AOS 101 Weather and Climate
Lisha M. Roubert
University of Wisconsin-Madison
Department of Atmospheric & Oceanic Sciences
Fronts: The Boundaries between Air
Masses
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A front is defined as the transition zone between two air masses of different
density.
Why do we have this transition zone between air masses?
Initially when air masses come in contact with each other they cannot mix
together as quickly because they posses different properties (such as temperature,
moisture, etc.). Upon coming in contact the air masses retain their properties for a
period of time and thus we observe fronts separating the air masses. Fronts can
exist as long as the air masses they separate remain distinct.
Fronts: The Boundaries between Air
Masses
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Different air masses which affect North America, as well as other
continents, tend to be separated by frontal boundaries.
In this image, the Arctic front separates the Arctic from Polar air
masses, while the Polar front separates Polar air from warm air
masses. (cA is continental arctic; cP is continental polar; mP is
maritime polar; cT is continental tropic; and mT is maritime tropic.)
Frontal Zone
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A frontal zone is a sloping surface that separates
two air masses. The area where the front meets the
ground is called the frontal zone, which is the region
where the contrasts between the air masses are
usually most prominent.
Front classifications
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Fronts are classified by the temperature changes
that result after an air mass passes over a given
location.
Weather map symbols:
1. cold front;
2. warm front;
3. stationary front;
4. occluded front;
5. surface trough;
6. squall/shear line;
7. dry line;
8. tropical wave
Cold Front: front in which cold air is
replacing warm air at the surface
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Some of the characteristics of cold fronts
include the following:
 Cold fronts tend to move faster than all
other types of fronts.
 Cold fronts tend to be associated with the
most violent weather among all types of
fronts.
 Cold fronts tend to move the farthest while
maintaining their intensity.
 Cold fronts tend to be associated with
cirrus well ahead of the front, strong
Cold fronts usually bring cooler weather,
thunderstorms along and ahead of the
front, and a broad area of clouds
clearing skies, and a sharp change in wind
immediately behind the front (although
direction.
fast moving fronts may be mostly clear
behind the front).
 Cold fronts can be associated with squall
lines (a line of strong thunderstorms
parallel to and ahead of the front).
Warm Front: front in which warm air
replaces cooler air at the surface
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Some of the characteristics of warm fronts include
the following:
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Warm fronts tend to move slowly.
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Warm fronts are typically less violent than
cold fronts.
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Although they can trigger thunderstorms, warm
fronts are more likely to be associated with
large regions of gentle ascent (stratiform
clouds and light to moderate continuous rain).
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Warm fronts are usually preceded by cirrus
first (1000 km ahead), then altostratus or
altocumulus (500 km ahead), then stratus and
possibly fog.
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Behind the warm front, skies are relatively
clear (but change gradually).
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Warm fronts are associated with a
frontal inversion(warm air overrunning cooler
air).
If a warm front exists on a weather map, it will be
northeast of the cold front and often, to the east of
a surface low pressure area.
Clouds and precipitation are quite prevalent to the
north of the warm front.
http://www.classzone.com/books/earth_science
/terc/content/visualizations/es2002/es2002pa
ge01.cfm?chapter_no=visualization
Stationary Fronts
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Stationary front- a front that does
not move or barely moves.
Stationary fronts behave like warm
fronts, but are more quiescent.
Many times the winds on both sides
of a stationary front are parallel to
the front.
Typically stationary fronts form
when polar air masses are modified
significantly so as to lose their
character (e.g., cold fronts which
stall).
Occluded Front
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Because cold fronts move faster than
warm fronts, they can catch up to and
overtake their related warm front.
When they do, an occluded front is
formed.
Occluded fronts are indicative of
mature storm systems (i.e., those about
to dissipate).
The most common type of occlusion in
North America is called a cold-front
occlusion and it occurs when the cold
front forces itself under the warm front.
The weather ahead of the cold
occlusion is similar to that of a warm
front while that along and behind the
cold occlusion is similar to that of a cold
front.
Development of an Occluded Front
Fronts on Weather Maps
Weather map symbols:
1. cold front;
2. warm front;
3. stationary front;
4. occluded front;
5. surface trough;
6. squall/shear line;
7. dry line;
8. tropical wave
Lab Setup
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Today we will be simulating a front in the rotating tank.
Dyed salty water is in the middle of the cylinder so we can create a
density difference where more dense water is in the cylinder and
less dense water is in the rest of the tank.
The table is set into rapid rotation at a speed of about 14 rpm.
We will observe what happens under 2 different scenarios: the first
time with rotation, the second time without rotation.
For next time:
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Lab report due with the same format as the
previous lab report you wrote.