Severe Weather Elements
There are some thunderstorms out there that, because of the conditions surrounding them, can create updrafts and downdrafts that are so in phase, that they can actually enhance the environment around themselves and last for hours longer than a normal thunderstorm. They are also capable of channeling powerful energy into the development of high winds, heavy rains, hail, lightning, and even tornadoes. A Severe Thunderstorm is on that is typically characterized by:
Interior Cyclonic rotation. Warm winds at the surface interact with
cooler winds from a different direction in the upper levels resulting in a
shearing cyclonic flow inside a developing severe thunderstorm. |
 Tornado
development. Tornadoes form out of that rotation inside a severe
thunderstorm. Not all severe thunderstorms produce tornadoes, and a 'weak'
severe thunderstorm is still capable of putting down a tornado. |
 Hail.
The typical quantifier for a severe thunderstorm would be hail that's
3/4" in diameter (about the size of pennies), but a National Weather
Service can still issue a warning about a severe thunderstorm even if it
may not rise to that occasion. |
 Wind.
A typical identifier for a severe thunderstorm would be for wind to exceed
50knots, however, some severe thunderstorms may not have winds that strong
and yet possess other characteristics of severe weather. Microbursts,
downdrafts, whatever you would like to call them, associated with a severe
thunderstorm can often exceed 100 miles per hour and not only seem like a
tornado went by, but in some cases be more powerful than winds associated
with a tornado that may spawn from the thunderstorm itself. |
There are two main types of severe thunderstorms: Supercells
and Squall-line Thunderstorms. Supercells are often out on their own
ahead of a 
line
of thunderstorms and create their own environment using the warm, moist air near
the surface and the cool air aloft. On a radar image, they look like blobs of
the most intense radar returns seemingly by themselves. Here is a radar loop of
a supercell thunderstorm that occurred in Australia in 1995. The strongest
activities is in red and white. The top picture is how it would look from above,
and the bottom image is a simulation of the view from the side. The squall-line
thunderstorms form a bit differently, where often ahead of an advancing cold
front small perturbations in the upper level wind flow will contribute to a
slight upward air motion downstream which explodes into mass convection given
the right level of instability. These often form in the Central Plains states,
and often bring gusty winds. Though these storms can form all elements of severe
weather, 
squall-line
thunderstorms usually aren't as awesome a spectacle as a supercell because all
incoming energy is not devoted to a single storm. The following radar image
depicts squall-line thunderstorms advancing on and through the Knoxville area of
Tennessee in 1999. None of these squall line storms produced a tornado, but
widespread wind speeds of over 70mph and 1-3" of rain caused many problems
and even a few deaths. Notice the "bow"-like appearance of the yellows
and reds on the radar imagery. This is usually a signifier in severe weather
cases of very gusty winds.
Lightning
Lightning is simply a discharge of electricity.
Under normal circumstances, we would just call this a spark, but when the amount
of electricity is on the order of hundreds of thousands of volts, 'spark' sounds
a bit wee. When lightning ionizes the air it travels through, it can heat it up
warmer than the surface of the sun (a balmy 54,000ºF). This electrical
discharge, as with any spark, is the result of attracting charges getting too
close to one another in large enough quantities. Lightning may strike from a
cloud to the same cloud or another cloud, from the cloud to the air, or from the
cloud to the ground. Not all lightning strikes are fatal, though around 100
people die each year from lightning strikes. Charges are present in all atoms,
but normally in a stably neutral arrangement. The electrification of clouds is
still somewhat a source of debate, but it's pretty well agreed that
precipitation movement has a lot to do with it. When water vapor condenses into
droplets, or freeze into ice crystals, there is some latent heat released that
adds to the temperature of the localized are and droplets themselves (especially
if a super-cooled water droplet freezes immediately onto a hailstone or piece of
graupel). When water droplets or water droplet systems (raindrop, hailstone,
etc.) of different temperatures collide, there is a net transfer of positive
ions from the warmer object to the colder object. It follows that the colder
precipitation 
and
colder regions of the cloud would be positively charged, and the middle layer
would be negatively charged. The part of the cloud close to the surface would be
a mix of charges, but often there's a small core of positive charges inside
falling precipitation near the melting level (altitude where air temperature is
32ºF).
Opposite charges attract one another, so whatever is roaming
around underneath the cloud in terms of a net charge gathering will also cause
all opposing charge ions to follow along the ground almost as a shadow.
Generally it would be negatively charged ions in the base of the cloud, and
positively charged ground ions. The charges would seek to get as close to each
other as possible, which is why trees, antennae, and buildings often get struck
by lightning. Once the electric potential is large enough (higher charge over a
shorter distance), any insulating properties of the air separating the two
charges is broken down (ionization) and a current flows (lightning) designed to
ease the charge buildup. A typical cloud-to-ground lightning strike carries with
it an electric field of 3,000,000 volts per meter along a path of about 50
meters (that's a lot). A discharge of electrons would then move down toward the
ground in a "stepped fashion" covering about 50 meters at a time. This
'stepped leader' is very faint and often invisible to the human eye. Eventually,
the flow of electrons along the stepped leader gets sufficiently close to the
ground that the potential breaks down at the surface and all the positive
charges rise to meet the leader, at which point the huge flash of lightning is
typically seen as a channel is now opened up in a jagged pattern between the
cloud and ground. This return stroke is the most visible element of
lightning. Once this channel is open, many more leaders and return strokes can
occur along exactly the same path (the resistance is l
east
here once the air has been ionized). Sometimes the eye can perceive the many
different strokes as a flicker, but other times it all happens so fast it is
indiscernible. Since lightning is as random as it is lazy, it is nearly
impossible to figure out in advance where lightning will strike other than to
say it follows the path of least resistance (usually the shortest way to the
ground). Sometimes the only way to be sure your house won't get struck by
lightning is to install a "lightning rod", which ironically encourages
lightning to strike, but in a more manageable manner (lightning strikes the rod
which channels the electricity away from the house). If you are outside and
close to being struck by lightning, you will soon notice though not have much
time, because the ionizing air would make you look a bit like this: A situation
like this one is not a time for smiling, as you may have less than a few seconds
to do whatever you can to avoid being the main highway of all charges in the
area to move about 300,000 volts.
When lightning strikes, the thunder is the sound of
the shockwave created by the ionizing of the air. A normal spark may make a
small "tick" sound, but the roaring boom of lightning can have quite a
loud crackling noise when close to the actual strike. Whereas lightning will
travel at the speed of light, thunder travels at the speed of sound, and can be
muffled or reverberated by bouncing off clouds and buildings and such. This is
why it takes thunder about 5 seconds for every mile longer to reach a person
after lightning is witness, and why often a thunderclap can be a rolling series
of booms that last several times longer than the lightning strike itself. If you
can hear thunder, you can be struck by lightning when in an unsafe place.
Sometimes thunder can be muffled completely, or a lightning strike could happen
far enough away in a moist environment that the light from it can be reflected
long distances. This is called Heat Lightning because it often occurs on
hot, summer nights, but it really is lightning, but it's just too far away for
the thunder to be heard.
Hail
Within an severe thunderstorm, the updrafts and downdrafts of a thunderstorm are
quite powerful, and in between the two is often a region of vertical
circulation. Needless to say, precipitation and the like get caught up in this
circulation and go up and down in this circulation system, Each time a rain drop
goes up into the upper regions of a thunderstorm, it either freezes or has
super-cooled water droplets or ice crystals freeze onto it. On it's way down, it
will attract more water droplets before riding again up the updraft. Sometimes
in more powerful thunderstorms, individual hailstones will actually stick
together and form one unit, which is quite a deadly one. Once enough frozen
matter has collected onto the droplet mass, it starts to get a white-ish hue and
becomes more opaque. As it goes through the cycle, it can sometimes evidence the
rings of expansion
,
similar to what a tree would do along it's growth path. Once the updraft can no
longer support another trip high up into the cloud, the hailstone will fall to
the ground, sometimes at speeds over 100 mph. Hail does not fall from a
thunderstorm immediately as rain might, but it takes some time for some hail to
form, and, depending upon the severity of the system, grow large enough to fall.
One thing is for sure: It is never a good idea to be outside during a hailstorm,
because often inside a 'hail streak' of smaller hail falling locally, there
could be a small region in which hail the size of golf balls will fall, breaking
windows, denting cars, and striking deadly blows on those who do not take cover.
Microbursts
Microbursts, and the larger scale wind gust Downburst
are very high winds associated with an advancing severe 
thunderstorm.
Within a thunderstorm there is often very heavy rain. In this rain, water that
is quite cold is falling through air that is much warmer. The temperature
difference causes a wind, and moreover also causes the colder air to be
accelerated towards the ground. Compounding this is the large onrush of mass
(think of many many millions of raindrops all pushing ground-ward with small
amounts of air inbetween being propelled out of the way. Just like tilting a
large piece of plywood onto the ground, there is a large rush of air 
compounded
by the dwindling amount of room for it all to fit under the large barrage of
rain-water. It all combines for a cool gust of wind that flies outward from the
storm and wreaks havoc on the ground below. Often these winds are stronger than
a tornado that may form in the same thunderstorm were to be. In the midst of a
downburst, an intense core of winds may be a few times as strong as the larger
downburst itself, often over 100 mph. These microbursts are quite deadly and
often mistook for a tornado.
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Tornado Damage vs. Microburst Damage |
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Tornadoes
Tornadoes are by far the most awesome specimen in
meteorology, and also the most often discussed by weather enthusiasts and
Average Joes alike. 
They
form in severe thunderstorms in which rotation is present due to vertical wind
shear in the atmosphere. Warm wind flowing in from the southwest, coupled with
cold upper level winds from the northwest will generate a cylinder of rotating
air oriented horizontally. When this air comes across a powerful severe
thunderstorm, there are additional wind influences put in place which are part
of the thunderstorms updraft and downdraft. The direction and speed of wind in a
system is the result of all influences, so in this case what would result would
be for the horizontally oriented rotating column of air to be tilted upward
locally into an upside-down "U" shape, aided by the updraft of the
thunderstorm. Next, since the ambient horizontally-oriented wind shear still
exists, it will act to diminish one side of the "U" shaped spinning,
while enhancing the other, gradually creating only one main column of spinning
air (although in a severe thunderstorm, it is possible to have multiple
updrafts, areas of rotation, and/or tornadoes). Since air moves as a constant
resultant of all forces acting on it, all of these steps are often taken
simultaneously, so certain steps may not be as obvious before the detection of a
cyclonic circulation within a thunderstorm core. When this happens, it can be
detected by modern radar, allowing some time for an alert to be issued. All
tornadoes begin as a rotating column of air within a severe thunderstorm, and
can often be seen before toughing down as a Funnel Cloud, resembling a
low, rotating, elephant trunk hanging down from the base of the storm. If the
ground is flat enough, and the shear strong enough, a positive feedback system
occurs, whereby rushing air into the cyclonic circulation deepens the pressure
at the core and continues to further tighter circulation and convergence. Like a
figure skater pulling in her arms on a twirl, the circulation gets tighter as
the general circulation is drawn into a tighter radius. A 30 mph wind out of the
south 10 miles away from a 30 mph wind from the north can get quite dramatic
when it is brought into a tight spin of less than a mile using the conservation
of angular momentum.
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Dangerous Tornado Elements |
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As air is drawn into the circulation, it condenses, creating clouds almost from the ground up depending upon the circulation strength. Moreover, the convergence causes a powerful lifting to take place, and what may begin as dirt and sand swept up to darken the newly forming tornado, can quickly turn into a powerful vortex that can pick up a car and throw it hundreds of yards. Sometimes the circulation can generate multiple vortices all rotating around each other. In the center of the tornado, much like a hurricane, all wind forces are balanced and the result is not only no wind, but calm conditions. But unlike a hurricane, getting to the center of a tornado is more like a blade of grass venturing to the middle of the mower blade.
Tornadoes are measured chiefly by the damage they cause, and the Fujita Scale, named for the late and great meteorologist who developed it. He actually created a 12 level scale calculated from damage and wind speeds out to Mach1 (around 750 mph), but it is nearly inconceivable not only to get a tornado that strong, but to ever prove the strength, as nearly anything man can create can be ripped to shreds, moved hundreds of yards from the original spot, and the whole area flattened bare of even grass in the path of a tornado half-way up the scale. To compensate, level "F"(for Fujita)"0" was inserted for those real weakling tornadoes, and nothing is really used past F5, although F6 is kept on stand-by in case something really mythical shows up. Here are the representative description of each stage of the Fujita Scale (adapted from Cappella, 1999)
| F-0 Gale tornado (40-72 mph): Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages sign boards. Damage estimates with a F0 tornado not convincing with photographs, but rather observation and small scale debris pattern. |
 F-1
Moderate tornado (73-112 mph): The lower limit is the beginning of
hurricane wind speed; peels surface off roofs; mobile homes pushed off
foundations or overturned; moving autos pushed off the roads; attached
garages may be destroyed. |
 F-2
Significant tornado (113-157 mph): Considerable damage. Roofs torn off
frame houses; mobile homes demolished; boxcars pushed over; large trees
snapped or uprooted; light object missiles generated. |
 F-3
Severe tornado (158-206 mph): Roof and some walls torn off
well-constructed houses; trains overturned; most trees in forest uprooted. |
 F-4
Devastating tornado (207-260 mph): Well-constructed houses leveled;
structures with weak foundations blown off some distance; cars thrown and
large missiles generated. |
 F-5
Incredible tornado (261-318 mph): Strong frame houses lifted off
foundations and carried considerable distances to disintegrate; automobile
sized missiles fly through the air in excess of 100 meters; trees
debarked; steel-reinforced concrete structures badly damaged. |
F-6 Inconceivable tornado (319-379 mph): These winds are very unlikely. The small area of damage they might produce would probably not be recognizable along with the mess produced by F-4 and F-5 wind that would surround the F-6 winds. Missiles, such as cars and refrigerators would do serious secondary damage that could not be directly identified as F-6 damage. If this level is ever achieved, evidence for it might only be found in some manner of ground swirl pattern, for it may never be identifiable through engineering studies.