1. FrontsFronts

2. Fronts and Air MassesFronts and Air Masses
 Air massesAir masses: a huge: a huge
body of air withbody of air with
similarsimilar temperature,temperature,
humidityhumidity, and, and
pressurepressure..
 In America, we haveIn America, we have
four air masses thatfour air masses that
affect us. They areaffect us. They are
based onbased on
temperaturetemperature andand
humidity.humidity.

3. Air Masses and Global WindsAir Masses and Global Winds
 In America air masses are pushed by theIn America air masses are pushed by the
prevailing westerlies.prevailing westerlies.
 So they tend to moveSo they tend to move westwest toto easteast..

4. Types of Air MassesTypes of Air Masses
 TropicalTropical: form over the: form over the
tropics, so they aretropics, so they are
warmwarm and will haveand will have lowlow
air pressure.air pressure.
 PolarPolar: form north/south: form north/south
of 50º latitude. So theyof 50º latitude. So they
areare coldcold and will haveand will have
highhigh pressurepressure..
 MaritimeMaritime: form over the: form over the
oceans so they areoceans so they are
humidhumid..
 ContinentalContinental: form over: form over
the middle ofthe middle of continentscontinents
(land) so they are dry.(land) so they are dry.

5. Air Masses ComparedAir Masses Compared
TypeType TempTemp HumidityHumidity PressurePressure
MaritimeMaritime
tropicaltropical
(mT)(mT)
WarmWarm HumidHumid LowLow
MaritimeMaritime
polar (mP)polar (mP)
ColdCold HumidHumid HighHigh
ContinentalContinental
tropical (cT)tropical (cT)
WarmWarm DryDry LowLow
ContinentalContinental
polar (cP)polar (cP)
ColdCold DryDry highhigh

6. Weather FrontsWeather Fronts
 FrontsFronts: where two: where two
air masses meet andair masses meet and
don’t mix.don’t mix.
 Front is a militaryFront is a military
term where twoterm where two
armies meet toarmies meet to
fight.fight.
 The collision ofThe collision of
fronts often causesfronts often causes
storms andstorms and
changeable weather.changeable weather.
 There areThere are fourfour
kinds of fronts.kinds of fronts.

7. Cold FrontsCold Fronts
 ColdCold fronts have cold,fronts have cold,
dense air.dense air.
 They will slide underThey will slide under
warm, less dense air.warm, less dense air.
 So theSo the warmwarm air isair is
pushed up, whichpushed up, which
causes it tocauses it to coolcool..
 This means it can holdThis means it can hold
lessless waterwater vapor, sovapor, so
cloudsclouds form as itform as it
reaches itsreaches its dew pointdew point..
 This can causeThis can cause rainrain andand
storms behindstorms behind thethe
front.front.
 They move quickly, andThey move quickly, and
bring cool, dry airbring cool, dry air
after the front passes.after the front passes.

8. Warm FrontsWarm Fronts
 MovingMoving warmwarm air collides withair collides with coldcold air.air.
 Tend to move moreTend to move more slowlyslowly than cold fronts.than cold fronts.
 Can bring cloudy skies, showers, and light rainCan bring cloudy skies, showers, and light rain
preceding the frontpreceding the front..
 Weather will beWeather will be warmwarm andand humidhumid after itafter it
passes.passes.

10. StationaryStationary FrontsFronts
 Occur when cold andOccur when cold and
warm fronts meetwarm fronts meet
and neither is strongand neither is strong
enough to force theenough to force the
other to move.other to move.
 It is aIt is a “standoff.”“standoff.”
 The warm air willThe warm air will
condensecondense and formand form
rain, fog, snow, orrain, fog, snow, or
clouds.clouds.
 Can causeCan cause manymany daysdays
ofof wetwet weather.weather.

11. OccludedOccluded FrontsFronts
 Occur when aOccur when a warmwarm airair
mass ismass is trappedtrapped
between two cold airbetween two cold air
masses.masses.
 TheThe warmwarm air is pushedair is pushed
up when the two coldup when the two cold
air masses meet.air masses meet.
 It is cut off from theIt is cut off from the
ground, orground, or occludedoccluded..
 Tend to produce strongTend to produce strong
thunderstormsthunderstorms..

13. Weather Map SymbolsWeather Map Symbols
 These are theThese are the
symbols yousymbols you
would see on awould see on a
weather map.weather map.
 The arrows orThe arrows or
bumps point inbumps point in
the directionthe direction
the front isthe front is
moving.moving.

14. Low Pressure AreasLow Pressure Areas
 Low pressure areas willLow pressure areas will
have warm, rising air.have warm, rising air.
 Cool air around it willCool air around it will
spiral in counterspiral in counter
clockwise (Coriolisclockwise (Coriolis
effect).effect).
 As the air rises, it coolsAs the air rises, it cools
and reaches its dewand reaches its dew
point.point.
 So clouds and rain willSo clouds and rain will
form.form.
 Called cyclones from aCalled cyclones from a
Greek word meaningGreek word meaning
wheel.wheel.

15. High Pressure AreasHigh Pressure Areas
 High pressure centers ofHigh pressure centers of
dry air.dry air.
 Spiral outward towardsSpiral outward towards
areas of lower pressure.areas of lower pressure.
 Coriolis effect causesCoriolis effect causes
them to spiral clockwisethem to spiral clockwise
in the Northernin the Northern
Hemisphere.Hemisphere.
 Made of cool, falling air,Made of cool, falling air,
so its relative humidityso its relative humidity
drops.drops.
 Produces clear, dryProduces clear, dry
weather.weather.
 Called anticyclones.Called anticyclones.