Tornado
A tornado is defined
as: A violently rotating column of air, either pendant from a
thunderstorm or underneath a thunderstorm, in contact with the
ground, and often (but not always) visible as a funnel cloud.
Tornadoes come in many sizes, shapes, and colors. They can range in
width from several yards to more than a mile. Their lifetimes can be
as short as a few seconds or they can last several hours. They can
move forward at speeds of up to 70 miles per hour, or stand nearly
still.
Research is ongoing as
to how tornadoes form and why similar atmospheric conditions may or
may not produce tornadoes. We provide some basic information
with links to research facilities. Much of the tornado
research within the United State is confined to the National Weather
Service (and its federal partners) along with academic institutions.
There is always emerging information regarding tornadoes. At
the 2003 Severe Weather Workshop, new information was provided on
how the temperature of rear-flank downdrafts assist or retard
tornado-genesis. More information on tornadoes is provided
below, courtesy of the National Weather Service.
Tornadoes -
It's easy for
most people to recognize the funnel-shaped cloud of a typical
tornado, but the funnel can vary considerably in appearance. Some
are long and slender while others are wider than they are tall.
Some of the worst tornadoes in history, such as the Tri-State
tornado family outbreak of March 18, 1925 which killed 740 people
in Missouri, Illinois and Indiana, were not recognized as
tornadoes since they were so large or were wrapped in rain. Most
people reported an area of black skies to the west or a wall of
dark clouds - they never saw the tornado coming.
Some tornadoes do
not even have a visible funnel, and their presence is indicated only
by a swirl of dust near the ground. Other tornadoes are made up of
several funnels, with smaller funnels rotating around a central
vortex. Colors range from jet black to brown or reddish, when dust
is lifted into the funnel, to almost white when sunlight falls on
the funnel and dark clouds are in the background.
In addition to
having various sizes, shapes, and colors, some tornadoes have
distinctive sounds. Those who have been close enough to hear a
tornado usually describe the sound as a loud roar like that of a jet
aircraft or a freight train. Not all tornadoes have this roar, but
at night, this may be one of the few clues that a tornado is
approaching.
Why is Oklahoma
Tornado Alley? -
While scientists
are still trying to understand exactly how a tornado forms, the
atmospheric ingredients that lead to supercell thunderstorms -
long-lived thunderstorms with rotating updrafts - are fairly well
understood. Typically it is with these well organized thunderstorms
that the most violent weather occurs, including giant hail,
destructive winds and strong and violent tornadoes. Unfortunately,
not all tornadoes come from supercell thunderstorms, and not every
rotating updraft produces a tornado.
Near the ground
lies a layer of warm and humid air along with strong south winds.
Colder air and strong west or southwest winds lie in the upper
atmosphere. Temperature and moisture differences between the surface
and the upper levels create what we call instability, while the
change in wind with height is known as wind shear. This shear is
linked to the eventual development of storm-scale rotation, which
may eventually contribute to a tornado.
A third layer of
very warm dry air becomes established between the warm moist air at
low levels and the cool dry air aloft. This very warm layer acts as
a cap and allows the atmosphere below it to warm further, making the
air even more unstable. Things start to happen when a storm system
aloft moves east and begins to lift the various layers. Through this
lifting process the cap is removed, and sets the stage for explosive
thunderstorm development as strong updrafts develop.
Complex
interactions between the updraft and the surrounding winds both at
storm level and near the surface may cause the updraft to begin
rotating, and a supercell thunderstorm is born.
The plains of the
central United States are uniquely suited to bring all of these
ingredients together, and this region has become known as "tornado
alley." The main factors are the Rocky Mountains to the west, the
Gulf of Mexico to the south, and a terrain that slopes downward from
west to east.
During the spring
and summer months southerly winds prevail across the plains. Windy
springs and summers are well known to Oklahomans. At the origin of
those south winds lie the warm waters of the Gulf of Mexico, which
provide the moisture at low levels. Very warm, dry air forms over
the higher elevations to the west, and becomes the cap as it spreads
eastward over the moist gulf air.
Where the dry air
and the gulf air meet the ground, a boundary known as a dry line
forms, often in western Oklahoma or the Texas or Oklahoma
panhandles. A storm system moving out of the southern plains or
Rockies may push the dry line eastward, with severe thunderstorms,
possibly producing tornadoes, forming along the dry line or in the
moist air ahead of it.
Tornado Watch and
Warning -
A tornado watch is
issued by the
Storms Prediction Center when
conditions are favorable for tornadoes. The coverage of a
watch can be several counties to two or three states.
A tornado warning
is issued by a local National Weather Service Forecast Office (WSFO)
when a tornado has been spotted or indicated by RADAR.
Credits -
National
Weather Service - Norman, Oklahoma
Research Reference
-
National Severe Storms Laboratory
Purdue University
University of
Oklahoma
