In this lecture we'll
briefly look at the internal structures of severe
and supercell thunderstorms. Then we'll look at some general
storms are more likely to form when there is vertical wind shear.
shear (pt 1) is changing wind direction and/or wind speed with
this case, the wind speed is increasing with increasing altitude, this
is vertical wind shear.
A thunderstorm that forms in this kind of an environment
will move; we'll assume it moves at an average of the speeds at
the top and bottom of the
The thunderstorm will move to the right more rapidly than the air at
which is where the updraft begins. Rising air that is situated at
front bottom edge of the thunderstorm will find itself at the back edge
storm when it reaches the top of the cloud.
This movement from front to back produces a
updraft (pt. 3). The downdraft is situated at the back of the
ground. In a moving thunderstorm like this the updraft is
continually moving to the right and
from the downdraft. The updraft and downdraft coexist and do not
"get in each others way" as was the case in air mass
thunderstorms. A severe thunderstorm can last longer and get
larger and stronger than an air mass thunderstorm.
shear can also cause the
called a mesocyclone
(pt. 4). Meso refers to medium size
means winds spinning around low pressure. Low pressure in the
core of the
mesocyclone creates an inward pointing
gradient force needed to keep the updraft winds spinning in circular
pressure also keeps winds spinning in a tornado).
The cloud that
below the cloud base and surrounds the mesocyclone
called a wall cloud (pt.
5). The largest and strongest tornadoes
generally come from the wall cloud.
Note (pt. 6) that a tilted updraft provides a way of keeping growing
inside the cloud. Hailstones get carried up toward the top of the
where they begin to fall. But they then fall
the strong core of the updraft and get carried back up toward the top
cloud. Formation of large hailstones (3/4 inch or larger) is one
of the criteria meterologists use to identify a severe thunderstorm.
A wall cloud can form a little bit
below the rest of the base of the thunderstorm; the figure above
tries to explain why that is true. Clouds normally form when air
rises, expands, and cools as shown above at left. The rising air
expands because it is moving into lower pressure surroundings at higher
At right the air doesn't have to rise to as high an altitude to
experience the same amount of expansion and cooling. This is
because it is moving into the core of the rotating updraft where the
pressure is a little lower than normal for this altitude. Cloud
forms a little bit closer to the ground. We'll see later that the same
kind of thing happens when moist air moves horizontally into the low
pressure core of a tornado.
part of a supercell thunderstorm.
Here is a
drawing showing some of the key features in a supercell
thunderstorm. In a supercell the
updraft (shown in red above) is moving upward with enough momentum that
it is able to penetrate a little ways into the stratosphere. This
produces the overshooting top or dome feature
above. A wall cloud and a tornado are shown at the bottom of the mesocyclone. The
is a line of new cells trying to form alongside the supercell
thunderstorm. This is basically downdraft winds colliding with
prexisting winds just like what can occur along a gust front.
is a second slightly more complicated drawing of a supercell
thunderstorm. A typical air mass thunderstorm (purple) has been
drawn in to give you an idea of relative size.
This second image is from a May 3, 1999 storm that produced an F5
(perhaps an F6) tornado
that hit Oklahoma City. Winds in the tornado may have exceeded
300 MPH at an altitude about 100 feet above the ground (wind speeds
were measured with a mobile doppler radar).
with rotating updrafts often have a distinctive radar signature called
a hook echo.
A hook echo is sketched above (shaded in brown), normally this
about all you would see on a radar image of the storm (remember the
radar signal is reflected by precipitation particles but not cloud
droplets or ice crystals). Some of the other thunderstorm
features have been added here, however. FFD and RFD show the
locations of the front
flank and rear flank downdrafts, respectively. U is the rotating
updraft T in the figure shows where tornado
formation is most likely. Point G indicates something that
resembles a tornado, a
gustnado, that will sometimes form along the gust front (flanking
line). These are not true tornadoes and often do not extend all
the way from the ground to the cloud. Though winds in a gustnado
are sometimes strong enough to produce light damage. The cold
front symbol has been used to show a gust front where existing winds
collide with downdraft winds. This is the flanking line.
Storm motion is toward the ENE. Someone interested in
observing this storm (a "storm chaser") should position themselves SE
of the storm.
There they would be able to view the wall cloud and any tornadoes that
form. Many of the interesting features would be hidden by rain if
you were on the NW side of the storm.
Here are some actual radar images of supercell
thunderstorms with prominent
Note a portion of the flanking line seems to be visible in this
While the hook echo shape on a conventional radar suggests
rotation, Doppler radar can confirm it (Doppler radar uses a shift in
the frequency of the reflected signal to determine wind direction and
speed). The sketches below shows
simplified images of a supercell thunderstorm obtained with both
conventional and Doppler radar (this is the May 3 1999 Oklahoma City
The orange and yellow colors on the left image indicate the intensity
of the reflected radar signal and tell you something about
precipitation amount or intensity. The colors on the Doppler
image tell you about wind direction and speed. The green, blue,
and purple colors on the Doppler image indicate winds
blowing toward the radar (the radar was east of the image).
The red, orange, and yellow show winds blowing away from the
different colors correspond to different wind speeds. Winds
blowing in opposite directions in such close proximity indicate
rotation and are called a velocity couplet or a tornado vortex
signature. In this case the winds are spinning in a
counterclockwise direction (the Doppler radar isn't able to detect the
north and south components of the spin because those winds aren't
pointing toward or away from the radar).
This is a nice picture showing what is probably a relatively weak
tornado extending downward from the bottom edge of a wall cloud (from
has more tornadoes in an
average year than
other country in the world (over 1000 per year). The
conditions. (T. Fujita,
"Tornadoes Around the World, Weatherwise, 26, 56-83, 1973)
In the spring, cold
air can move all the way
Canada to the Gulf Coast (with being blocked by mountains) and collide
warm moist air
from the Gulf of Mexico. Strong thunderstorms can form along the
resulting cold fronts. It also helps if winds change direction
and speed with altitude (vertical wind shear).
It seems counterintuitive, but a mid-level inversion layer can
also contribute to severe thunderstorm development. An inversion
layer keeps a lot of relatively weak storms from forming. Instead
only a few, stronger-than-average storms form. If they are able
to "punch through" the inversion layer they encounter cold dry and
unstable air above and grow explosively. Jet stream winds
overhead can help by providing upper level divergence.
every state in the US, but tornadoes are most frequent in the central
plains, a region referred to as "Tornado Alley" (highlighted in red,
orange, and yellow above; the numbers are tornadoes per year within a
circle of one degreee latitude by longitude)
Here are some basic
1. About 2/3rds of tornadoes
are F0 or F1 tornadoes (the F refers to the Fujita scale, we'll learn
more about that in Lecture 32) and have spinning
about 100 MPH or less. Microburst winds can also reach 100
MPH. Microbursts are fairly common in Tucson in the summer,
tornadoes are rare. Microbursts can inflict
the same level of damage as most tornadoes.
2. A very strong inwardly directed pressure
gradient force is needed to keep winds spinning in a circular
path. The pressure in the center core of a tornado can be 100 mb
the pressure in the air outside the tornado. This is a very large
pressure difference in such a short distance. The
Force (CF) and the CF can be
3. Tornadoes can spin clockwise or
counterclockwise rotation is more common.
4, 5, 6. Tornadoes usually last only a few
minutes, leave a path
on the ground that is a few miles
long, and move at a few 10s of MPH. There are exceptions, we'll
look at one shortly.
7, 8. Most tornadoes
move from the SW toward the NE. This is because tornado-producing
thunderstorms are often found just ahead of a cold front. Winds
ahead of a cold front often blow from the SW. Most
mile have been observed.
9, 10. Tornadoes
Tornadoes are most common in the late
afternoon when the atmosphere is most unstable.
The figure traces out
the path of the 1925 "Tri-State
Tornado" . The
tornado path (note the SW to NE orientation) was 219 miles long, the
tornado lasted about 3.5 hours and
killed 695 people. The tornado was traveling over 60 MPH over
much of its path. It is the deadliest single tornado ever in the United
Tornadoes often occur
The paths of 148
during the April 3-4, 1974 "Jumbo Tornado
Outbreak" are shown above. Note the first tornadoes were
in the upper left corner of the map. The tornadoes were produced
by thunderstorms forming along a cold front (see the weather map
below). During this two day
period the front moved from the NW part toward the SE part of the
that all the tornado paths have a
SE toward NE
The recent April
is now being called the largest tornado outbreak in US history. A
total of 327 tornadoes in 21 states have been confirmed. At least
people were killed during the outbreak.
At the present time about 75 people
every year in the
United States. This is about a factor of ten less than a century
ago due to improved methods of detecting tornadoes and severe
thunderstorms. Modern day communications also make easier to warm
people of dangerous weather situations. Lightning and flash
floods (floods are the most
severe weather hazard) kill slightly more people. Hurricanes kill
fewer people on average than tornadoes.
Tornadoes begin in and descend from
thunderstorm. You would usually see a funnel cloud dropping from
of the thunderstorm. Spinning winds will probably be present
between the cloud and ground before the tornado cloud becomes
visible. The spinning winds can stir up dust at ground
level. The spinning winds might also be strong enough at this
point to produce some minor damage. Here's a pretty good example
of the beginning stages of a tornado: tornado
In Stage 2, moist air moves horizontally toward the low pressure
core of the tornado. This sideways moving air will expand and
cool just as rising air does (see figure below). Once the air
cools enough (to the
dew point temperature) a cloud will form.
Tornadoes can go from Stage 2 to Stage 3 (this is what the
tornadoes do) or
directly from stage 2 to stage 5. Note a strong tornado is
usually vertical and thick as shown in Stage 3. "Wedge tornadoes"
actually appear wider than they are tall.
The thunderstorm and the top of the tornado will move faster than
surface winds and the bottom of the tornado. This will tilt and
stretch the tornado. The rope like appearance in Stage 5 is
usually a sign of a weakening (though still a dangerous) tornado.
The tornado cloud forms when moist
air moves into lower
in the core of the tornado. The air expands and cools to the dew
point and a cloud forms. This is just like the cloud that forms
when air rises (and moves into lower pressure and expands).