associated with the warm air’s being
forced upward
Remember, fronts, like all aspects of
nature, do not always behave as we
expect them to
Formation of an
Occluded Front
Middle-Latitude Cyclones
The main weather producers in the U.S. are
middle-latitude cyclones, denoted by the letter L
Middle-Latitude Cyclones – large centers of
low pressure that generally travel from west to
east and cause stormy weather
The air in these weather systems moves in a
counterclockwise direction and in toward the
center of the low
Most middle-latitude cyclones have a cold front,
and frequently a warm front, extending from the
central area
Forceful lifting causes the formation of clouds
that drop abundant precipitation
MiddleLatitude
Cyclone
Model
Cloud
Patterns
Associated
With
MiddleLatitude
Cyclones
Concept Check
How do middle-latitude cyclones form and
develop?
The first stage is the development of a front.
Over time, the front takes on a wave shape.
Changes in air flow and pressure result in a
counterclockwise flow of air. The cold front
eventually closes in on the warm front to
produce an occluded front. As the cold front lifts,
an occluded forms. Eventually, the cyclone
weakens.
The Role of Airflow Aloft
Airflow aloft plays an important role in maintaining
cyclonic and anticyclonic circulation
The rotating wind systems are actually generated
by upper-level flow
Surface convergence must be offset by outflow
somewhere higher in the atmosphere
As long as the spreading out of air high up is equal
to or greater than the surface inflow, the lowpressure system can be sustained
More often than not, air high up in the atmosphere
fuels a middle-latitude cyclone
In an anticyclone, air spreading out at the surface is
balanced by air coming together from high up
Role of Airflow Aloft in Cyclonic Activity
Assignment
Read Chapter 20, Section 2 (pg. 564-570)
Do Chapter 20 Assessment #1-37 (pg. 583584)
Print Out “Investigation 20A – Analyzing
Severe Weather Data”