AOS 101 Weather and Climate
Lisha M. Roubert
University of Wisconsin-Madison
Department of Atmospheric & Oceanic Sciences
Severe Weather
• Severe weather refers to any
dangerous meteorological pheno
mena with the potential to cause
damage, serious social
disruption, or loss of human
life. Types of severe weather
phenomena vary, depending on
the latitude, altitude, topograph
y, and atmospheric conditions.
• High winds, hail,
excessive precipitation,
and wildfires are forms of severe
weather.
Wind events
•
•
•
•
•
•
•
•
Downburst
Squall lines
Tornadoes
Tropical cyclones
Waterspouts
Strong extratropical cyclones
Dust storm
Wildfires
Tornadoes
•
•
•
Tornadoes are defined as
violently rotating columns of
high wind that rotate around
regions of low pressure
beneath a thunderstorm.
These rotating columns of air
are called “funnels”. These
funnels are visible due to
moisture that condenses in
the rising and cooling air
within a tornado and also due
to the dust and debris that
have been sucked into the
vortex.
Tornadoes usually rotate
cyclonically and move towards
the northeast along with the
thunderstorm.
Wedge Tornado
Rope Tornado
Funnel clouds
• There are tornadoes in which the
circulation does not extend to the
ground. These are called funnel
clouds instead of tornadoes.
• Funnel clouds are rotating columns
of air not in contact with the ground.
However, the violently rotating
column of air may reach the ground
very quickly - becoming a tornado! If
there is debris being picked up or
blown around by the "funnel cloud" the rotating column of air has already
reached the ground and it's a
tornado!
What causes Tornados?
• Tornadoes in the winter and early spring are
often associated with strong frontal systems
that form in the Central US and move east
• During the spring in the Central Plains,
thunderstorms develop along a dryline –
separates very warm, moist area to the east
from hot, dry air to the west.
• Tornadoes may form as thunderstorms fire
along the dryline during the afternoon hours
Tornado Development
• Tornadoes typically form in
association with supercell
thunderstorms.
• A supercell thunderstorm is a
special type of thunderstorm
that can persist for several
hours due to its organized
internal structure.
• Supercell thunderstorms are
characterized by a single,
rotating updraft. They form in
regions of strong vertical
wind shear.
Wind Shear
• Vertical wind shear is the change
in wind speed and/or direction
with height.
– Directional wind shear- the
change of wind direction with
height
– speed shear-the change of wind
speed with height.
• Vertical wind shear induces a
"rolling" effect in the
atmosphere, similar to the
diagram below. This rolling effect
becomes important when a
thunderstorm forms because it
is tilted upward and causes a
thunderstorm to rotate.
Directional Wind Shear
Speed Shear
Supercell Tornadogenesis
•
•
•
A rotating updraft is a key to the development of
a supercell, and eventually a tornado. There are
many ideas about how this rotation begins. One
way a column of air can begin to rotate is from
wind shear – when winds at two different levels
above the ground blow at different speeds or in
different directions.
An example of wind shear that can eventually
create a tornado is when winds at ground level,
often slowed down by friction with the earth's
surface, come from the southwest at 5 mph. But
higher up, at 5000 feet above the same location,
the winds are blowing from the southeast at 25
mph! An invisible "tube" of air begins to rotate
horizontally. Rising air within the thunderstorm
tilts the rotating air from horizontal to vertical –
now the area of rotation extends through much
of the storm.
Once the updraft is rotating and being fed by
warm, moist air flowing in at ground level, a
tornado can form. There are many ideas about
this too.
Mesocyclones
•
•
When the combination of wind speeds and
changing directions in a super cell is right the air
will rotate, causing a mesocyclone to form. So the
air near a mesocyclone is rotating and rising. Where
air rises there aren't water droplets, so there won't
fall any rain in that area. Since tornadoes are made
of rising air, they never form in the neighbourhood
of any rain. When the mesocyclone is powerful
enough the actual tornado will form.
Often, a tornado is located on the edge of the
updraft, next to air that's coming down from the
thunderstorm with falling rain or hail. This is why a
burst of heavy rain or hail sometimes announces a
tornado's arrival. Although the air is rising in a
tornado, the funnel itself grows from the cloud
toward the ground as the tornado is forming. When
the funnel cloud touches the ground, it becomes a
tornado.
How big are tornadoes?
• Most tornadoes are less than 1/4 of a
mile wide on the ground - but they can
also can exceed 1 mile in width! The
McColl tornado in March 1984 (moving
from the McColl S.C. area into North
Carolina) was over 1 1/2 miles on the
ground!
• Most tornadoes are on the ground 10
minutes or less - but in 1925 a tornado
traveled 219 miles across Missouri,
Illinois, and Indiana in 4 hours!
• In 1924, a tornado that started in Aiken
County South Carolina traveled 135
miles into Florence County!
How strong are tornadoes?
• While most tornadoes (69%) have
winds of less than 100 miles an hour,
they can be much much stronger.
Violent tornadoes (winds greater than
205 miles an hour) account for only
2% of all tornadoes, they cause 70%
of all tornado deaths.
– In 1931, a tornado in Minnesota lifted
an 83-ton railroad train with 117
passengers and carried it more than
80 feet!
– Once a tornado in Oklahoma carried
a motel sign 30 miles and dropped it
in Arkansas!
– In 1975 a Mississippi tornado carried
a home freezer more than one mile!
An F2 tornado near Seymour,
Texas, in April 1979. F2 and F3
tornadoes are considered
strong, packing winds of 113-206
mph that can cause major to
severe damage.
Tornado Wind and Damage Scale
Tornado Scale Wind Speeds
Damage
Frequency of
Occurrence
F0
40 to 72 MPH
Some damage to chimneys, TV antennas, roof shingles,
trees, and windows.
29%
F1
73 to 112 MPH
Automobiles overturned, carports destroyed, trees
uprooted
40%
F2
113 to 157 MPH
Roofs blown off homes, sheds and outbuildings
demolished, mobile homes overturned.
24%
F3
158 to 206 MPH
Exterior walls and roofs blown off homes. Metal buildings
collapsed or are severely damaged. Forests and farmland
flattened.
6%
F4
207 to 260 MPH
Few walls, if any, standing in well-built homes. Large steel
and concrete missiles thrown far distances.
2%
F5
261 to 318 MPH
Homes leveled with all debris removed. Schools, motels,
and other larger structures have considerable damage with
exterior walls and roofs gone. Top stories demolished
Less than 1%
How fast can tornadoes move?
• The average speed of a
tornado is around 35 miles
an hour - but they can
remain almost stationary
or move as fast as 70 miles
an hour!
• The average tornado
moves from southwest to
northeast - but they can
move in any direction and
even change direction.
Tornado Video
http://www.weather.com/outlook/videos/vort
ex2-watching-tornado-develop-live-14517
Where do tornadoes occur?
•
•
•
Because a tornado is part of a severe convective storm,
and these storms occur all over the Earth, tornadoes are
not limited to any specific geographic location. In fact,
tornadoes have been documented in every one of the
United States, and on every continent, with the exception
of Antarctica (even there, a tornado occurrence is not
impossible). Wherever the atmospheric conditions are
exactly right, the occurrence of a tornadic storm is
possible.
However, some parts of the world are much more prone
to tornadoes than others. Globally, the middle latitudes,
between about 30° and 50° North or South, provide the
most favorable environment for tornadogenesis. This is
the region where cold, polar air meets against warmer,
subtropical air, often generating convective precipitation
along the collision boundaries. In addition, air in the midlattitudes often flows at different speeds and directions at
different levels of the troposphere, facilitating the
development of rotation within a storm cell.
Interestingly, the places that receive the most frequent
tornadoes are also considered the most fertile agricultural
zones of the world. This is due in part to the high number
of convective storms delivering needed precipitation to
these areas.
Regions of the world with
increased likelihood of
experiencing tornadoes.
Florida and “Tornado Alley”
•
•
In the United States, there are two regions with a
disproportionately high frequency of tornadoes.
Florida is one and "Tornado Alley" in the southcentral U.S. is the other. Florida has numerous
tornadoes simply due to the high frequency of almost
daily thunderstorms. In addition, several tropical
storms or hurricanes often impact the Florida
peninsula each year. When these tropical systems
move ashore, the embedded convective storms in
the rain bands often produce tornadoes.
Tornado Alley is a nickname given to an area in the
southern plains of the central U.S. that consistently
experiences a high frequency of tornadoes each year.
Tornadoes in this region typically happen in late
Spring and occasionally the early fall. The Gulf Coast
area has a separate tornado maximum nicknamed
"Dixie Alley" with a relatively high frequency of
tornadoes occurring in the late fall (October through
December).
Tornado Alley
Dixie Alley
When are tornadoes most likely?
• In the southern
United States the
peak occurrence
of tornadoes is March
through May - but
tornadoes can occur any
time of year.
Most tornadoes
occur between 3
and 9 PM - but
tornadoes can occur at
any time of day or night
Hail
•
•
•
•
Hail is a form of solid precipitation which consists of
balls or irregular lumps of ice, that are individually
called hail stones.
Hail stones consist mostly of water ice and measure
between 5 millimetres (0.20 in) and 150 millimetres
(5.9 in) in diameter, with the larger stones coming
from severe thunderstorms.
Hail is possible with most thunderstorms as it is
produced by cumulonimbi (thunderclouds), usually
at the leading edge of a severe storm system.
Hail formation requires environments of strong,
upward motion of air with the parent thunderstorm
(similar to tornadoes) and lowered heights of the
freezing level. Hail is most frequently formed in the
interior of continents within the mid-latitudes
of Earth, with hail generally confined to higher
elevations within the tropics.
The largest recorded hailstone in the
United States by diameter (8 inches)
and weight (1.93 pounds). The
hailstone fell in Vivian, South Dakota
on July 23, 2010.
Hail Structure
• Unlike ice pellets, hail stones are
layered and can be irregular and
clumped together. Hail is
composed of transparent ice or
alternating layers of transparent
and translucent ice at least
1 millimeter (0.039 in) thick,
which are deposited upon the hail
stones.
• What causes hail to have this
structure?
Hail stone that has been cut in
half
Hail Formation
•
•
•
•
•
Inside of a thunderstorm are strong updrafts of warm
air and downdrafts of cold air.
If a water droplet is picked up by the updrafts it can be
carried well above the freezing level. With
temperatures below 32F the water droplet freezes.
As the frozen droplet begins to fall, carried by cold
downdrafts, it may thaw as it moves into warmer air
toward the bottom of the thunderstorm
But, this small half-frozen droplet may also get picked
up again by another updraft, carrying it back into very
cold air and re-freezing it. With each trip above and
below the freezing level the frozen droplet adds
another layer of ice.
Finally, the frozen water droplet., with many layers of
ice - much like the rings in a tree, falls to the ground - as
hail!
Is there a way to estimate hail size...or do
I have to go outside and measure it?
• It's often difficult to get an
accurate measurement of
hail diameter - especially
when it's falling
• The table to the right helps
observers estimate the size
of hail based on average
diameters of common items
• When in doubt - play it safe
and wait until the
thunderstorm has moved
away before going outside to
measure the size of hail.
Estimating Hail Size
Pea = 1/4 inch diameter
Marble/mothball = 1/2 inch diameter
Dime/Penny = 3/4 inch diameter - hail penny
size or larger is considered severe
Nickel = 7/8 inch
Quarter = 1 inch
Ping-Pong Ball = 1 1/2 inch
Golf Ball = 1 3/4 inches
Tennis Ball = 2 1/2 inches
Baseball = 2 3/4 inches
Tea cup = 3 inches
Grapefruit = 4 inches
Softball = 4 1/2 inches
Detecting Hail in a storm
• There are methods available to
detect hail-producing thunderstorms
using weather satellites and radar
imagery.
• When large hail is present in a
thunderstorm a signature can be
seen on a radar image called a Three
Body Scatter Spike or TBSS. This is
caused by the radar beam being
reflected off of the hail causing it to
take longer to be returned to the
radar. This extra time causes radar
reflectivity to be plotted beyond the
main reflectivity of the storm, seen as
a spike on the radar screen.
Radar reflectivity during the time of
baseball size hail in downtown
Wichita.
Detecting Hail in a storm
The 2238Z composite reflectivity from Maxwell Air Force Base (KMXX)
on February 18, 2009. This supercell in west-central Alabama had split
into two cells, while a spurious hail spike flared to the west of the
storm.
Is there a way to simply look at a
thunderstorm and tell if it will produce hail at
the ground?
• There is no positive way to look at a
thunderstorm in the distance and
tell if it will produce hail reaching
the ground.
• Meteorologists use weather radar to
"look" inside a thunderstorm. Since
hail reflects more energy back to the
radar than raindrops it often shows
up in red shades.
• The WSR-88D Doppler Radar can
also estimate size of the hail based
on the amount of energy reflected
back.
Should I be concerned about
tornadoes when hail is observed?
• The presence of large hail
indicates very strong updrafts and
downdrafts within the
thunderstorm. These are also
possible indicators of tornadic
activity. Often large hail is
observed immediately north of a
tornado track - but the presence
of hail doesn't always mean a
tornado and the absence of hail
doesn't always mean there isn't a
risk of tornadoes.
Hail Frequency
• Large hail results in nearly $1 billion in damage
annually to property and crops in the U.S.
Today in the Lab
• Today in the lab we will be using upper air soundings to
detect severe weather.
• Upper air soundings are plotted data from balloons.
• The balloon records temperature, humidity and winds
and these are plotted versus pressure/height to give
details on the vertical structure of the atmosphere.
• The type of plot is called a SkewT and a set of SkewTs
are available for most US stations. These are updated
once every 12 hours at roughly 10:00 EST.
Loaded Gun Sounding
•
•
•
•
Severe weather sounding
(large CAPE, very
unstable LI)Large hydrolapse in
mid-levels (mT air in boundary
layer and capped by cT air).
There must be an inversion above
mT air.
Most common in Great Plains,
Midwest and SE US.
Most common severe
weather: Large
hail, tornadoes, convective wind
gusts of 58mph or greater.
If speed /directional wind
shear and strong low-level jet are
present on sounding, severe
weather chances are enhanced.
Homework
• Study for final quiz. Labs covered:
– Lab 6: Global Cloud Cover Trends
– Lab 8:El Niño
– Lab 10: Convection in the Atmosphere and Oceans