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

2. Meteorology Weather-current state of the atmosphere.
Climate- describes the average weather over a long period of time

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. Three Types of Thunderstorms
1) air mass thunderstorm-results from air rising due to unequal heating in an air mass.
2) sea breeze thunderstorms-results from extreme temperature differences air over land and water.
3) frontal thunderstorms-results from advancing cold fronts.

5. How Thunderstorms Form
For a thunderstorm to form, three conditions must exist.
Abundant source of moisture
2. Mechanism to lift air so that the moisture can condense and release latent heat.
3. Instability in the atmosphere

6. Cumulus Stage Air rises vertically.
Moisture condenses into visible cloud.
Height at which condensation occurs is called lifted condensation level (LCL)
Coalesence

7. Mature Stage
Precipitation falls, cooling the air surrounding it and creating a downdraft.
Strong updraft of warm, moist air, AND a strong downdraft of wind and precipitation
Equal amount of updraft and downdraft creates a cumulonimbus cloud.

9. Dissipation Stage
The surface has been cooled, cutting off the warm, moist air that “fuels” the storm. Updraft ceases.
Lingering downdraft persists.
Once the precipitation has fallen from the sky, the storm is over

10. Limit of Thunderstorm Growth
The air in a thunderstorm will keep rising until:
1. Stable air is reached that it cannot overcome
2. Rate of condensation is not able to generate enough latent heat to keep the cloud warmer than the surrounding air

11. 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

12. 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”

13. 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)

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

15. Tornadoes
A tornado is a violent, whirling column of air in contact with the ground.
Before touching ground it is called a funnel cloud.

16. Tornadoes
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.

17. Supercell Structure

18. Tornado Development Less than 1% of thunderstorms produce tornadoes.
Most intense tornadoes are associated with supercells (big, cumulonimbus cloud structures).

19. Tornadoes Tornadoes are visible due to dust, debris, and condensation.
Created when wind speed and direction change suddenly with height, a process called wind shear.
Flash

20. Tornado Development

21. 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)

22. 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.

23. Tornado Frequency in the US

24. Tornado Frequency Around the World

25. Cool Tornado Pictures

31. Hurricanes

32. 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

33. Tropical Cyclones
Tropical cyclones are large, rotating, low-pressure storms that form over water during summer and fall in the tropics.
Two things to form:
Abundance of warm water
Disturbance to lift air and keep it rising

34. 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

35. 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

36. 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

37. 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

38. 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 (& given a name)
If the system continues to gain strength and winds speeds reach 74 mph, it is officially classified as a hurricane

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. 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

41. Hurricanes Eye-calm center of the storm
Eyewall-band immediately surrounding the eye that contains the strongest winds in a hurricane.
A hurricane will last until it can no longer produce enough energy to sustain itself. This usually happens when:
1) Hurricane runs over land.
2) Hurricane runs into colder water

42. 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