1
Severe Winds and Tornadoes
Think About It
Page F113
 What is a tornado?
 How likely is that
your community will
experience a tornado?
Date
Page
2
Severe Winds and Tornadoes
Investigate Part A
Pages F114-115
1a. Which five states
reported the fewest
tornadoes?
1b. Which five states
reported the most
tornadoes?
1c. How many tornadoes
occurred in your state?
1d. How many tornadoes
does your state average
per year?
2c. How do places of
high tornado frequency
compare to high
thunderstorm frequency?
2d. How many tornadoes
does your state average
per year?
2a. Record on the map
the total number of
tornadoes reported in
each state from 1950 to
Date
Page
3
1994.
2b. Shade in the tornado
frequency for each state
using the color scale:
•25 or more (red)
•20 – 25 (orange)
••1100 –– 2200 ((yyeelllloow
w))
•1 – 10 (green)
•less than 1 (blue)
2c. How do places of
high tornado frequency
compare to places of
high thunderstorm
frequency?
2d. How many
thunderstorms does your
state have per year?
2e. How many tornadoes
would you expect to
occur annually in your
state?
2f. Calculate the
percentage of all U.S.
tornadoes that occurred
in your state from 1950
4
to 1994 (34,349).
3a. In which four months
is tornado frequency the
highest?
3b. In which season is
the frequency of
tornadoes the highest?
3c. In which season is
the frequency of
tornadoes the lowest?
3d. In which season does
your state have the
highest number of
thunderstorms?
3e. How does timing of
local thunderstorm
activity compare to the
season of highest tornado
occurrence?
5
Severe Winds and Tornadoes
Digging Deeper
Pages F116-122
Microburst
Date
Page
an intense downdraft impacting a
relatively small area (4 km or less
across)
http://www-frd.fsl.noaa.gov/mab/microburst/vu_micro.html
Microbursts over short distances
are particularly hazardous to
aircrafts taking off or landing
Wind shear
a sudden change in wind speed or
direction with distance
6
http://esminfo.prenhall.com/science/geoanimations/animati
ons/Tornadoes.html
Downburst winds
downburst winds blow outward
from a central area
Tornadic winds tend to swirl in circles
Violent tornadoes
the updraft near the center of the
system may top 160 km/hr (100
mph), strong enough to lift a
house off its foundation
Most destructive
tornadoes
are large systems made up of two
or more whirling masses of air
(multi-vortex systems)
Fujita Scale
Tornado intensity:
Weak
F 0-1
Strong F 2-3
Violent F 4-5
7
Tornadoes
come in a variety of shapes,
ranging from cylindrical cloud
masses having nearly uniform
lateral dimensions to long, slender
rope-like pendants
8
Weak tornadoes
are less than 1.5 km (1.0 mi) long
and 100 m (330 ft) wide, with a
life expectancy of only one to
three minutes winds less than 180
km/hr (110 mph)
Intense tornadoes
can exceed 160 km (100 miles)
long and hundreds of meters wide,
with a life expectancy of two
hours
Violent tornadoes
range up to 500 km/hr (300 mph)
Tornado path
usually runs from southwest to
northeast
The average forward speed is about 48 km/h (30
Winds in the vast majority of
Northern Hemisphere tornadoes
blow in a counterclockwise
direction when viewed from above
9
Tornadoes can occur on
mountainous terrain
Frequency
the U.S. can anticipate between
700 and 1100 tornadoes each year
Only 1% of these are rated as
violent (F4-5)
Month
February
April
May-June
June-Aug.
Tornado
development
High Tornado
Frequency
Central Gulf States
Southeast Atlantic States
Southern Great Plains
Northern Plains and the
Great Lakes region
most intense tornadoes develop in
supercell thunderstorms
10
Mesocyclone
A counterclockwise (viewed from
above) circulation that develops in
a supercell thunderstorm; may
evolve into a tornado
About 60% of mesocyclones
produce tornadoes
Humid air
expands and cools as it is drawn
inward toward the low pressure
center of the whirling system
Cooling air
causing water vapor to condense
into water droplets, forming a
funnel-shaped cloud extending
downward from the parent
cumulonimbus cloud
11
Cyclone
a large low-pressure weather
system in which surface winds
blow counterclockwise and inward
viewed from above in the
Northern hemisphere
Cyclone + warm humid air mass = tornadoes an
http://esminfo.prenhall.com/science/geoanimations/animati
ons/Tornadoes.html
http://www.usatoday.com/weather/graphics/tornadoes/flash
.htm
http://news.bbc.co.uk/2/hi/5328524.stm
Tornado alley
severe thunderstorms and
tornadoes are most frequent in a
north-south belt in the center of
the nation, stretching from east
Texas, northward through
Oklahoma, Kansas, Nebraska, and
into southeast South Dakota
12
Formation of
tornadoes
cold air masses + warm humid air
masses = sharply defined cold
front for thunderstorms
Thunderstorm + warm humid air
masses ahead of the cold front
Tornado season
tornadoes are most common in
spring and early summer because
the temperature contrast in
between air masses is greatest
National
Weather Service
issues watches and warnings when
conditions are favorable for severe
weather
Watch
indicates that conditions in the
atmosphere appear favorable for
the development of severe weather
and be prepared to take action
13
Warning
means severe weather is spotted
and take action because severe
conditions (thunderstorm, tornado)
have been spotted or detected on
radar
The Doppler
was named after Johann Christian
Doppler the Austrian physicist
who explained the phenomenon in
1842
Doppler effect
refers to a shift in frequency of
sound waves or electromagnetic
waves when a source is moving
http://www.kettering.edu/~drussell/Demos/doppler/carhorn
.wav
http://www.wwnorton.com/college/geo/egeo/flash/1_1.swf
14
Doppler radar
monitors the speed of precipitation
particles as they move directly
toward or away form the radar
antenna
Doppler radar
color code
green and blues indicate motion
directly toward the radar
Red and yellow indicate motion
directly away from the radar
Tornado safety
tips
•seek shelter
•go to a tornado shelter or steel
framed or substantially reinforced
concrete building, basement
15
•shelter under a mattress
•do not go near a window
•go to an interior hallway avoid
auditoriums, gymnasium and
supermarket
•never out run a tornado in a car
•never seek shelter in a motor
vehicle
16
Severe Winds and Tornadoes
Check Your Understanding
Page F122
1. What is the relationship
between a severe
thunderstorm and a
tornado?
2. What is the relationship
between a mesocyclone
and a tornado?
3. Compare the diameter
and life expectancy of a
tornado with that of a
supercell thunderstorm.
4. What is the difference
between a weather watch
and warning?
Date
Page
17
Severe Winds and Tornadoes
Understanding and Applying
Page F122
1. Is a thunderstorm
that produces hail also a
good candidate to spawn
a tornado?
2. How does the pattern
of property damage
caused by a downburst
compare to a tornado?
3. Why is a tornado
potentially so destructive?
4. What is the value of
Doppler weather radar in
safeguarding the public?
5a. Where do you seek
shelter from a tornado in
your school?
5b. Where do you seek
shelter from a tornado in
your home?
5c. Where would the
entertainment company
seek shelter during a
Date
Page
18
tornado?
6. During what time of
the year is the threat of a
tornado in your
community greatest?
19
Severe Winds and Tornadoes
Inquiring Further
Page F123
1. Direction of cyclones
Conduct research to find
out why surface winds in
a cyclone blow
clockwise in the Southern
Hemisphere and
counterclockwise in the
Northern Hemisphere.
2. Wind shear and
airplanes
Find information about
airplanes that crashed
because of wind shear.
Two examples are the
Delta Airlines L-1011 in
Dallas in 1985, and a
USAir DC-9 in Charlotte,
NC in 1994.
•Choose one and write a
paragraph about how the
weather affected the crash.
Date
Page
20
•Research how the FAA
has reduced the chances
that aircraft will
encounter dangerous
wind shear. What are
some devices that help
pilots and air traffic
controllers detect wind
shear?
3. Tornadoes and
Hollywood
Watch a non-documentary
movie like Twister.
Prepare an analysis of
how the movie is or is
not scientifically accurate.