1. Severe Weather

2. Air masses are classified on the basis of their source region

3. Air masses and weather Continental polar (cP)
From northern Canada and interior of Alaska
Winter – brings cold, dry air
Summer – brings cool relief
Responsible for lake-effect snows
cP air mass crosses the Great Lakes
Air picks up moisture from the lakes
Snow occurs on the leeward shores of the lakes

4. Air masses and weather Maritime tropical (mT)
From the Gulf of Mexico and the Atlantic Ocean
Warm, moist, unstable air
Brings precipitation to the eastern United States

5. Air masses and weather Continental tropical (cT) Southwest and Mexico
Hot, dry
Seldom important outside the source region

6. Air masses and weather Maritime polar (mP)
Brings precipitation to the western mountains
Occasional influence in the northeastern United States causes the "Northeaster" in New England with its cold temperatures and snow

7. Fronts Types of fronts Warm front Warm air replaces cooler air
Shown on a map by a line with semicircles
Small slope (1:200)
Clouds become lower as the front nears
Slow rate of advance
Light-to-moderate precipitation

8. Warm Front / High pressure system
Cold Front / Low
pressure system
Rain!

9. Fronts Types of fronts Cold front Cold air replaces warm air
Shown on a map by a line with triangles
Twice as steep (1:100) as warm fronts
Advances faster than a warm front
Associated weather is more violent than a warm front
Weather behind the front is dominated by
Cold air mass
Subsiding air
Clearing conditions

10. Warm Front / High pressure system
Cold Front / Low
pressure system
Warm Front / High pressure system
Rain/Snow
Thunderstorms

11. Fronts Types of fronts Stationary front Occluded front
Flow of air on both sides of the front is almost parallel to the line of the front
Surface position of the front does not move
Occluded front
Active cold front overtakes a warm front
Cold air wedges the warm air upward
Weather is often complex
Precipitation is associated with warm air being forced aloft

12. Middle-latitude cyclone
Primary weather producer in the middle-latitudes

13. Middle-latitude cyclone
Idealized weather
Middle-latitude cyclones move eastward across the United States
First signs of their approach are in the western sky
Require two to four days to pass over a region
Largest weather contrasts occur in the spring

14. Cloud patterns typically associated with a mature middle-latitude cyclone

15. Stages in the development of a thunderstorm

16. Severe weather types Tornadoes
A rapidly spinning column of air descending from a large thunderstorm
Have the highest wind speeds of any other weather phenomenon.
In the U.S., tornadoes move northeast at speeds up to 100 km/hr (62 mph)
Rotating wind speeds sometimes in excess of 500 km/hr (310 mph)

17. Severe weather types Tornadoes How a Tornado Forms
Moist air from Gulf of Mexico
Fast moving cold, dry air mass from Canada
Jet stream moving east at 150 mph
Sets up shearing conditions

18. Severe weather types Tornadoes How a Tornado Forms
Warm moist Gulf air releases latent heat, creates strong updraft
Updraft sheared by polar air, then twisted in a different direction by jet stream

19. Severe weather types Tornadoes
Why do some thunderstorms spawn tornadoes while others do not?
Super Cell Thunderstorms

20. Severe weather types Tornadoes Scales
The Fujita-Pearson Scale measures wind speed
The size of a tornado is not necessarily an indication of its intensity
Large tornadoes can be weak, and small tornadoes can be violent

22. Severe weather types Hurricanes
Only natural disaster that is given a human name
Actually large tropical cyclones
Convert heat in the ocean into winds
Exports excess heat from the tropics to the midlatitudes

23. Severe weather types Hurricanes How a Hurricane Works
Tropical disturbance
Low pressure zone develops and draws in clusters of thunderstorms and winds

24. Severe weather types Hurricanes How a Hurricane Works
Tropical disturbance
Tropical depression
Surface winds strengthen, move about the center of the storm
Central core funnels warm moist air up towards stratosphere
Air cools, vapor condenses, latent heat released
Fuels more updrafts, cycle repeats, storm grows

25. Severe weather types Hurricanes How a Hurricane Works
Tropical disturbance
Tropical depression
Tropical Storm
Storm has sustained surface wind speeds of +39 mph

26. Severe weather types Hurricanes How a Hurricane Works
Tropical disturbance
Tropical depression
Tropical Storm
Hurricane
Surface winds consistently over 74 mph

27. Severe weather types Hurricanes How a Hurricane Works
Tropical disturbance
Tropical depression
Tropical Storm
Hurricane
The Eye
As wind speed increases, winds are spiraled upwards prior to reaching the center
A distinctive clear “eye” is formed
Strongest winds are located on the walls of the eye

29. Hurricane Origins Form in the tropics ~ 5° and 20 ° latitude
Cannot form at the equator (Coriolis effect = 0)

30. Hurricanes Hurricane Damages Storm Surges
Large mound of water builds up beneath the eye
Reaches land as a surge of water

31. Hurricanes Hurricane Damages Storm Surges
Wind speed varies depending upon which side of the hurricane you’re on
Amount of damage on the coastline will vary accordingly

32. Hurricanes Hurricane Damages Heavy Rains
Mudflows and Debris Avalanches
Flooding

33. ~ End ~