1. Severe Weather Thunderstorms, Tornadoes, Hurricanes, etc…

2. In this section, we will learn…
Tornadoes, hurricanes, blizzards, and thunderstorms are severe weather phenomena that impact society and ecosystems. Hazards include downbursts (wind shear), strong winds, hail, lightning, heavy rain, and flooding.
The movement of air in the atmosphere is due to differences in air density resulting from variations in temperature. Many weather conditions can be explained by fronts that occur when air masses meet.
E4.3A Describe the various conditions of formation associated with severe weather (thunderstorms, tornadoes, hurricanes, floods, waves, and drought).
E4.3B Describe the damage resulting from and the social impact of thunderstorms, tornadoes, hurricanes, and floods.
E4.3C Describe severe weather and flood safety and mitigation.
E4.3D Describe the seasonal variations in severe weather.
E4.3E Describe conditions associated with frontal boundaries that result in severe weather (thunderstorms, tornadoes, and hurricanes).

3. Thunderstorms
Thunderstorms are most often associated with a Cumulonimbus Cloud
This is a cloud that forms when air is forced high into the atmosphere (against its will)
Cold fronts often create this condition
This produces an UNSTABLE environment
The process of cooling, condensing, cloud formation, saturation and precipitation occurs quickly
Cold fronts often create the proper conditions for this to occur

4. Thunderstorm Formation
Thunderstorms have three stages…
The Cumulous Stage: Warm air rising and condensing to form clouds
The Mature Stage: Strong updraft of warm, moist air, AND a strong downdraft of wind and precipitation (hail and lightning can occur in this stage too)
The Dissipation Stage: The surface has been cooled, cutting off the warm, moist air that “fuels” the storm. Once the precipitation has fallen from the sky, the storm is over

5. Lightning
Lightning is believed to be created by friction between the rising updraft and the falling downdraft in a cumulonimbus cloud
This friction creates a separation of charges (kinda like rubbing your feet across the carpet in your socks)
Eventually there is a discharge between positive and negative charges, creating a “spark” (kinda like touching that doorknob and getting a shock)
On a larger scale, that “spark” is a bolt of lightning

7. Thunder
When lightning strikes, the “bolt” can be as hot as 30,000 degrees
The air surrounding that bolt of electricity expands rapidly (remember… warm air expands), and then contract quickly as it cools.
Because of the extremely high temperatures involved, the air expands and contracts rapidly enough to break the sound barrier (767 mph)
The thunder is a “sonic boom”

8. Wind & Downdrafts What goes up, must come down!
Thunderstorms are no different, when warm air rises high into the atmosphere, it eventually cools and comes back down toward the surface
Cumulonimbus clouds can be as tall as 40,000 feet, so…
It can fall very quickly toward the surface, producing strong winds (called downdrafts)

9. Hail

10. Thought to be the 1st photograph of a tornado (1884)
Tornadoes
Thought to be the 1st photograph of a tornado (1884)

11. Tornadoes
Violent windstorms that take the form of a rotating column of air, or vortex, that extends downward from a cumulonimbus cloud.
Because of the lower pressure in the center vortex, air near the ground rushes into the tornado from all directions.
Air streams inward, it is spiraled upward around the core until it eventually merges with the airflow or the parent thunderstorm deep in a cumulonimbus tower.

12. Tornadoes Some tornadoes consist of a single vortex.
However, within many stronger tornadoes are smaller intense whirls known as suction vortices, that orbit the center of the larger tornado → Multiple Vortex Tornadoes.
Usually die out in less than a minute.

13. Tornadoes
Suction vortices are responsible for most of the narrow, short waves of extreme damage that sometimes are through tornado tracks.
Most reports of several tornadoes at once actually were multiple vortex tornadoes.

14. Supercell Structure

15. Tornado Development Less than 1% of thunderstorms produce tornadoes.
Most intense tornadoes are associated with supercells (big, cumulonimbus cloud structures).
Tornado formation begins with the development of a mesocyclone.
Mesocyclone- Vertical cylinder of rotating air that develops in the updraft of a severe thunderstorm.

16. Tornado Development

17. Tornado Development *See supercell diagram for location.
The mesocyclone within the cloud stretches vertically and narrows horizontally, causing winds speeds to accelerate.
Air stretches downward until a portion of the cloud protrudes below the cloud base, producing a dark, slow rotating wall cloud.
*See supercell diagram for location.

18. Tornado Classification
Fujita Intensity Scale-
Assesses the damage produced by a storm as it relates to wind speed.
F0- Moderate; mph
F5- Severe; mph
Path of Destruction
Duration
Problem: Doesn’t take into account for structural integrity of objects.
The ENHANCED Fujita Scale (EF – Scale) is now used in the US. It DOES take into account structural damage (but still uses a 0-5 numerical scale)

19. Watches & Warnings Tornado Watch Tornado Warning
Conditions are ideal for a tornado to be created.
Tornado Warning
An actual tornado has been sighted in the area or is indicated by weather radar.

20. Tornado Frequency in the US

21. Tornado Frequency Around the World

22. Cool Tornado Pictures

31. Hurricanes

32. Hurricane Formation
The hurricanes that strike the Caribbean, Gulf of Mexico and the US originate in Africa
They begin as low pressure storm systems over land
The Trade Winds (one of the systems of global winds) blow them out over the warm water of the Atlantic Ocean

33. Hurricane Growth
Remember that low pressure pulls air in at the surface…
and pushes it to higher levels in the atmosphere
Air temperatures at high levels in the atmosphere are colder,
and the warm, moist air over the ocean begins to form clouds (BIG Cumulonimbus clouds)
Lots of Latent Heat released during condensation
Creates a very unstable environment

34. Continued Growth
Remember that the water over the Atlantic Ocean near Africa is warm (80°F) and has lots of moisture in it (mT air masses)
As precipitation begins, the rain and cool downdrafts of the Cumulonimbus system are not enough to cut off the updraft (it’s too warm)
The warm, moist air continues to fuel the system of Cumulonimbus clouds, making them bigger, and bigger, and bigger

35. Hurricane Movement
Once the growing low pressure system is out over the warm water of the Atlantic Ocean, the Trade Winds blow it to the west
Because the oceans are relatively smooth, there isn’t much to stand in its way and slow it down
With 2,000 to 3,000 miles of warm ocean water to move across…
the system has the time and the fuel to become severe

36. Tropical Depressions, Tropical Storms and Huricanes
Low pressure begins as a Tropical Disturbance
The low pressure systems that blow out over the waters of the Atlantic Ocean begin as Tropical Depressions (a low pressure system is in fact a “depression” of pressure)
If winds in the system reach 39mph, the system becomes classified as a Tropical Storm (and it is given a name)
If the system continues to gain strength and winds speeds reach 74 mph, it is officially classified as a hurricane

37. Hurricanes, Typhoons and Cyclones
We give different names to Tropical Storms that develop, depending on their location
In the Atlantic Ocean we call them Hurricanes
In the Pacific Ocean we call the Typhoons
In the Indian Ocean we call them Cyclones
They are all basically the same type of storm

38. Hurricane Dangers
The most dangerous component of a hurricane is the “Storm Surge”
As the storm makes landfall, a mound of ocean water (driven by strong winds AND the “sucking” power of the strong low pressure system) is pushed on shore
More people die as a result of drowning, than do of strong winds

39. Hurricane Classification The Safir-Simpson Scale
Type
KNOTS
MPH
millibars
inches of mercury
Feet
Meters
 Damage
Depression
less than 34
less than 39
------
Tropical Storm
Category 1
greater than 980
greater than 28.94
3 - 5
minimal
Category 2
6 - 8
moderate
Category 3
9 - 12
extensive
Category 4
extreme
Category 5
136+
156+
less than 920
less than 27.17
19+
5.7+
catastrophic

40. A Hurricane’s “Death”
As a hurricane makes landfall, the supply of warm, moist air that was fueling it is cut off
As the downdrafts and rain cool the land, the strength of the updraft slows
Without an updraft, the system will dissipate, and eventually die out (just like a thunderstorm)
This may take days and 100’s of miles
Hurricane Ike comes to Michigan in 2008 (my front yard in Commerce Township) >

41. Hurricane Katrina
In August of 2005, Tropical Storm Katrina developed over the Bahamas
As it approached Florida, it developed into a Category I hurricane
It lost some strength over the land (turned back into a Tropical Storm), but picked right back up when it moved back over the warm water of the Gulf of Mexico

42. Hurricane Katrina
As the storm moved across the Gulf of Mexico, it RAPIDLY developed (from a Category II to a Category V in only 9 hours)
Wind speeds reached 175 mph
A mound of water was pushed toward Louisiana and Mississippi…
even though wind speeds slowed (to a Category III) by the time it made landfall

43. Flooding
While the storm surge associated with Katrina was estimated to be 28 feet above normal sea level
In addition, much of the city is BELOW sea level
A systems of levees and dams were supposed to protect the city, but this was too high
85% of the city flooded

44. The Aftermath
The most destructive and costliest natural disaster in the history of the US
More than $81 billion dollars in damage
More that 1800 people died (700 still listed as missing)
More than 90,000 square miles declared a Federal Disaster Area
More than 3 million people were left without power

45. In Addition… There were unforeseen economic effects
Gas and Oil production shut-downs
Unemployment
Insurance Company Bankruptcies
Relocation/Flight from the region
Environmental Effects
Oil Spills
Erosion
Sewage
Loss of Marine and Animal life
Looting and Violence