CH21: Severe Weather
Earth Science Spring, 2016
Thunderstorms
A thunderstorm is a form of
weather with lightning,
thunder, wind, and rain
Forecasting Thunderstorms
Satellite imagery is used to
track the movement of weather
systems
Computer model data Surface and upper air observations
determines where
to find low level moisture
favorable thunderstorm and instability. It determines how
formation might be
winds aloft might storm
development.
Recipe for Thunderstorms
Recipe for T-storms:
1) Moisture (liquid or vapor H2O) in air
2) Rising unstable, warm air
3) Lifting mechanism (either cold or dry air bumps
into warm air, “nudging” the warm air upward
Life
Cycle-Shaped
Like
an
Anvil
Feet
40,000
30,000
20,000
10,000
Cumulus stage
Mature stage Dissipating stage
A storm may dissipate
because of a downburst
causing the inflow to be
cut off and cause an
outflow boundary
Two winds converge
forcing air upwards
Masses of moisture are
lifted upwards
The moisture rapidly
cools into liquid drops
of water because of the
adiabatic temp change
The air is forced
to spread out
because it can’t
rise any farther
Classification
Air-mass
Thunderstorm that
are generally weak
and usually not
severe
Lifting source is
uneven heating of
the surface
Multicell
Supercell
The most common Very large, severe
storm
type
Strong cold fronts strong, rotating
updraft
Moderate-sized
At least 90 percent
hail, flash flooding, of the time severe
and weak
Destructive tornadoes
tornadoes
The most powerful
thunderstorm
Squall line
Severe thunderstorms along
or ahead of a cold front
Hail, frequent lightning,
strong straight-line winds
Shelf Cloud in front of
the winds
Weather
Underground
A Derecho is a widespread
and long-lived line of storms
Possibly tornadoes
Bow echo
Lightning
Electrons are stimulated
during a storm: electrons are
moving in clouds FROM
ground (+) TO cloud (-) the
air in a zigzag motion
Electricity flows UP TO
cloud, causing a
lightning bolt to strike
and travel for a quarter
of a second on avg.
54,000 °F
140,000 mph
Types
Ground to Cloud
Cloud-to-cloud
lightning
Discharges
occur between
The best known and second areas of cloud
most common type of
lightning
Discharge
between cloud
and the ground.
Hail formation requires strong As hail is pushed upward it grows
upward motion of air (updraft) as droplets of water hit it. This
and a low freezing level
happens until the updraft can’t
support the hail. Then it falls to the
Ping-pong ball
ground.
1.50 inches
Pea 0.25 inches
Dime 0.705 inches
1.75 inches
1.00 inch
Quarter
Volleyball 8.00 inches
Baseball
ball Golf ball
2.75 Tennis
2.50 inches
inches
1.93 pounds
4.50 inches The largest recorded
hailstone in the United
Softball
States
Vertical draft
Updraft
Downdraft
Warm air will rise until it
reaches air warmer or less
dense than itself
Low pressure area will attract
air from the surrounding area.
It will move to the center and
then rise
High pressure region will
attract air from the
surrounding area. It will move
towards the center and sink
Both can cause airplane crashes
Microburst / Downburst
High winds that can knock
over fully grown trees.
A couple of seconds to
several minutes.
Column of sinking air
producing damaging
straight-line winds
Is capable of producing
damaging straight-line winds
of over 150 mph
Very rare
Heat burst
Typically occur
during night-time
Gusty winds, rapid increase Caused when rain evaporates
in temperature, and decrease into a parcel of cold dry air
high in the atmosphere. They
in dew point
descends rapidly, warming due
Temperatures reach 90 °F
to compression.
Reports up to 120 °F
Associated with
dying thunderstorms
Tornadoes
• Shortest lived, most violent thunderstorm with
whirling funnel shaped cyclone
• Cyclone: circular moving weather mass that
follows Coriolis Effect (So, CCW in Northern
Hemisphere and CW in Southern Hemisphere) and
has a low pressure center
RECIPE: YOU WILL NEED:
 Thunderstorm
 High Altitude, horizontal winds
 Rotation
 Vortex (the spinning part that touches the
ground)
If this happens over water, it is called Water Spout, not
Tornado
A tornado is a violent,
dangerous, rotating
column of air
Most have wind speeds less
than110 mph
Tornado
Tornadoes are
usually located on a
hook echo at the
southwest part of the
storm.
May 1999
May 1999
The strongest ever.
318 MPH winds
Doppler reflecting
off debris swept up
in the tornado.
Warm and
Come in moist inflow
many to power it.
August
24, 2006
shapes
and sizes
Wedge Tornado
Tornadoes-When and Where
Do They Occur?
• Tornadoes can occur in any state and any
time of year
• They typically occur from March to August
• The most common tornadoes are in
“Tornado Alley”
• “Tornado Alley” - Arkansas, Iowa, Kansas,
Louisiana, Minnesota, North Dakota, Ohio,
Oklahoma, South Dakota, and Texas
A Tornado that contains two
or more small vortexes as part
of the main vortex
The only times they are visible
are when the tornado is first
forming.
Watches
Tornado watch
Severe thunderstorm watch
Issued when conditions are
Issued when weather conditions
favorable for a tornado to
are favorable for the
form
Map of weather advisories development of severe
thunderstorms. Tornadoes can
still form.
(NWS website)
Warnings
Tornado warning
Issued after a tornado
or funnel cloud has been
spotted or if radar
indications of tornado
formation. (Red box)
Tornado emergency
A large violent tornado
expecting to cause significant
damage and a high likelihood
of numerous fatalities.
Severe thunderstorm warning
Is issued when trained storm
spotters or a radar indicate a
strong thunderstorm. (Yellow
Box)
58 mph wind and 1 inch hail
Tornado Classifications:
The Enhanced Fujita Scale
EF 0
EF 1
EF 2
EF 3
EF 4
EF 5
65 to 85 MPH
86 to 110 MPH
111 to 135 MPH
136 to 165 MPH
166 to 200 MPH
Over 200 MPH
Tornado Safety What To DO:
Get indoors
Get to the lowest point in your house
AWAY from windows
No basement, get in your bathtub
If time, put something soft and heavy
over you (mattress/blanket)
If time, grab your animals
If Caught Outside:
Get to a low lying area (ditch/river/etc.)
Lay as flat as you can and protect your
head/face/neck
Bands of
Heavy
Rainfall
Warm
Ocean