Tornadoes
Outline
Motivation
Definition
When and Where
Ingredients
Forecasting
Historical Events
Most
tornado
outbreak
s result
in less
economi
c losses
than
other
natural
#Injuries
Estimate
d
damage
Year
Deaths
In mobile
homes
1985
94
30%
1986
15
47%
1987
59
41%
1989
50
24%
2490
$1088M
1990
53
13%
1150
$668M
1991
39
51%
854
$798M
1992
39
51%
1300
$764M
1993
33
39%
3990
$368M
1994
69
38%
1067
$518M
1995
30
647
$408M
However,
tornados
account
for a
higher
number of
fatalities
on
This is largely due to two reasons:
1)Warning time
2)Catastrophic nature of damage
In
general,
the
weakest
tornados
have wind
speeds of
hurricane
force.
 The overall number of fatalities
associated with tornados continues to
decrease
This decrease in fatalities is being
realized even as the number of reported
tornados is increasing
This decrease in fatalities is being
realized even as the number of reported
tornados is increasing
What is a tornado?
A tornado is a violently rotating
column of air extending
between, and in contact with, a
cloud and the surface of the
earth.
The stronger tornadoes attain
an awe-inspiring intensity, with
wind speeds that exceed 200
mph and in extreme cases may
How do we know how strong
the wind speeds in a
tornado are?
The most common way to identify the strength of a
tornado is through an examination of the damage
caused. This concept led to the development of the
Fujita and Enhanced Fujita Scales.
EF Scale Damage
EF1 – 31.6% of all
reports
EF2 – 10.7% of all
reports
EF Scale Damage
EF3 – 3.4% of all
reports
EF4 – 0.7% of all
reports
EF Scale Damage
EF5 – .1% of all
reports
How do we know how strong
the wind speeds in a
tornado are?
Another way is to remotely measure the wind
speeds by using a mobile doppler radar.
How do we know how strong
the wind speeds in a
tornado are?
Doppler radars can measure the speed of wind
because the beam that is received by the radar
will have a different frequency depending on the
motion of the water drops
How do we know how strong
the wind speeds in a
tornado are?
This is an
image of
the wind
velocities
from a
doppler
radar
scan.
The circle
indicates a
possible
Map of World-Wide Tornado
Fromation
Over 85% of world-wide
tornado reports occur in
North America
The geography of
North America is
uniquely favorable
for the formation of
tornados.
Warm-moist air
source
Cold air source
All significant
mountain ranges are
oriented north-
The Gulf of Mexico and the western North Atlantic Ocean are important sources of water vapour for
North American thunderstorms
States with the largest number
of tornadoes are located in
‘tornado alley’
Tornado alley corresponds to a northeast-
southwest orientation that corresponds to the
orientation of strong fronts traveling across
the central United States in the spring and
early summer, and to the orientation of the
upper-level ‘jet stream’
These states include Texas, Oklahoma,
Nebraska, Kansas, Iowa, Missouri, Illinois,
Indiana, Mississippi, and Florida
States with the largest number
of tornadoes are located in
‘tornado alley’
However, the
traditional
tornado alley
isn’t where
deaths are most
frequent.
Instead, most
deaths occur in
the deep south.
Why?
Tornadoes (when?)
Most frequent in the spring and summer
Tornadoes (when?)
Most frequent in the spring and summer
Tornadoes (when?)
Most frequent in the spring and summer
Tornadoes (when?)
Most common in the evenings
All tornados form within
thunderstorms
These thunderstorms can be
found in:
Landfalling hurricanes
Within supercell thunderstorms
In squall lines, often located
ahead of cold fronts
Thunderstorms
Defined by its scale
(short and shortlived)
Typically 5-10 km
horizontally and
vertically
An individual cell
lasts only 30-60
minutes
How do thunderstorms
form?
Require three elements:
1. A source of moisture
2. A conditionally unstable atmosphere
3. A mechanism to trigger a thunderstorm
updraft, either through forced lifting or
heating
Try this: In order to get a thunderstorm you have to go out on a “lim”. l=lift, i=instability, m=moisture
How do thunderstorms
form?
Typically, the source
of moisture is the
Gulf of Mexico.
Cool dry air aloft
often comes from
the Plains or the
Rockies.
This combination of
warm moist air at
the surface and cool
dry air aloft leads to
So what do
we mean by
instability?
What is a lapse rate?
A lapse rate is
defined as the
rate of change in
temperature
observed while
moving upwards
through the
Earth's
atmosphere.
Key points
Rate at which a temperature decreases with
height.
Units: generally C/km, sometimes K/km
Stability of
Atmosphere
 This describes the tendency for the atmosphere
to either resist or enhance vertical motions. The
stability of the atmosphere is directly related to
the changes of temperature with height.
First we need to consider the temperature
changes experienced by rising air. As a parcel of
air rises it moves into regions of lower pressure.
This means that the surrounding air is pushing on
the parcel with less force. So the air in the parcel
will expand, and the volume will become larger.
When the air expands, the molecules must now cover a larger volume. This means that the air in the parcel must perform work to inhabit the increased volume. The work done by the parcel will result in lower kinetic energy, and the temperature must fall. Now, what happens when air
becomes saturated?
Consider: condensation causes a release
of latent heat.
So the expansion of the air will induce a
cooling, but this will be partially offset by
heat release from the condensation of
water.
As a result, the rate of temperature
change of rising air that is saturated is
smaller than for dry air. This is called the
Moist Adiabatic Lapse Rate, and it is
not a constant value. This is because the
rate of condensation changes with height.
Stability and Saturated
Recall that Air
when rising air becomes saturated, latent heat is released, and slows the rate of cooling. The moist adiabatic rate is variable, but always less negative than the dry adiabatic Conditional Instability
Air is stable
to a certain height, however, if a
“lifting mechanism” can cause air to rise, to a level where condensation is reached the air is now
saturated.
What is a “trigger” or lifting
mechanism?
Any mechanism which lifts a parcel of air up
to the point where it becomes buoyant is can
trigger a thunderstorm.
Examples of lifting mechanisms
Tornadoes migrate
northward from spring to
summer:
Triggering in the
form of forced lift
occurs along the
front range of the
Rocky Mountains
and along the seabreeze
convergence zone
in Florida
Airmass thunderstorms
occur in three stages
1. Cumulus (warm, buoyant plume
with updrafts)
2. Mature (combination of both
updrafts and downdrafts)
3. Dissipating (falling precipitation
shuts off the updrafts…all downdrafts
and system self destructs
Stages of a thunderstorm
Severe thunderstorms
However, airmass thunderstorms don’t
typically produce tornados and almost never
produce strong tornados.
The reasons that airmass thunderstorms don’t
produce thunderstorms are two fold:
Updrafts are suppressed by the weight of the
water
There is no inherent rotation to work with
Both liabilities can be
overcome by shear
Speed shear
Directional Shear
The updraft then tilts the rotation
from the horizontal axis to the
vertical axis
Also the updraft is tilted by the shear,
removing the updraft from the precipitation
area. At this point the thunderstorm is
called a “supercell”.
Eventually, this rotating updraft
produces a small scale low pressure
system called a mesocyclone
About 10% of the time, this rotation
becomes “stretched” by the rear flank
downdraft and concentrated into a tornado
About 10% of the time, this rotation
becomes “stretched” by the rear flank
downdraft and concentrated into a tornado
Overhead view of a
supercell
T
Tornado forecasting
Tornado
forecasting is based
largely on the
recognition of the
patterns which are
conducive to
tornado formation.
Tornado forecasting
However, we can
not pinpoint actual
locations of tornado
formation because
the process is
somewhat
stochastic.
Consequently,
forecasts are for
generalized regions
with probabilities of
events.
•88 tornadoes touched down in areas from Alabama to Pennsylvania
•36 fatalities
•one of the most extensive tornado outbreaks in several years
•the outbreak is especially unusual during the month of November
•Much of the activity occurred between the hours of 3PM and 9PM
EST during the afternoon and evening of November 10, 2002
A map summarizing the
severe weather reports for
Nov. 10, 2002
An image of a tornado near
West Mansfield, Ohio on Nov.
10, 2002:
Vehicles having been thrown
across Main Street in Clark,
Pennsylvania:
An evacuation of patrons at a movie
theatre just minutes prior to the
tornado saved lives in Van Wert,
Ohio:
There was very little left of a
mobile home in Fruitvale,
Tennessee:
Canada’s most lifethreatening tornadoes:
 Regina, Saskatchewan; June 30, 1912; 28 dead, hundreds
injured
 Edmonton, Alberta; July 31, 1987; 27 dead, hundreds injured
 Windsor, Ontario; June 17, 1946; 17 dead, hundreds injured
 Pine Lake, Alberta; July 14, 2000; 12 dead, 140 injured
 Valleyfield, Quebec; August 16, 1888; 9 dead, 14 injured
 Windsor, Ontario; April 3, 1974; 9 dead, 30 injured
 Barrie, Ontario; May 31, 1985; 8 dead, 155 injured
 Sudbury, Ontario; August 20, 1970; 6 dead, 200 injured
 St-Rose, Quebec; June 14, 1892; 6 dead, 26 injured
 Buctouche, New Brunswick; August 6, 1879; 5 dead, 10
injured
The Edmonton tornado of
July 31, 1987
Wind speeds up to 420 km per hour (a
minimal F5) and stayed on the ground for an
hour
27 people were killed with several hundred
injuries
300 homes destroyed
$330 million in total property damage
One of the worst natural disasters in Canadian
history
The tornado
droppped down
from a supercell
thunderstorm at
approximately
1458 and stayed
on the ground for
about an hour;
the path was
about 40 km long.
An aerial
damage
survey of
several of
the
industrial
sites in
listed in
the
previous
slide:
A view of the Edmonton tornado from
the shores of the North Saskatchewan
River:
A view of a residential area being
affected:
And just after the damage
was done:
Mitigation
Residents accessing either
radio or television were
warned as much as 15
minutes prior to the tornado’s
impact
What about other more
recent cases?
First, what is a tornado
warning?
A tornado warning means that a tornado is
believed to be present in the warning area
The warning may be based upon a Doppler
radar signature, or on public spotter reports
Storm spotters can be the first line of defence
against tornadoes, particularly in regions
outside of Doppler radar coverage
Warning times for
tornadoes:
Longest warning times are based upon
mesoscale vortex signatures seen only one
hour or more in advance
The best way to be informed is to listen to
a Weather radio (20 dollars and up)
The theatre manager in van Wert, Ohio
evacuated his customers after hearing the
warnings being broadcast on the weather
radio
This alert action saved many lives
Tornado watches provide
more potential lead time:
A tornado watch states that
conditions are favorable for
tornadoes to form
These watches may be given as
much as 12 hours in advance of
an anticipated event
Tornado safety
measures:
When a tornado watch is issued, listen
to a weather radio
If at home, move to a designated
shelter, such as a basement
If there is no basement, move to an
interior room and avoid windows (put as
many walls as possible between you and
the tornado)
If outside, move to a low elevation away
from any possible airborne object
(including vehicles and mobile homes!)
Tornado myths:
The
skyscrapers
in cities
prevent
tornados
from forming
in or striking
downtown
areas.
Tornado myths:
Highway
overpasses
are a safe
place to
shelter if
you are on
the road
when you
see a
tornado.
Tornado myths:
It is
commonly
and
mistakenly
thought that
if the
condensation
funnel of a
tornado does
not reach the
ground, then
the tornado
can not cause
substantial
damage.
Tornado myths:
It has been
thought in
the distant
past that
tornadoes
moved
almost
exclusively
in a
northeasterl
y direction.
Readings:
http://www.mb.ec.gc.ca/air/summersevere/
tornadoes.en.html
http://www.nssl.noaa.gov/edu
http://www.fema.gov/library/tornado.htm
Bluestein, H. B., 1999: Tornado alley:
Monster storms of the Great Plains. Oxford
University Press.
Zebrowski, Chapter 8.
Rauber, Walsh, and Charlevoix, chapters
17-18.