Download Front - Mrs. Plough`s Classroom

Objectives
1) Describe the weather conditions associated with
different types of fronts.
2) Describe the “life cycle” of a mid-latitude low.
Introduction
 Changes in weather
result mostly from the
movement of lowpressure systems and
their associated frontal
systems
What is a “Front”?
 Front: the boundary
that separates opposing
air masses
 Most common in midlatitudes
 Southward-moving
polar air masses and
northward-moving
tropical air masses often
meet
What is a “Front”? Cont’d
 Air masses on either side of a front may differ in temperature
and/or humidity
 Less-dense air mass is forced to rise over the denser air mass
 Rises high into the troposphere
 Clouds and precipitation form if there is humidity
 Usually bring precipitation
Kinds of Fronts
 The weather associated with a particular front depends
on the types of air masses involved and the speed at
which the front is moving
Kinds of Fronts cont’d
 Sometimes happen
between air masses that
have the same
temperature but
different humidity
 Fronts are usually
defined by temperature
of the advancing air
mass
Kinds of Fronts cont’d
 4 basic kinds of fronts
 Cold
 Warm
 Occluded
 Stationary
Cold Fronts
 Cold front: the
boundary between an
advancing cold air mass
and the warmer air mass
it is displacing
 Cold air slides
underneath the warm air
and forces it upward
Cold Fronts cont’d
 Friction causes the lower part of the cold air mass to lag
behind the upper part
 Have steep slopes
Cold Fronts cont’d
 Weather depends to a
large degree on the type
of air mass it is
displacing
 Summer: cold cP air +
warm, humid mT air =
thunderstorms
 Summer: cold cP air +
hot, dry cT air = little to
no precipitation
Cold Fronts cont’d
 Winter
 A cold front may be marked by rain or snow showers
 The precipitations associated with the front covers only a
narrow band of ground b/c of the steep slope
 Moves quickly—precipitation usually ends shortly after
the front passes
Warm Fronts
 Warm front: the boundary between air masses when
warm air displaces cold air
 Slope is more gradual
 Weather changes associated with the warm front are
less dramatic
Warm Fronts Cont’d
 First signs of the approach
of a warm front
 1—High cirrus clouds
 2—Cirrostratus and lower
thicker stratiform clouds
 Thunderstorms
occasionally form
 Not typical
 Weather warms after front
passes
Occluded Fronts
 Cold fronts typically move about twice as fast as warm fronts
 Occluded front: when a cold front “catches up” to a warm
front
 Warm air that is caught between the 2 colder air masses is
forced to rise

Air cools causing cloudiness and precipitation
Stationary Front
 Stationary front: a front
not moving forward
 Flooding can occur if a
front is stationary too
long
Life Cycle of a Mid-Latitude Low
 Fronts are usually
connected to midlatitude low-pressure
systems
 A low-pressure system
often begins when cold
polar air meets warm
tropical air
 Can grow into an
intense storm system
Life Cycle of a Mid-Latitude Low
 Usually takes 12-24 hours
Cont’d
for a low-pressure system
to pass through the 1st 2
stages
Surface air pressure at
the polar front drops and
a wave forms on the
front
2) In the Northern
Hemisphere, winds blow
counterclockwise
around the low
3) After reaching the
occluded stage, the low
can last for 3+ days
1)
a)
Still have high winds and
rains
Upper-Air Flow
 Upper-air flow is what
controls the surface
low’s path and intensity
 Air is constantly
spiraling into the lowpressure system
Weather Associated with Pressure
Systems
 Weather you experience depends on where the center of
the low passes in relation to you
 If a low passes north of you, a warm front may move over
followed by a cold front
 You may have steady snow or rain if a low passes south
Weather Associated with Pressure
Systems Cont’d
 High-pressure areas are
associated with fair
weather
 Skies are clear
 Days may be hot
 Nights may be cold
 Winds blow outward in
a clockwise direction
 Little or no wind in the
center of the high
Weather Associated with Pressure
Systems Cont’d
 Still air takes on the characteristic temperature and
humidity of the area
 Where air masses usually form
20.2 Exit Ticket