Weather Patterns & Severe Weather Patterns & Severe StormsStorms

Chapter 20Chapter 20

Air MassesAir Masses

Section 20.1Section 20.1

Air Masses & WeatherAir Masses & Weather

Air mass is immense body of air that is Air mass is immense body of air that is characterized by similar temp & amount of characterized by similar temp & amount of moisture @ any given altitudemoisture @ any given altitude– Can be 1600km or more across & several km thickCan be 1600km or more across & several km thick– May take days to move over an areaMay take days to move over an area

Movement of air massesMovement of air masses– As it moves out of area, carries conditions w/itAs it moves out of area, carries conditions w/it– Characteristics of air mass changes & so does weather Characteristics of air mass changes & so does weather

in area over which it movesin area over which it moves

Classifying Air MassesClassifying Air Masses

Gets it characteristic properties of temp & moisture Gets it characteristic properties of temp & moisture from source region (area over which it forms)from source region (area over which it forms)

Named according to their source regionsNamed according to their source regions– Polar (P) air mass – high latitudes near poles – COLDPolar (P) air mass – high latitudes near poles – COLD– Tropical (T) air mass – low latitudes – WARMTropical (T) air mass – low latitudes – WARM

Classified by surface over which they formClassified by surface over which they form– Continental (c) air mass – over land – DRYContinental (c) air mass – over land – DRY– Maritime (m) air mass – over water – HUMIDMaritime (m) air mass – over water – HUMID

4 basic types4 basic types 1. Continental polar (cP) – dry & cool1. Continental polar (cP) – dry & cool 2. Continental tropical (cT) – dry & warm/hot2. Continental tropical (cT) – dry & warm/hot 3. Maritime polar (mP) – humid & cool3. Maritime polar (mP) – humid & cool 4. Maritime tropical (mT) – humid & 4. Maritime tropical (mT) – humid &

warm/hotwarm/hot

Weather in N.A.Weather in N.A.

East of Rocky mtns influenced by cP East of Rocky mtns influenced by cP (Alaska & Canada) & mT (Gulf of (Alaska & Canada) & mT (Gulf of Mexico)Mexico)

Continental Polar Air MassesContinental Polar Air Masses– Cold & dry winter; cool & dry summerCold & dry winter; cool & dry summer– Not assoc w/heavy precip.Not assoc w/heavy precip.– Cause lake effect snows when passing over Cause lake effect snows when passing over

Great LakesGreat Lakes Air gets heated & moistened by waterAir gets heated & moistened by water Drops off heat & moisture when it hits land againDrops off heat & moisture when it hits land again

Maritime Tropical Air MassesMaritime Tropical Air Masses– Warm, inc. moisture, unstableWarm, inc. moisture, unstable– Source of most precip. in U.S.Source of most precip. in U.S.– High heat & humidity in summerHigh heat & humidity in summer

Maritime Polar Air MassesMaritime Polar Air Masses– Mild, humid, unstableMild, humid, unstable– Low clouds & shower before it hits mtnsLow clouds & shower before it hits mtns– Heavy rain/snow on windward slopes of mtns Heavy rain/snow on windward slopes of mtns – Nor’easter – when New England is on N or NW side of Nor’easter – when New England is on N or NW side of

passing low-pressure center, causes counterclockwise passing low-pressure center, causes counterclockwise rotation of mP airrotation of mP air

Continental Tropical Air MassesContinental Tropical Air Masses– Least influence on weatherLeast influence on weather– Occasionally affect weather outside of source regionOccasionally affect weather outside of source region– Indian summer in fall Indian summer in fall

FrontsFronts

Section 20.2 Section 20.2

Formation of FrontsFormation of Fronts

Front is boundary btwn 2 air massesFront is boundary btwn 2 air masses Formed when 2 air masses meetFormed when 2 air masses meet Often assoc w/precip.Often assoc w/precip. Narrow – 15-200 km wideNarrow – 15-200 km wide One air mass advances into another One air mass advances into another

causing some mixing of aircausing some mixing of air

Types of FrontsTypes of Fronts

Warm frontsWarm fronts– Red line w/semicircles pointing in direction of windRed line w/semicircles pointing in direction of wind– Warm air moves into area formerly covered by cooler airWarm air moves into area formerly covered by cooler air– Produce light-to-moderate precipProduce light-to-moderate precip– Wind shifts from E to SWWind shifts from E to SW

Cold FrontCold Front– Blue line w/triangles pointing toward warm airBlue line w/triangles pointing toward warm air– Cold, dense air moves into region occupied by warmer Cold, dense air moves into region occupied by warmer

airair– More violent weatherMore violent weather– Weather soon calms after it passesWeather soon calms after it passes

Stationary FrontStationary Front– Blue triangles on 1 side; red semicircles on otherBlue triangles on 1 side; red semicircles on other– Flow of air on either side of front isn’t toward either cold Flow of air on either side of front isn’t toward either cold

or warm air massesor warm air masses– Sometimes gentle-to-moderate precipSometimes gentle-to-moderate precip

Occluded FrontOccluded Front– Active cold front overtakes slower moving warm frontActive cold front overtakes slower moving warm front– Complex weather patternsComplex weather patterns

Mid-Latitude CyclonesMid-Latitude Cyclones Large centers of low pressure that generally travel from WLarge centers of low pressure that generally travel from WE and E and

cause stormy weathercause stormy weather Air moves counterclockwise & in toward the low pressure areaAir moves counterclockwise & in toward the low pressure area Most have cold front extending from central area; frequently have Most have cold front extending from central area; frequently have

warm fronts toowarm fronts too FormationFormation

– 2 air masses w/diff temp move in opposite directions2 air masses w/diff temp move in opposite directions– Front creates wave shapeFront creates wave shape– Warm air moves towards poles; cold air moves toward equatorWarm air moves towards poles; cold air moves toward equator– Cold front lifts warm front (occlusion) causing storm to get strongerCold front lifts warm front (occlusion) causing storm to get stronger

DestructionDestruction– As more warm air is uplifted, pressure change lessensAs more warm air is uplifted, pressure change lessens– Entire amount of warm air is displacedEntire amount of warm air is displaced– Horizontal temp diff disappearsHorizontal temp diff disappears– Nrg source is gone Nrg source is gone

Role of Airflow AloftRole of Airflow Aloft

Maintains cyclonic & anticyclonic circulationMaintains cyclonic & anticyclonic circulation Cyclones generally last for a week or longerCyclones generally last for a week or longer Air in upper atmosphere fuels middle-lat Air in upper atmosphere fuels middle-lat

cyclonecyclone Cyclones bring stormy weatherCyclones bring stormy weather Anticyclones are found next to cyclone; air Anticyclones are found next to cyclone; air

moves clockwisemoves clockwise

Severe StormsSevere Storms

Section 20.3 Section 20.3

Thunderstorms (T-Storm)Thunderstorms (T-Storm)

Storm that generates lightning & thunderStorm that generates lightning & thunder– Frequently produce gusty winds, heavy rain, & hailFrequently produce gusty winds, heavy rain, & hail

May be produced by single cumulonimbus cloud; May be produced by single cumulonimbus cloud; affect small areaaffect small area

May be assoc w/clusters of cumulonimbus clouds; May be assoc w/clusters of cumulonimbus clouds; affect larger areaaffect larger area

Occurrence of t-stormsOccurrence of t-storms– @ any given time, 2000+ storms on earth@ any given time, 2000+ storms on earth– Higher # in tropics where warm, moist air & instability Higher # in tropics where warm, moist air & instability

are commonare common– 45,000 t-storm per day; 16 mill annually45,000 t-storm per day; 16 mill annually– Most frequent in FL & east Gulf CoastMost frequent in FL & east Gulf Coast– Least frequent in western margin of U.S.Least frequent in western margin of U.S.

Development of t-stormsDevelopment of t-storms– Arise when warm, humid air rises in an unstable Arise when warm, humid air rises in an unstable

environmentenvironment– 3 stages3 stages

1. Cumulus stage – updrafts supply warm, moist air 1. Cumulus stage – updrafts supply warm, moist air to cloudto cloud

2. Mature stage – heavy precip, most active2. Mature stage – heavy precip, most active 3. Dissipating stage – downdrafts dominate, causing 3. Dissipating stage – downdrafts dominate, causing

storm to die downstorm to die down

– Life span = 1-2 hrsLife span = 1-2 hrs Fresh supplies of warm, humid air generate new cells Fresh supplies of warm, humid air generate new cells

to replace dying onesto replace dying ones

TornadoesTornadoes

Violent windstorms that take the form of a rotating column Violent windstorms that take the form of a rotating column of air called a vortex which extends down from of air called a vortex which extends down from cumulonimbus cloudcumulonimbus cloud– May consist of single or multiple vortices w/in a funnelMay consist of single or multiple vortices w/in a funnel

Occurrence & development of tornadoesOccurrence & development of tornadoes– 770 reported annually in US770 reported annually in US– Most frequent Apr-JuneMost frequent Apr-June– Most form in assoc w/severe t-stormMost form in assoc w/severe t-storm– Most impt part in development is formation of mesocycloneMost impt part in development is formation of mesocyclone

Mesocyclone is vertical cylinder of rotating air located in updraft of t-Mesocyclone is vertical cylinder of rotating air located in updraft of t-stormstorm

Formation of vortex begins as winds in higher atmos cause lower Formation of vortex begins as winds in higher atmos cause lower atmos to roll atmos to roll

Rolling air tilts to vertical forming mesocycloneRolling air tilts to vertical forming mesocyclone Mesocyclone formation does NOT mean tornado will developMesocyclone formation does NOT mean tornado will develop

Tornado IntensityTornado Intensity– Atmos pressure is lower w/in storm which Atmos pressure is lower w/in storm which

causes air to rush into the storm from all causes air to rush into the storm from all directionsdirections

– Air eventually merges w/cloudAir eventually merges w/cloud– Max winds can approach 480 kmMax winds can approach 480 km– Intensity measured on Fujita tornado intensity Intensity measured on Fujita tornado intensity

scalescale Developed by assessing damage – NOT speedDeveloped by assessing damage – NOT speed

Tornado SafetyTornado Safety– Storm Prediction Center (SPC) in Norman OK Storm Prediction Center (SPC) in Norman OK

monitors severe weathermonitors severe weather– Watch – conditions are right for formation Watch – conditions are right for formation – Warning – tornado spotted by person or radar Warning – tornado spotted by person or radar

HurricanesHurricanes

Tropical cyclones that produce winds of @ Tropical cyclones that produce winds of @ least 119km/hr are known in U.S. as least 119km/hr are known in U.S. as hurricaneshurricanes– Known in other parts of the world as: typhoon, Known in other parts of the world as: typhoon,

cyclone, tropical cyclonecyclone, tropical cyclone Most powerful storms on earthMost powerful storms on earth

– 15m high waves15m high waves– Strong windsStrong winds– FloodingFlooding

Hurricane Floyd Sept 1999Hurricane Floyd Sept 1999– Deadliest since Hurricane Agnes in 1972 (* H. Deadliest since Hurricane Agnes in 1972 (* H.

Katrina surpassed)Katrina surpassed)– 2.5 mill people were evacuated2.5 mill people were evacuated– Most deaths were due to drowning in floodsMost deaths were due to drowning in floods

Growing threat b/c most people live near Growing threat b/c most people live near coastcoast– 50% of U.S. population is 75km from coast50% of U.S. population is 75km from coast

Occurrence of hurricanesOccurrence of hurricanes– Form btwn 5Form btwn 5oo -20 -20oo N & S latitude N & S latitude– N. Pacific (most active) 20 per yearN. Pacific (most active) 20 per year– U.S. fewer than 5, on averageU.S. fewer than 5, on average– Many tropical disturbances do not make it to Many tropical disturbances do not make it to

hurricane levelhurricane level

Development of hurricanesDevelopment of hurricanes– Late summer when water temps are warm Late summer when water temps are warm

enough to provide necessary heat & moisture to enough to provide necessary heat & moisture to the airthe air

– Begins as tropical disturbance – unorganized Begins as tropical disturbance – unorganized group of clouds & t-stormsgroup of clouds & t-storms

– Only a few t.d. become hurricanesOnly a few t.d. become hurricanes– Parts of a hurricaneParts of a hurricane

Eye wall – donut-shaped wall in center of storm; Eye wall – donut-shaped wall in center of storm; highest wind speed & heaviest rainfallhighest wind speed & heaviest rainfall

Eye – very center of storm; no precip & wind Eye – very center of storm; no precip & wind subsides; warmest part of stormsubsides; warmest part of storm

Hurricane intensityHurricane intensity– Saffir-Simpson Hurricane ScaleSaffir-Simpson Hurricane Scale– Storm surge causes most devastating damage; Storm surge causes most devastating damage;

dome of water 65-80km wide that sweeps dome of water 65-80km wide that sweeps across coast where hurricane’s eye moves onto across coast where hurricane’s eye moves onto landland

– Weakens when it moves over cool ocean Weakens when it moves over cool ocean waters (decrease in moisture) or land (inc in waters (decrease in moisture) or land (inc in friction = blockage of air flow)friction = blockage of air flow)