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Unit 9 Section 4: Fronts
Objectives
 Compare the characteristic weather
patterns of cold fronts with those of warm
fronts.
 Describe how a midlatitude cyclone forms.
 Describe the development of hurricanes,
thunderstorms, and tornadoes.
Fronts
 A cool air mass is dense and does not mix with the
less-dense air of a warm air mass.
 Thus, a boundary, called a front, forms between air
masses.
 Changes in middle-latitude weather usually take
place along the various types of fronts.
 Fronts do not exist in the Tropics because no air
masses that have significant temperature
differences exist there.
Fronts, continued
Cold Fronts
 cold front the front edge of a moving mass
of cold air that pushes beneath a warmer air
mass like a wedge
 If the warm air is moist, clouds will form.
Fronts, continued
Cold Fronts, continued
 Large cumulus and cumulonimbus clouds typically form along fastmoving cold fronts.
 A long line of heavy thunderstorms, called a squall line, may occur in
the warm, moist air just ahead of a fast-moving cold front.
 A slow-moving cloud front typically produces weaker storms and lighter
precipitation than a fast-moving cold front does.
Fronts, continued
Warm Fronts
 warm front the front edge of advancing warm air mass
that replaces colder air with warmer air
 The slope of a warm front is gradual.
 Because of this gentle slope, clouds may extend far
ahead of the surface location, or base, of the front.
 A warm front generally produces precipitation over a
large area and may cause violent weather.
Fronts, continued
Stationary and Occluded Fronts
 stationary front a front of air masses that moves either
very slowly or not at all
 occluded front a front that forms when a cold air mass
overtakes a warm air mass and lifts the warm air mass of
the ground and over another air mass
 Sometimes, when air masses meet, the cold moves
parallel to the front, and neither air mass is displaced.
Cold Front
Warm Front
Occluded Front
Polar Fronts and Midlatitudes
Cyclones
 Over each of Earth’s polar regions is a dome of
cold air that may extend as far as 60° latitude.
 The boundary where this cold polar air meets the
tropical air mass of the middle latitudes, especially
over the ocean, is called the polar front.
 Waves commonly develop along the polar front.
 A wave is a bend that forms in a cold front or
stationary front.
Polar Fronts and Midlatitudes
Cyclones, continued
 midlatitude cyclone an area of low pressure that
is characterized by rotating wind that moves
toward the rising air of the central low-pressure
region
 Waves are the beginnings of low-pressure storm
centers called midlatitude cyclones or wave
cyclones.
 These cyclones strongly influence weather
patterns in the middle latitudes.
Polar Fronts and Midlatitudes
Cyclones,
continued
Stages of a Midlatitude Cyclones
 A midlatitude cyclone usually last several days.
 In North America, midlatitude cyclones generally travel
about 45 km/h in an easterly direction as they spin
counterclockwise.
 They follow several storm tracks, or routes, as they
move from the Pacific coast to the Atlantic coast.
 Midlatitude Cyclone = Low Pressure = Stormy!
Polar Fronts and Midlatitudes
Cyclones, continued
The diagram below shows the different stages
of a midlatitude cyclone.
Polar Fronts and Midlatitudes
Cyclones, continued
Anticyclones
 Unlike the air in the midlatitude cyclone, the air of
an anticyclone sinks and flows outward from a
center of high pressure.
 Because of the Coriolis effect, the circulation of air
around an anticyclone is clockwise in the Northern
Hemisphere.
 Anticyclones bring dry weather, because their
sinking air does not promote cloud formation.
 Anticyclone = High Pressure = Good Weather!
Reading Check
How is the air of an anticyclone different from
that of a midlatitude cyclone?
The air of an anticyclone sinks and flows
outward from a center of high pressure. The
air of a midlatitude cyclone rotates toward
the rising air of a central, low-pressure
region.
Severe Weather
Thunderstorms
 thunderstorm a usually brief, heavy storm that consists of
rain, strong winds, lightning, and thunder
 Thunderstorms develop in three distinct stages.
 The thunderstorm dissipates as the supply of water vapor
decrease.
Severe Weather, continued
Lightning
 During a thunderstorm, clouds discharge electricity
in the form of lightning.
 The released electricity heats the air, and the air
rapidly expands and produces a loud noise known
as thunder.
 For lightning to occur, the clouds must have areas
that carry distinct electrical charges.
Thunderstorm
 A thunderstorm, also known as an
electrical storm, a lightning storm, a
hailstorm, or simply a storm is a form of
weather characterized by the presence
of lightning and thunder.
 The meteorologically-assigned cloud
type associated with the thunderstorm is
the cumulonimbus.
Thunderstorm cont…
 Thunderstorms are usually accompanied by strong winds,
heavy rain and sometimes snow, hail, or no precipitation at
all.
 Those which cause hail to fall are known as hailstorms.
 Thunderstorms can generally form and develop in any
geographic location, perhaps most frequently within areas
located at mid-latitude when warm moist air front collides
and border cool air fronts.
Thunderstorms cont…
 Thunderstorms, and the phenomena that occurs
along with it, can produce numerous risks and
hazards to populations and landscapes.
 Damages that result from thunderstorms are
mainly inflicted by downburst winds, large
hailstones, and flash flooding caused by heavy
precipitation.
Thunderstorm dangers
 Cloud-to ground lightning.
– These cloud-to- ground lightning frequently occurs during a
thunderstorm.
– These strikes can pose a significant hazard due to possible ignition
of wildfires.
 Hail
– A large chunk of ice raining death from the sky, enough said right?
 Tornadoes and Waterspouts
– We talked about this earlier.
Thunderstorm dangers cont…
 Flash Flood
– Flash flooding is the process where a landscape, most
notably urban environments, is subjected to rapid
floods.
– Flash flooding can frequently occur in slow-moving
thunderstorms and are usually caused by the heavy
liquid precipitation that accompanies it during country
music concerts.
Thunderstorm dangers cont…
 Downburst
– Downburst winds can produce numerous hazards to
landscapes experiencing thunderstorms.
– Downburst winds can generally be extremely powerful,
and are often mistaken for wind speeds produced by
tornadoes.
– Downburst winds can be hazardous to unstable,
incomplete, or weakly-constructed infrastructures and
buildings
Thunder and Lightning
 Lightning is the most spectacular element of a
thunderstorm.
 Lightning is a giant spark. A single stroke of lightning can
heat the air around it to 30,000 degrees Celsius (54,000
degrees Farhenheit)!
 This extreme heating causes the air to expand at an
explosive rate.
 The expansion creates a shock wave that turns into a
booming sound wave, better known as thunder. Thus the
name thunderstorm.
How to Estimate Lightning's
Distance
 The time between seeing a lightning flash
and hearing the thunder it produces is a
rough guide to how far away the lightning
was.
 Normally, thunder can be heard up to 10
miles from the lightning that makes it.
Estimation cont…
 Since light travels at 186,000 miles per second,
you see the lightning the instant it flashes.
 But sound, including thunder, travels about a mile
in five seconds near the ground.
 If 15 seconds elapse between seeing a lightning
bolt and hearing its thunder, the lightning was
about three miles away.
Estimation cont…
 Lightning closer than about three miles
away is a warning to take shelter
immediately.
 Successive lighting strikes are often two to
three miles apart. If the first stroke is three
miles away, the next one could hit you.
Thunderstorm safety
 If you are outdoors:
– Attempt to get into a building or a hardtop car and keep the
windows closed.
– If no structure is available, as quickly as possible go to an open
space and squat low to the ground. If in the woods, find an area
protected by a low clump of trees; never stand beneath a large tree
in the open. Be aware of the potential for flooding in low-lying
areas.
– Kneel or crouch with hands on knees.
– Avoid tall structures, such as towers, tall trees, fences, telephone
lines, or power lines.
Thunderstorm safety Cont…
– Stay away from natural lightning rods, such as golf
clubs, tractors, fishing rods, bicycles, or camping
equipment.
– Stay away from rivers, lakes or other bodies of water.
– If you are boating or swimming, get to land and find
shelter immediately.
– If you are isolated in a level field and feel your hair stand
on end, which indicates that lightning is about to strike,
drop your knees and bend forward, putting your hands
on your knees. Do not lie flat on the ground
Thunderstorm safety Cont…
 If you are in a vehicle:
– Pull safely onto the shoulder of the road, away from any trees that
could fall on the vehicle.
– If flash flooding is possible, go to higher ground.
– Stay in the vehicle and turn on the emergency flashers until the
heavy rains subside. If flash flooding is possible, abandon the
vehicle and climb to higher ground.
– Avoid flooded roadways.
Thunderstorm safety Cont…
 If you are indoors:
– Secure outdoor objects, such as lawn furniture, that
could blow away or cause damage or injury.
– Listen to a battery-operated radio or television for the
latest storm information.
– Do not handle any electrical equipment or telephones,
because lightning could follow the wire. Television sets
are particularly dangerous at this time. Use telephones
ONLY in an emergency.
– Do not take a bath or shower. Metal pipes can transmit
electricity.
– Turn off air conditioners. Power surges from lightning
can overload the compressors.
Severe Weather, continued
Hurricanes
 hurricane a severe storm that develops over tropical
oceans and whose strong winds of more than 120 km/h
spiral in toward the intensely low-pressure storm center
 A hurricane begins when warm, moist air over the ocean
rises rapidly.
 When moisture in the rising warm air condenses, a large
amount of energy in the from of latent heat is released.
This heat increase the force of the rising air.
Severe Weather, continued
Hurricanes, continued
 A fully developed hurricane consists of a series of thick
cumulonimbus cloud bands that spiral upward around
the center of the storm.
 The most dangerous aspect of a hurricane is a rising sea
level and large waves, called a storm surge.
 Every hurricane is categorized on the Saffir-Simpson
scale by using several factors. These factors include
central pressure, wind speed, and storm surge.
Hurricanes
 Hurricanes are giant, spiraling tropical storms that
can pack wind speeds of over 160 miles per hour,
and unleash more than 2.4 trillion gallons of rain a
day.
 These same tropical storms are known as
cyclones in the northern Indian Ocean and Bay of
Bengal, and as typhoons in the western Pacific
Ocean.
Hurricanes, cont.
 Katrina
– The Atlantic Ocean’s hurricane season peaks
from mid-August to late October and averages
five to six hurricanes per year.
Hurricanes, cont.
 Hurricanes begin as tropical disturbances in warm
ocean waters with surface temperatures of at least
80 degrees Fahrenheit.
 These low pressure systems are fed by energy
from the warm seas.
 If a storm achieves wind speeds of 38 mph it
becomes known as a tropical depression.
Hurricanes, cont.
 A tropical depression becomes a tropical
storm, and is given a name, when its
sustained wind speeds top 39 mph.
 When a storm’s sustained wind speeds
reach 74 mps it becomes a hurricane and
earns a category rating of 1 to 5 on the
Saffir-Simpson scale.
Hurricanes, cont.
 Hurricanes are enormous
heat engines that generate
energy on a staggering
scale.
 They draw heat from warm,
moist ocean air and release
it through condensation of
water vapor in
thunderstorms.

Hurricanes,
cont.
Hurricanes spin around a low-pressure center known as
the “eye”.
 Sinking air makes this 20 to 30 mile wide area
notoriously calm.
 But the eye is surrounded by a circular “eye wall” that
hosts the storm's strongest winds and rain.
Hurricanes, cont.
 These storms bring destruction ashore in
many different ways.
 When a hurricane makes landfall it often
produces a devastating storm surge that
can reach 20 ft high and extend nearly 100
miles.
 Ninety percent of all hurricane deaths
result from storm surges.
Hurricanes, cont.
 A hurricane’s high winds are also destructive
and may spawn tornadoes.
 Torrential rains cause further damage but
spawning floods and landslides, which may
occur many miles inland.
Safety
 The best defense against a hurricane is an
accurate forecast that gives people time to
get out of its way.
 The National Hurricane center issues
hurricane watches for storms that may
endanger communities, and hurricane
warning for storms that will make landfall
within 24 hrs.
Reading Check
Where do hurricanes develop?
Over warm tropical seas
Severe Weather, continued
Tornadoes
 tornado a destructive, rotating column of air that has very
high wind speeds and that maybe visible as a funnelshaped cloud
 The smallest, most violent, and shortest-lived severe storm
is a tornado.
 A tornado forms when a thunderstorm meets high-altitude
horizontal winds. These winds cause the rising air in the
thunderstorm to rotate.
Severe Weather, continued
Tornadoes, continued
 A storm cloud may develop a narrow, funnel-shaped rapidly
spinning extension that reaches downward and may or
may not touch the ground.
 If the funnel does touch the ground, it generally moves in a
wandering, haphazard path.
 The destructive power of a tornado is due to mainly the
speed of the winds. These winds may reach speeds of
more than 400 km/h.
Tornadoes
 Tornadoes are vertical funnels
or rapidly spinning air.
 Their winds may top 250 mph
and can clear-cut a pathway a
mile wide and 50 miles long.
 Twisters are born in
thunderstorms and are often
accompanied by hail.
 Giant, persistent thunderstorms
called super cells spawn the
most destructive tornadoes.
Tornadoes Cont…
 These violent storms occur around the
world, but the United States is a major
hotspot with about a thousand tornadoes
every year.
 “Tornado Alley” is a region that includes
several planes states and West
Michigan.
 It is the home to the most powerful and
destructive of these storms.
 In the U.S. tornadoes cause 80 deaths
and more than 1500 injuries per year.
Tornado Formation
 A tornado forms when changes in wind
speed and direction create a horizontal
spinning effect within a storm cell. This
effect is then tipped vertical by rising air
moving up through the thunderclouds.
Tornado Formation, cont.
 The meteorological factors that drive
tornadoes make them more likely at some
times than others.
 They occur more often in late afternoon,
when thunderstorms are common, and more
prevalent in spring and summer.
 However, tornadoes can and do form at any
time of the day and year.
Tornado Formation, cont.
 Tornadoes’ distinctive funnel clouds are
actually transparent.
 They become visible when water
droplets pulled from a storm’s moist air
condense or when dust or debris are
taken up.
 Funnels typically grow about 660 feet
wide.
Tornado Formation, cont.
 Tornadoes move at speeds of about 10 to
20 miles per hour, although they’ve been
clocked in bursts up to 70 miles per hour.
 Most don’t get very far though.
 They rarely travel more than about six miles
in their short lifetimes.
Deaths and Injuries
 Violent tornadoes make up only two
percent of all tornadoes, but they
cause 70% of all tornado deaths
and may last an hour or more.
 People, cars, and even buildings
may be hurled aloft by tornadoforce winds-or simply blown away.
 Most injuries and deaths from
tornadoes are actually caused by
flying debris.
Prediction
 Tornado forecasters can’t
provide the same kind of
warning that hurricane
watchers can, but they can
do enough to save lives.
 Today the average warning
time for a tornado alert is 13
minutes.
 Tornadoes can also be
identified by warning signs
that include a dark, greenish
sky, large hail, and a
powerful train-like roar.
Types of Fronts
Today’s Weather Map for the
U.S.
 Satellite map of the U.S.
– http://www.usatoday.com/weather/satpic/wsatusa.htm
 Precipitation map for U.S.
– http://www.usatoday.com/weather/wrain.htm
 Temperature map of the U.S.
– http://www.weather.com/maps/maptype/currentweatherusnational/uscurren
ttemperatures_large.html
 Wind Speed map for the U.S.
– http://www.weather.com/maps/maptype/currentweatherusnational/uscurren
twindsgusts_large.html
 Fronts map of the U.S.
– http://www.weather.com/maps/maptype/currentweatherusnational/index_lar
ge.html
Michigan’s Atmospere
 http://weather.cnn.com/weather/forecast.jsp
?locCode=MI24&zipCode=48846