This document provides information about three types of severe weather associated with cumulonimbus clouds: thunderstorms, tornadoes, and hurricanes. It examines how each forms from different lifting mechanisms in the atmosphere and discusses their characteristics. Thunderstorms form from warm, moist air that is lifted violently by cold fronts. Tornadoes are rotating columns of air that form from wind shear. Hurricanes are intense rotating storms that form over tropical oceans from converging winds and require warm ocean waters to strengthen. The document outlines the damage potential of each, including lightning, hail, strong winds, and flooding from thunderstorms; winds and rotation from tornadoes; and winds, storm surge, and inland flooding from hurricanes.

1. Severe Weather
Thunderstorms
Tornadoes
Hurricanes

2. Severe Weather
• Refers to any dangerous meteorological
phenomena with the potential to cause damage,
serious social disruption, or loss of human life.
We will be examining 3 types of sever weather associated with
Cumulonimbus clouds that form in completely different ways.
Thunderstorms Tornadoes Hurricanes

3. What are the 4 different ways in which air can
be lifted to create condensation?
Warm air rises.
Convectional
Lifting
Air rises up over a
mountain.
Orographic Lifting
A cold front
lifts air up as it
moves.
Frontal Lifting
Wind hits from
opposite directions
and goes up.
Convergence

4. What is a Severe Thunderstorm?
• Thunderstorms are storm that produces…
– Lightning
– Thunder
– Heavy Rain/Precipitation/Hail/Flooding
– Strong Winds
• These storms last approximately 15-60 minutes.
• A Severe Thunderstorm is classified as a thunderstorm with sustained
winds over 58 mph.

5. How do thunderstorms form?
• When warm moist air is lifted up violently (cold front).
• The air rises to form a cumulonimbus cloud.

6. How does lightning form?
• Updrafts and downdrafts of wind and rain cause parts of a cloud to
become electrically charged.
• Molecules rub together (like when you rub your feet over carpet)
– The colder falling air gains electrons from the warm rising air.
– (+) charge at top of cloud
– (-) charge at bottom of cloud
• The charges build up and are released.
• The release of energy finds the nearest positive charge.

7. How does lightning form?
• Lightning and the Van de Graf Generator

8. Where does lightning strike?
• From Cloud to Cloud • From Cloud to Ground
• There is NO such thing as “Heat Lightning” = FAR AWAY Thunderstorm

9. What is thunder?
• Lightning bolt heats the air that surrounds it rapidly.
– Air expands from the gain of energy.
– The thunder is the noise of the air molecules moving rapidly.

10. How does hail form?
• We have discussed this before…here is the animation!

11. Tornadoes

12. How does a tornado form?
Tornadoes are rotating columns of air formed in cumulonimbus clouds.
1. Strong updrafts and downdrafts are formed in a cold front.
2. Wind shear at surface
3. Downdrafts split the wind shear in half creating two columns of
opposite spinning air.
4. The counter-clockwise wind shear is forced up with the updrafts forming
a vortex of spinning air.
5. Condensation forms around the vortex and touches ground.

13. How are tornadoes classified?
Fujita Scale
• A scale for rating tornado intensity, based primarily on the damage
tornadoes inflict on human-built structures and vegetation.
• Official scale is rated after a tornado when damage is assessed by
meteorologists data and engineers surveys.

14. What is a hurricane?
A hurricane is an intense
rotating storm that
forms over the tropical
oceans.
• “Hurricane” is a specific regional
name. In general, these storms
are known as “tropical cyclones”:
– Tropical depression:winds < 39 mph
– Tropical storm: winds < 74 mph
– Hurricane: winds > 74 mph
( gets named! )

15. - Atlantic and Eastern Pacific Oceans (hurricanes)
- Western North Pacific Ocean (typhoons)
- South Pacific and Indian Oceans (cyclones)
Where do tropical cyclones form?
(10)
(26)
(14)
(15)
(5)
(9)
(nn) Number of storms per year

16. Hurricanes affecting the U.S., 1950-2009

17. Outline
• What hurricanes are and where they form
• How hurricanes work
• How hurricanes cause damage
• How we forecast hurricanes

18. Why are the winds so strong in a hurricane?
Winds always move from areas
of higher pressure to areas of
lower pressure. The bigger the
difference between the high
and low pressure, the stronger
the winds.
H L
The winds from
Hurricane Fabian
battering Bermuda on
September 5, 2003.

19. What causes the winds to rotate around a hurricane?
Hurricane Isabel making landfall
on September 18, 2003
H
H H
H
L
The Coriolis Effect: A change in the
direction of moving objects due to
those objects moving on a rotating,
spherical planet.

20. • Show Vis/IR satellite loop of Katrina…

21. How is a lawn mower like a hurricane?
Gas provides the
fuel for the
mower’s engine
The engine uses
the fuel to
perform work
and move the
grass-cutting
blade
The starter rope
and spark plug are
the “triggers” for
making the mower
start
Exhaust fumes
are expelled
out away from
the engine

22. The hurricane heat engine: how it works
3. Having lost most of its heat and
energy, the air is now expelled outward
away from the center of the storm in the
cooler hurricane outflow at high levels.
In mature hurricanes, an eye forms
in the center. Here, the air is
descending and the region is
generally cloud-free.
2. The warm, moist air rises in
thunderstorms surrounding the
eye, supplying the fuel that
helps strengthen the storm and
intensify the winds.
1. As air rushes into the hurricane’s low pressure center, the air picks
up heat and moisture (fuel) from the warm ocean surface.
H HL

23. …but where’s the trigger?
• Hurricanes don’t just form out of thin air.
They typically grow from clusters of smaller
thunderstorms that move across the oceans.

24. Hurricane formation
• Hurricanes grow from clusters of smaller
storms and become more organized and
intense as they develop.
Daily images of
Hurricane
Frances (2004),
from a weak
tropical
depression to a
Category-4
hurricane… to
landfall

25. Outline
• What hurricanes are and where they form
• How hurricanes work
• How hurricanes cause damage
• How we forecast hurricanes

26. The triple threat of damage from a hurricane:
• Wind Damage: Wind gusts can approach 200 mph in
the strongest hurricanes.
• Storm Surge: The winds of a hurricane pile up ocean
water that can inundate coastlines with water levels up
to 30 feet higher than normal.
• Inland Flooding: Today, this is the greatest risk to
life from a hurricane. Hurricanes and tropical storms,
especially slow-moving ones, can drop huge amounts of
rain in a very short time, even inland far from where
the storm made landfall. From 1970-2000, 9 out of
every 10 fatalities in tropical cyclones were due to
drowning from inland flooding.

27. Wind Damage examples
A 1x4 board of
wood was
pierced
through a palm
tree by
Hurricane
Andrew’s winds
in August,
1992.
A mobile home park near Miami, FL was completely
destroyed by Hurricane Andrew in August, 1992.
An historic home along the Gulf Coast before and after the passage of
Category 5 Hurricane Camille in August, 1969.

28. The Saffir-Simpson Hurricane Wind Scale: A
method for categorizing hurricanes based on wind
speed
Category 1 Category 2 Category 3 Category 4 Category 5
Wind
speed (mph)
74-95 96-110 111-130 131-155 >155
Storm
surge (ft)
4-5 6-8 9-12 13-18 >18
Damage
description
Minimal Moderate Extensive Extreme Catastrophic

29. Storm Surge examples
Storm surge
from
Hurricane
Carol
completely
inundates a
yacht club in
August, 1954.
Apartment building in Pass Christian, Mississippi before and after the passage
of Category 5 Hurricane Camille in August, 1969. Thirty-two people ignored
warnings and stayed for a “hurricane party”. Only two survived.
Hurricane Katrina’s 30-foot storm surge obliterated
every home within several blocks of the shoreline in
Pass Christian and Waveland, Mississippi.

30. Storm Surge examples
Waveland, MS
David & Kimberly King’s house, 4 blocks
from the beach, on August 28
(1 day before Katrina hit)
…the same property, same view, on
September 2
(4 days after Katrina hit)

31. • Show car flooding video…

32. How are hurricanes different from tornadoes?
Hurricanes Tornadoes
Where they form Hurricanes form over warm
water in the tropical oceans
and develop best when far
from the jet stream.
Tornadoes form over land and
form within storms that are
often very close to the jet
stream
How big they are Up to several hundred miles wide No more than ¼ mile wide
How long they last Average of 9 days (up to 3
weeks)
No more than an hour
How strong the winds are Less than 200 mph Up to 300 mph
Occurrences per year An average of 10 tropical storms
in the Atlantic Ocean
In the United States, 800-1000
Advance warning from
forecasters
Several days No more than 15-30 minutes

33. Outline
• What hurricanes are and where they form
• How hurricanes work
• How hurricanes cause damage
• How we forecast hurricanes

34. Two NOAA P3
“Hurricane
Hunter” aircraft
View from a
Hurricane Hunter
aircraft inside the
eye of Hurricane
Isabel while a
category 5 storm.
Aircraft:
Commercial aircraft
and “Hurricane
Hunters”
Forecasting a hurricane’s track and intensity
1. Observations: Measuring the atmosphere
Before we can predict what a storm will do in the future, we need to
know what the storm and the atmosphere look like right now.
Radiosondes:
These instruments
are attached to
weather balloons
Satellites
Surface

35. On board a recon flight into Hurricane Rita….

36. On board a recon flight into Hurricane Rita….

37. On board a recon flight into Hurricane Rita….

38. Forecasting a hurricane’s track and intensity
2. Running a computer model

40. Forecasting a hurricane’s track and intensity
3. Making heads or tails of all the model forecast data
Sometimes
the
models
agree with
each
other…
This plot shows the computer model track
forecasts for Hurricane Katrina issued
almost 3 days before she made landfall.
The fact that the models agreed so well
with one another made it relatively easy for
the National Hurricane Center to forecast
her landfall location.
In this case, the forecast at this
particular time for Hurricane Kate
became a difficult one for the National
Hurricane Center because the different
computer model forecast tracks varied so
wildly.
…and sometimes they don’t!

41. Outlook for 2010 Atlantic Hurricane Season
• 85% chance of an
above-normal
hurricane season
• 14-23 named
storms, 8-14 of
which will become
hurricanes
2010 Storm Names
Alex
Bonnie
Colin*
Danielle
Earl
Fiona*
Gaston
Hermine
Igor*
Julia*
Karl
Lisa
Matthew
Nicole
Otto
Paula
Richard
Shary
Tomas
Virginie
Walter

42. Outline
• What hurricanes are and where they form
• How hurricanes work
• How hurricanes cause damage
• How we forecast hurricanes
• Hurricanes and climate

43. Hurricanes and climate
1. What is El Nino and does it have any effect on hurricanes?
El Nino and the accompanying Southern Oscillation, referred to
collectively as “ENSO”, occur in the tropical regions of the Pacific
Ocean. During an El Nino event, the trade winds weaken and sea
surface layers become anomalously warm in the central and eastern
Pacific Ocean.
A fairly strong relationship exists between El Nino (“ENSO warm
events”) and the suppression of hurricane activity in the Atlantic
Ocean.
El Nino
“ENSO
warm
event”
La Nina
“ENSO
cold
event”

44. Hurricanes and climate: Global Warming
2. Is global warming responsible for the very active Atlantic
hurricane seasons we’ve seen over the last 15 years?
Other research has shown the
existence of natural swings in Atlantic
hurricane activity that can last for
decades. These swings are tied to
changes in ocean circulations,
affecting not only ocean temperatures
but also the atmosphere above. The
1930’s to 1960’s were very active,
followed by a relatively inactive
period from 1970-1994. It appears
that in 1995 we entered into another
active period.
Recent research has indicated
that while the number of
hurricanes has not increased over
the last 50 years, the percentage
of storms that reach the more
intense categories has increased.
The authors point to global
warming as a likely explanation.
This is currently the hottest topic in hurricane
research. There are 2 opposing views
YES NO

45. Hurricanes and climate
3. Will the BP oil spill have any effect on hurricanes?
Hurricanes feed off of evaporation of sea surface water. Over calm
water, the oil on the surface could suppress evaporation of water.
But under the higher wind speeds of a mature hurricane, the oil layer
breaks up and has little effect on evaporation.
Therefore, the oil slick is NOT likely to have a significant impact on a
hurricane.

46. Outline
• What hurricanes are and where they form
• How hurricanes work
• How hurricanes cause damage
• How hurricanes differ from tornadoes
• How we forecast hurricanes
• Hurricanes and climate
• Hurricanes that have affected NJ and PA

47. Hurricanes Carol and Hazel
August & October, 1954
Hurricane Carol caused $460
million in damage and was
responsible for 60 deaths on
the U.S. East Coast
Hurricane Hazel produced the
strongest winds ever officially
recorded in Philadelphia (94 mph)
Storm surge from
Hurricane Carol pounds
the New England Coast

48. Hurricanes Connie and Diane
August, 1955
Floods from the storms affected Yardley, washing
out the Yardley-Trenton Bridge. Fifty other
bridges along the Delaware River were either
damaged or destroyed.
Flooding of the
Delaware River at the
Washington Crossing
Bridge, August, 1955.
The 2 storms hit within a week of each other, causing
record rainfall and flooding, killing 180 people (90
along the Delaware River) and causing $680 million in
damage on the U.S. East Coast.

49. Hurricane Agnes
June, 1972
The storm stalled over northern
Pennsylvania as a weakened tropical
depression, leading to record rainfall
amounts and disastrous flooding.
Costliest natural disaster in Pennsylvania
history ($2.1 billion in damage)
48 deaths in Pennsylvania from the
flooding (122 total in the United States)

50. Hurricane Floyd
September, 1999
Hurricane Floyd dumped 8-13 inches of rain in
parts of the Philadelphia suburbs
Eight people in southeastern Pennsylvania
drowned in the flooding from Hurricane Floyd.
In total, Floyd caused more than $4 billion in
damage in the U.S.
Satellite
image of
Hurricane
Floyd making
landfall in
North
Carolina,
September
16, 1999
Flooding of the
Raritan River
near
Flemington,
NJ, in the
aftermath of
Hurricane
Floyd.

51. Washington Crossing Bridge
August, 1955 September 19, 2004