11
Severe Winds and Tornadoes
Think About It Date
Page F113 Page
· What is a tornado?
· How likely is that
your community will
experience a tornado?
Severe Winds and Tornadoes
Investigate Part A Date
Pages F114-115 Page
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
1994.
2b. Shade in the tornado
frequency for each state
using the color scale:
•25 or more (red)
•20 – 25 (orange)
•10 – 20 (yellow)
•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
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?
Severe Winds and Tornadoes
Digging Deeper Date
Pages F116-122 Page
Microburst 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
http://esminfo.prenhall.com/science/geoanimations/animations/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 are large systems made up of two
tornadoes 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
Tornadoes come in a variety of shapes, ranging from cylindrical cloud masses having nearly uniform lateral dimensions to long, slender rope-like pendants
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 mph)
Winds in the vast majority of Northern Hemisphere tornadoes blow in a counterclockwise direction when viewed from above
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 / High Tornado FrequencyFebruary / Central Gulf States
April / Southeast Atlantic States
May-June / Southern Great Plains
June-Aug. / Northern Plains and the Great Lakes region
Tornado most intense tornadoes develop in development supercell thunderstorms
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
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 and thunderstorms
http://esminfo.prenhall.com/science/geoanimations/animations/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
Formation of cold air masses + warm humid air
tornadoes 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 issues watches and warnings when
Weather Service 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
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
Doppler radar monitors the speed of precipitation particles as they move directly toward or away form the radar antenna
Doppler radar green and blues indicate motion
color code directly toward the radar
Red and yellow indicate motion directly away from the radar
Tornado safety •seek shelter
tips •go to a tornado shelter or steel framed or substantially reinforced concrete building, basement
•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
Severe Winds and Tornadoes
Check Your Understanding Date
Page F122 Page
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?
Severe Winds and Tornadoes
Understanding and Applying Date
Page F122 Page
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
tornado?
6. During what time of
the year is the threat of a
tornado in your
community greatest?
Severe Winds and Tornadoes
Inquiring Further Date
Page F123 Page
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.
•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.