Middle School Academics

1. Chapter 3Chapter 3
Section 1Section 1
Air Masses andAir Masses and
FrontsFronts

2. Four Major Types of Air MassesFour Major Types of Air Masses
in North Americain North America
Maritime TropicalMaritime Tropical
Maritime PolarMaritime Polar
Continental TropicalContinental Tropical
Continental PolarContinental Polar

3. Maritime TropicalMaritime Tropical
 Warm, humid air masses form over oceans nearWarm, humid air masses form over oceans near
the tropicsthe tropics
 Influence weather in the central & eastern UnitedInfluence weather in the central & eastern United
States when formed over the Gulf of Mexico & theStates when formed over the Gulf of Mexico & the
Atlantic OceanAtlantic Ocean
 Influence weather on the West Coast when formedInfluence weather on the West Coast when formed
over the Pacific Oceanover the Pacific Ocean
 Summer – hot, humid weatherSummer – hot, humid weather
 Winter – humid air can bring heavy rain or snowWinter – humid air can bring heavy rain or snow

4. Maritime PolarMaritime Polar
 Cool, humid air masses form over the icyCool, humid air masses form over the icy
cold North Pacific & North Atlantic oceanscold North Pacific & North Atlantic oceans
 Affect the West Coast more than the EastAffect the West Coast more than the East
CoastCoast
 In summer, often bring fog, rain, & coolIn summer, often bring fog, rain, & cool
temperatures to the West Coasttemperatures to the West Coast

5. Continental TropicalContinental Tropical
 Hot, dry air masses form only in summerHot, dry air masses form only in summer
over dry areas of the Southwest & northernover dry areas of the Southwest & northern
MexicoMexico
 Covers a smaller area than other airCovers a smaller area than other air
massesmasses
 Move northeast, brings hot, dry weather toMove northeast, brings hot, dry weather to
the southern Great Plainsthe southern Great Plains

6. Continental PolarContinental Polar
 Large masses that form over central & northernLarge masses that form over central & northern
Canada & AlaskaCanada & Alaska
 Brings cool or cold airBrings cool or cold air
 In winter, bring clear, cold, dry air to much of NorthIn winter, bring clear, cold, dry air to much of North
AmericaAmerica
 Can bring bitterly cold weather with very lowCan bring bitterly cold weather with very low
humidity.humidity.
 In summer, storms may occur when air massesIn summer, storms may occur when air masses
move south & meet maritime tropical air massesmove south & meet maritime tropical air masses
moving northmoving north

7. Four Types of FrontsFour Types of Fronts
Cold FrontsCold Fronts
Warm FrontsWarm Fronts
Stationary FrontsStationary Fronts
Occluded FrontsOccluded Fronts

8. Cold FrontsCold Fronts
 When a rapidly moving cold air mass runs into aWhen a rapidly moving cold air mass runs into a
slowly moving warm air mass, the denser cold airslowly moving warm air mass, the denser cold air
slides under the lighter warm airslides under the lighter warm air
 The warm air is then pushed upward resulting in aThe warm air is then pushed upward resulting in a
cold frontcold front
 Cold fronts move quickly & can cause quickCold fronts move quickly & can cause quick
changes in the weather, including violentchanges in the weather, including violent
thunderstormsthunderstorms
 Once a cold front moves through an area, cool,Once a cold front moves through an area, cool,
dry air moves in and often brings clear skies &dry air moves in and often brings clear skies &
cooler temperaturescooler temperatures

9. Warm FrontsWarm Fronts
 A moving warm air mass collides with a slowly moving coldA moving warm air mass collides with a slowly moving cold
air massair mass
 As in a cold front, the warm air rises above the cold airAs in a cold front, the warm air rises above the cold air
 If the warm air is humid, showers & light rain fall along theIf the warm air is humid, showers & light rain fall along the
front when the two air masses meetfront when the two air masses meet
 If the warm air is dry, scattered clouds formIf the warm air is dry, scattered clouds form
 Warm fronts move much slower than cold fronts, therefore,Warm fronts move much slower than cold fronts, therefore,
the weather may be rainy or foggy for several daysthe weather may be rainy or foggy for several days
 After a warm front passes through an area, warm & humidAfter a warm front passes through an area, warm & humid
weather will followweather will follow
 In winter, warm fronts bring snowIn winter, warm fronts bring snow

10. Stationary FrontsStationary Fronts
 Sometimes cold & warm air masses meet,Sometimes cold & warm air masses meet,
but neither has enough force to move thebut neither has enough force to move the
other; the result a stationary frontother; the result a stationary front
 When the two air masses meet, water vaporWhen the two air masses meet, water vapor
in the warm air condenses into rain, snow,in the warm air condenses into rain, snow,
fog, or cloudsfog, or clouds
 If the front is stalled, the weather may beIf the front is stalled, the weather may be
days of clouds & precipitationdays of clouds & precipitation

11. Occluded FrontsOccluded Fronts
 The most complex of the four frontsThe most complex of the four fronts
 At an occluded front, a warm air mass is caughtAt an occluded front, a warm air mass is caught
between two cooler air massesbetween two cooler air masses
 The two cooler air masses meet & may mixThe two cooler air masses meet & may mix
 Temperature near the ground becomes coolerTemperature near the ground becomes cooler
 The warm air mass is cut off (occluded) from theThe warm air mass is cut off (occluded) from the
groundground
 As the warm air cools & its water vaporAs the warm air cools & its water vapor
condenses, the weather may turn cloudy & rainy orcondenses, the weather may turn cloudy & rainy or
snowysnowy

12. CyclonesCyclones
 An area of relatively low air pressure is called a cycloneAn area of relatively low air pressure is called a cyclone
 Low air pressure is represented on a weather map as aLow air pressure is represented on a weather map as a LL
 Cyclone spin counterclockwise in the Northern HemisphereCyclone spin counterclockwise in the Northern Hemisphere
 Cyclones play a large part in the weather of the UnitedCyclones play a large part in the weather of the United
StatesStates
 As air rises in a cyclone, the air cools, forming clouds &As air rises in a cyclone, the air cools, forming clouds &
precipitationprecipitation
 Cyclones & decreasing air pressure are associated withCyclones & decreasing air pressure are associated with
storms & precipitationstorms & precipitation

13. AnticyclonesAnticyclones
 Opposite of cyclones in most waysOpposite of cyclones in most ways
 Anticyclones are high-pressure centers of dry airAnticyclones are high-pressure centers of dry air
 High pressure areas are represented on a weather map asHigh pressure areas are represented on a weather map as
aa HH
 Anticyclones spin clockwise in the Northern HemisphereAnticyclones spin clockwise in the Northern Hemisphere
 Cool air moves downward from higher in the troposphereCool air moves downward from higher in the troposphere
 As the cool air falls, it warms up, so its relative humidityAs the cool air falls, it warms up, so its relative humidity
dropsdrops
 Descending air in an anticyclone causes dry, clear weatherDescending air in an anticyclone causes dry, clear weather