Download Severe Storms and Patterns

Weather Patterns & Severe
Storms
Chapter 20
Air Masses
Section 20.1
Air Masses & Weather
 Air mass is immense body of air that is
characterized by similar temp & amount of
moisture @ any given altitude
– Can be 1600km or more across & several km thick
– May take days to move over an area
 Movement of air masses
– As it moves out of area, carries conditions w/it
– Characteristics of air mass changes & so does weather
in area over which it moves
Classifying Air Masses
 Gets it characteristic properties of temp & moisture
from source region (area over which it forms)
 Named according to their source regions
– Polar (P) air mass – high latitudes near poles – COLD
– Tropical (T) air mass – low latitudes – WARM
 Classified by surface over which they form
– Continental (c) air mass – over land – DRY
– Maritime (m) air mass – over water – HUMID
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4 basic types
1. Continental polar (cP) – dry & cool
2. Continental tropical (cT) – dry & warm/hot
3. Maritime polar (mP) – humid & cool
4. Maritime tropical (mT) – humid &
warm/hot
Weather in N.A.
 East of Rocky mtns influenced by cP
(Alaska & Canada) & mT (Gulf of Mexico)
 Continental Polar Air Masses
– Cold & dry winter; cool & dry summer
– Not assoc w/heavy precip.
– Cause lake effect snows when passing over
Great Lakes
 Air gets heated & moistened by water
 Drops off heat & moisture when it hits land again
 Maritime Tropical Air Masses
– Warm, inc. moisture, unstable
– Source of most precip. in U.S.
– High heat & humidity in summer
 Maritime Polar Air Masses
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Mild, humid, unstable
Low clouds & shower before it hits mtns
Heavy rain/snow on windward slopes of mtns
Nor’easter – when New England is on N or NW side of
passing low-pressure center, causes counterclockwise
rotation of mP air
 Continental Tropical Air Masses
– Least influence on weather
– Occasionally affect weather outside of source region
– Indian summer in fall
Fronts
Section 20.2
Formation of Fronts
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Front is boundary btwn 2 air masses
Formed when 2 air masses meet
Often assoc w/precip.
Narrow – 15-200 km wide
One air mass advances into another
causing some mixing of air
Types of Fronts
 Warm fronts
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Red line w/semicircles pointing in direction of wind
Warm air moves into area formerly covered by cooler air
Produce light-to-moderate precip
Wind shifts from E to SW
 Cold Front
– Blue line w/triangles pointing toward warm air
– Cold, dense air moves into region occupied by warmer
air
– More violent weather
– Weather soon calms after it passes
 Stationary Front
– Blue triangles on 1 side; red semicircles on other
– Flow of air on either side of front isn’t toward either cold
or warm air masses
– Sometimes gentle-to-moderate precip
 Occluded Front
– Active cold front overtakes slower moving warm front
– Complex weather patterns
Mid-Latitude Cyclones
 Large centers of low pressure that generally travel from WE and
cause stormy weather
 Air moves counterclockwise & in toward the low pressure area
 Most have cold front extending from central area; frequently have warm
fronts too
 Formation
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2 air masses w/diff temp move in opposite directions
Front creates wave shape
Warm air moves towards poles; cold air moves toward equator
Cold front lifts warm front (occlusion) causing storm to get stronger
 Destruction
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As more warm air is uplifted, pressure change lessens
Entire amount of warm air is displaced
Horizontal temp diff disappears
Nrg source is gone
Role of Airflow Aloft
 Maintains cyclonic & anticyclonic circulation
 Cyclones generally last for a week or longer
 Air in upper atmosphere fuels middle-lat
cyclone
 Cyclones bring stormy weather
 Anticyclones are found next to cyclone; air
moves clockwise
Severe Storms
Section 20.3
Thunderstorms (T-Storm)
 Storm that generates lightning & thunder
– Frequently produce gusty winds, heavy rain, & hail
 May be produced by single cumulonimbus cloud;
affect small area
 May be assoc w/clusters of cumulonimbus clouds;
affect larger area
 Occurrence of t-storms
– @ any given time, 2000+ storms on earth
– Higher # in tropics where warm, moist air & instability
are common
– 45,000 t-storm per day; 16 mill annually
– Most frequent in FL & east Gulf Coast
– Least frequent in western margin of U.S.
 Development of t-storms
– Arise when warm, humid air rises in an unstable
environment
– 3 stages
 1. Cumulus stage – updrafts supply warm, moist air
to cloud
 2. Mature stage – heavy precip, most active
 3. Dissipating stage – downdrafts dominate, causing
storm to die down
– Life span = 1-2 hrs
 Fresh supplies of warm, humid air generate new cells
to replace dying ones
Tornadoes
 Violent windstorms that take the form of a rotating column
of air called a vortex which extends down from
cumulonimbus cloud
– May consist of single or multiple vortices w/in a funnel
 Occurrence & development of tornadoes
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770 reported annually in US
Most frequent Apr-June
Most form in assoc w/severe t-storm
Most impt part in development is formation of mesocyclone
 Mesocyclone is vertical cylinder of rotating air located in updraft of tstorm
 Formation of vortex begins as winds in higher atmos cause lower
atmos to roll
 Rolling air tilts to vertical forming mesocyclone
 Mesocyclone formation does NOT mean tornado will develop
 Tornado Intensity
– Atmos pressure is lower w/in storm which
causes air to rush into the storm from all
directions
– Air eventually merges w/cloud
– Max winds can approach 480 km
– Intensity measured on Fujita tornado intensity
scale
 Developed by assessing damage – NOT speed
 Tornado Safety
– Storm Prediction Center (SPC) in Norman OK
monitors severe weather
– Watch – conditions are right for formation
– Warning – tornado spotted by person or radar
Hurricanes
 Tropical cyclones that produce winds of @
least 119km/hr are known in U.S. as
hurricanes
– Known in other parts of the world as: typhoon,
cyclone, tropical cyclone
 Most powerful storms on earth
– 15m high waves
– Strong winds
– Flooding
 Hurricane Floyd Sept 1999
– Deadliest since Hurricane Agnes in 1972 (* H.
Katrina surpassed)
– 2.5 mill people were evacuated
– Most deaths were due to drowning in floods
 Growing threat b/c most people live near
coast
– 50% of U.S. population is 75km from coast
 Occurrence of hurricanes
– Form btwn 5o -20o N & S latitude
– N. Pacific (most active) 20 per year
– U.S. fewer than 5, on average
– Many tropical disturbances do not make it to
hurricane level
 Development of hurricanes
– Late summer when water temps are warm
enough to provide necessary heat & moisture to
the air
– Begins as tropical disturbance – unorganized
group of clouds & t-storms
– Only a few t.d. become hurricanes
– Parts of a hurricane
 Eye wall – donut-shaped wall in center of storm;
highest wind speed & heaviest rainfall
 Eye – very center of storm; no precip & wind
subsides; warmest part of storm
 Hurricane intensity
– Saffir-Simpson Hurricane Scale
– Storm surge causes most devastating damage;
dome of water 65-80km wide that sweeps
across coast where hurricane’s eye moves onto
land
– Weakens when it moves over cool ocean
waters (decrease in moisture) or land (inc in
friction = blockage of air flow)