1. Weather Patterns & Severe Storms
Chapter 20

2. Air Masses
Section 20.1

3. Air Masses & Weather
Air mass is immense body of air that is characterized by similar temp & amount of 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

4. 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

5. 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

7. 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

8. Maritime Tropical Air Masses
Warm, inc. moisture, unstable
Source of most precip. in U.S.
High heat & humidity in summer
Maritime Polar Air Masses
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

9. Fronts
Section 20.2

10. Formation of Fronts Front is boundary btwn 2 air masses
Formed when 2 air masses meet
Often assoc w/precip.
Narrow – km wide
One air mass advances into another causing some mixing of air

11. Types of Fronts Warm fronts Cold Front
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

12. Stationary Front Occluded 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

14. 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
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
As more warm air is uplifted, pressure change lessens
Entire amount of warm air is displaced
Horizontal temp diff disappears
Nrg source is gone

16. 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

18. Severe Storms
Section 20.3

19. 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, 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.

20. 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

22. 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
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 t-storm
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

25. 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

27. 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

28. Hurricanes
Tropical cyclones that produce winds 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

29. Growing threat b/c most people live near coast
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

31. 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

32. 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

34. 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)