SCIENCE
The Union City, Oklahoma tornado on May 24, 1973, was the first tornado captured by
the National Severe Storms Lab (NSSL) chase team using Doppler radar (source: NOAA
Photo Library, NOAA Central Library; OAR/ERL/NSSL).
Definition: The tornado (also called cyclone or twister) is a violently rotating column of
air, pendant from a cumulonimbus cloud, and nearly always observable as a “funnel
cloud” or tube. It is the most destructive of all atmospheric phenomena. The vortex,
having an average size of several hundreds of yards in diameter, whirls usually
cyclonically (a few cases anti-cyclonically) at a speed of 100 to more than 300 mph
(NOAA).
Causes: A tornado is produced inside a severe thunderstorm cloud called a
cumulonimbus, and is usually accompanied by hail, lightening and thunder. This
thunderstorm may be associated with frontal boundaries, squall lines, mesoscale
convective complexes, supercells, and tropical cyclones.
Types: Tornadoes come in all shapes and sizes, and can occur anywhere in the U.S. at
any time of the year. Tornadoes are classified according to a system known as the Fujita
or Fujita-Pearson scale, named after Prof. Theodore Fujita, former professor at the
University of Chicago, and Dr. Alan Pearson, former director of the National Severe
Storm Forecast Center (NSSFC), who devised the system in 1971. This scale relates the
wind speed of a tornado to the amount of damage done (NOAA).
See http://www.spc.noaa.gov/faq/tornado/f-scale.html for a description of the FujitaPearson Scale and photos of damage typical of each category.
(source: Tornado Project Online)
Other tornado or vortice-like phenomena include:
Waterspouts: A waterspout is a tornado over water -- usually meaning non-supercell
tornadoes over water. Waterspouts are common along the southeast U.S. coast -especially off southern Florida and the Keys -- and can happen over seas, bays and lakes
worldwide. Although waterspouts are always tornadoes by definition; they don't officially
count in tornado records unless they hit land. They are smaller and weaker than the most
intense Great Plains tornadoes, but still can be quite dangerous. Waterspouts can overturn
small boats, damage ships, do significant damage when hitting land, and kill people. The
National Weather Service will often issue special marine warnings when waterspouts are
likely or have been sighted over coastal waters, or tornado warnings when waterspouts
can move onshore (SPC, NOAA).
Waterspout over the Florida Keys, Sept. 10, 1969 (NOAA Photo Library).
Landspouts: These non-supercell tornadoes arise from the intensification of preexisting,
shallow vertical vortices near the surface, through simple vortex stretching when a
developing convectional updraft moves over them. These narrow vortices look like
waterspouts, hence the name.
Multi-vortex tornado: Multi-vortex (a.k.a. multiple-vortex) tornadoes contain two or
more small, intense subvortices orbiting the center of the larger tornado circulation.
When a tornado doesn't contain too much dust and debris, they can sometimes be
spectacularly visible. These vortices may form and die within a few seconds, sometimes
appearing to train through the same part of the tornado one after another. Subvortices are
the cause of most of the narrow, short, extreme swaths of damage that sometimes arc
through tornado tracks. From the air, they can preferentially mow down crops and stack
the stubble, leaving cycloidal marks in fields. Multi-vortex tornadoes are the source of
most of the old stories from newspapers and other media before the late 20th century
which told of several tornadoes seen together at once. However, on rare occasions,
separate tornadoes can form close to one another as satellite tornadoes (see below).
Multi-vortex tornado near Altus, OK., May 11, 1982 (NSSL,NOAA).
Gustnadoes: These very small scale, shallow vortices may develop near the surface
along outflow boundaries and/or cold fronts with or without deep convection aloft.
Tornadocane: This unusual cluster of thunderstorms, assumed a hurricane-like shape
complete with an eye, produced several damaging tornadoes in North Carolina in April,
1999. For more on this tornadocane, see:
http://www.spc.noaa.gov/coolimg/nc_storm/index.html
Frequency and seasonality of occurrence:
In an average year, about 1,000 tornadoes are reported across the United States, resulting
in 80 deaths and over 1,500 injuries. Peak tornado season is March through May in
southern states and during the summer in northern states.
(Source: NSSL, NOAA)
For the United States as a whole, tornadoes peak from April through July, with May
normally having the largest numbers.
source: Storm Prediction Center
The frequency and seasonality of tornadoes vary from year to year as shown below.
Large tornado outbreaks, like in May, 2003, skew the averages if we only look at a few
year.
(source: Storm Prediction Center, NOAA)
The largest outbreak of tornadoes, when 148 tornadoes touched down in one day, April 34, 1974, is shown in the map below.
(source: SPC, NOAA)
The total number of tornadoes reported each year has increased (see below). This is
probably due to better detection and more sitings due to population increases in many
parts of the country.
Tornado Detection:
The National Weather Service uses several methods to help detect tornadoes either
forming or on the ground. Storm spotters are volunteers that call in to their local forecast
office with reports of severe weather and can be valuable life-savers with their timely
reports. See http://www.cimms.ou.edu/~doswell/spotter_history.html for a historical look
at the role of storm spotters in tornado detection.
Radar.
Conventional radar transmits microwave pulses that when they strike an object, a fraction
is scattered back to an antenna. Precipitation particles are large enough to bounce back an
“echo” on a radar screen. Tornadoes often form a “hook echo” in the southwest portion of
a thunderstorm. However, not all tornadoes produce this hook shape on a radar screen.
Doppler radar can see not only the precipitation in a thunderstorm (through its ability to
reflect microwave energy, or reflectivity), but motion of the precipitation along the radar
beam. In other words, it can measure how fast rain or hail is moving toward or away from
the radar. In the dual image below, the radar was unusually close to an F5 tornado in
northern Moore, Oklahoma -- close enough to make out signatures of the tornado itself.
This large, violent and extremely destructive tornado was hurling many tons of debris
high into the air as it approached I-35. The debris, which reflects radar energy much
stronger than rain does, caused the brightest white blocks of reflectivity within the hook
echo. To make a storm-relative velocity image, the radar system's computer programs
take away thunderstorm movement to give a truer look at the motions inside the storm.
Here, the mesocyclone and tornado in northern Moore are represented by progressively
brighter greens (toward the radar) and reds (away from the radar).
(source: SPC, NOAA)
Turtles are small, squat, heavy, aerodyamic instrument packages which were designed to
withstand tornado wind speeds while measuring temperature, pressure and humidity at
ground level. During the VORTEX program, they were sometimes placed on the ground
at 100-250 yard intervals in the path of tornadic mesocyclones. Scientists are still
analyzing data from those deployments. [Turtles do not measure winds.]
TOTO (TOtable Tornado Observatory) was a white metal barrel (painted orange in its
last few years of service), weighing from 250-350 pounds. The acronym TOTO was, of
course, adapted from the name of Dorothy's dog from The Wizard of Oz. TOTO was
outfitted with a variety of weather instruments -- anemometers, pressure sensors and
humidity sensors. The trick was to get TOTO in harms way, however it was never
successful at capturing a direct hit. TOTO was retired after 1987 because of safety issues
and the logistical difficulty of getting such a large, heavy, cumbersome object in front of
a tornado.