1. Chapter 20: Weather Patterns and Severe Storms

2. Air Masses and Weather Air masses: large bodies of air that move
similar temperatures and amounts of moisture at any given altitude
area experiences constant weather
as it moves, characteristics of air mass change and weather in the area changes

3. Classifying Air Masses
Named according to source region
Polar (P) masses (cold)
form at high latitudes toward Earth’s poles
Tropical (T) masses (warm)
form at low latitudes
Continental (c) masses (dry)
form over land
Maritime (m) masses (humid)
form over water

4. 4 Types of Masses continental polar (cP): dry and cool
continental tropical (cT): dry and warm/hot
maritime polar (mP): colder
maritime tropical (mT): hotter

5. Weather in North America
Influenced by continental polar (N. Canada, Alaska, and Arctic) and maritime tropical (Gulf of Mexico, Caribbean, or Atlantic) air masses

6. Continental polar air masses (cP)
cold and dry in winter
cool and dry in summer
not associated with heavy precipitation
lake-effect snow
localized storms
large difference in temperatures between land and water

7. Maritime polar air masses (mP)
come from North Pacific
accompanied by low clouds and showers
uplift produces heavy rain or snow
Maritime tropical air masses (mT)
warm with moisture
usually unstable
source of most precipitation

8. Continental tropical air masses (cT)
least influence on weather
hot and dry
begins in SW and Mexico during summer
can cause drought-like conditions

9. Formation of Fronts Front: a boundary that separates 2 air masses
occurs when 2 air masses meet
narrow ( km wide)
Warm, less dense air overlies cooler, denser air

10. Types of Fronts Classified according to temperature of advancing front

11. Warm Front warm air moves into area formerly covered by cooler air
Diagram:
warm air moves into area formerly covered by cooler air
represented by red arrows and semicircles
point toward cooler air
low, gradual slope
as warm air rises, it cools to produce clouds and precipitation
slow rate of movement
gradual increase in temperature

12. Cold Front cold, dense air moves into region of warmer air
shown by blue line with blue triangles
point toward warmer air
becomes steeper as it moves
moves rapidly
more violent weather
temperatures drop
winds shift
Diagram:

13. Stationary Front position of front does not move
shown by blue triangles on 1 side and red semicircles on the other
Diagram:
can produce gentle to moderate precipitation
Diagram:

14. Occluded Front active cold front overtakes warm front
cold air wedges warm front upward
Diagram:

15. Middle-Latitude Cyclones
main weather producers
represented by “L”
large centers of low pressure
gradually travel from west  east in counterclockwise direction
warm air moves toward poles
cool air moves toward equator
cause stormy weather
cold front pushes warm air up, strengthening storm
weakens pressure
stops cyclone

16. Role of Airflow Aloft maintains cyclonic and anticyclonic circulation
air high up in atmosphere fuels middle-latitude cyclone
surface air that feeds cyclone generally originates as air flowing out of an anticyclone
cyclones and anticyclones found next to each other

17. Thunderstorms

18. Thunderstorms generates lightning and thunder
produce gusty winds, heavy rain, and hail
can be produced by a single cumulonimbus cloud or clusters of them
can influence small area or km wide along a cold front

19. greater numbers in the tropics: warm, moist, and unstable
45,000 a day
16 million a year in the world
100,000 a year in the US
form when warm, humid air rises in an unstable environment

20. Cumulus Stage strong updrafts or upward movement of air
cloud grows vertically

21. Mature Stage
amount and size of precipitation too great for updrafts to support
heavy precipitation is released from cloud
most active stage
gusty wind, lightning, heavy precipitation, and hail

22. Dissipating Stage downdrafts/downward movement of air
cooling effect of falling precipitation
flowing in of cooler air
storm dies down

23. lifespan 1-2 hours
if storm moves, it can regenerate

24. Tornadoes

25. violent windstorms as a rotating column of air
called a vortex
extends downward from a cumulonimbus cloud
770 a year in the US
can occur at any time during the year
greatest from April to June
rare in December and January

26. most form with severe thunderstorms
development of mesocyclone
vertical cylinder of rotating air
develops in updraft of a thunderstorm

27. strong winds high up cause winds lower in atmosphere to roll
strong updrafts cause rolling to tilt
low pressure within tornado
air near ground rushes in and spirals upward around core

28. Tornado Safety
tornado watches: alert to possibility in a specific area for a time period
tornado warnings: actually been sighted in an area of indicated by radar

29. Hurricanes

30. whirling tropical cyclones
produce winds of at least 119 km/hr
called typhoons, cyclones, and tropical cycles outside the US

31. most powerful storms on Earth
most form between 5° and 20° N and S latitudes
formed/fueled by energy given off when huge amounts of the water vapor condense
develop mostly in late summer
water temperatures warm enough to provide heat and moisture to the air
most powerful storms on Earth
most form between 5° and 20° N and S latitudes
formed/fueled by energy given off when huge amounts of the water vapor condense
develop mostly in late summer
water temperatures warm enough to provide heat and moisture to the air

32. begins as tropical disturbance
disorganized clouds and thunderstorms
low pressures
little or no rotation

33. inward rush of warm, moist air moves toward core
turns upward and rises to cumulonimbus clouds
surrounds center of storm (eye wall)
greatest wind speeds
heaviest rainfall

34. Eye
center of storm
no rain or wind
warmest part of the storm

35. Storm Surge dome of water 65-80 km wide
sweeps across coast where eye moves onto land

36. weakens when it moves over cool ocean water or over land
Saffir-Simpson Scale: describes intensity (range from 1-5)