Severe Weather

Tornadoes

A Tornado is a violently rotating column of air that is associated with a thunderstorm.
No other weather phenomenon can match the fury and destructive power of tornadoes.
They can destroy large buildings, leaving only the bare concrete foundation,
lift 20-ton railroad cars from their tracks, or drive straw and blades of grass
into trees and telephone poles.
Tornado damage to man-made structures is the result of high wind velocity and the
associated windblown debris. Tornadic winds have been measured well in excess of 225 mph (375 km/h).
When viewed from above, the majority of
tornadoes rotate counterclockwise (at least in the Northern Hemisphere).
About 1% of tornadoes in the Northern Hemisphere rotate clockwise.
The center of a tornado is characterized by low pressure, which is typically 10-20 percent
lower than the surrounding air pressure. This pressure differential occurs over a very
short distance, resulting in a large pressure gradient force that generates
high wind speeds. As air spirals into the low pressure center, it expands and cools.
If the air cools enough condensation occurs and the characteristic condensation funnel
can be seen. If condensation does not take place, the only visual sign of the tornado
is circulating dust and debris lifted by the strong winds.
There is a common misconception that the strong winds are only found inside the funnel,
however, in reality, the strong winds can extend a considerable distance away from
the visible funnel.
A funnel cloud is a tornado whose circulation remains above
the ground.

Each year, tornadoes take the lives of many people. The yearly
average is about 100 (60 in the United States), although over 100 may die in a single day.
The deadliest tornadoes are those that occur in families;
that is, different tornadoes all spawned within a region with
favorable atmospheric conditions for tornado development.

When a large number of tornadoes (typically 6 or more) forms
over a particular region, this constitutes a tornado
outbreak. One of the most violent occurred on April 3 and 4,
1974. During a 16 hour period, 148 tornadoes cut through parts of
13 states, killing 307 people, injuring more than 6000, and
causing an estimated $600 million in damage.

An
extreme tornado outbreak occured in the southeastern United States
from April 25 – 28, 2011. There were 359 confirmed tornadoes in 21 states and
322 tornado deaths in six states. A record number of 208 tornadoes were observed
on April 27 alone and four of those have been classified as EF-5 tornadoes, the
strongest category for tornadoes. About one month later on May 22, the devastating EF5
Joplin tornado struck
Joplin, Missouri, killing 155 people. This was the deadliest single tornado to strike the US since 1947.
In 2011 there were 550 tornado deaths in the US (4th highest annual total) and 1691 reported tornadoes
(2nd highest annual total behind 1817 tornadoes in 2004).
Thus, 2011 will go down as one of the worst tornado years in US history.

Note that 2012 has been a very quiet tornado year ranking in the bottom 20% for total number
of reported tornadoes.

Tornado Occurrence

Tornadoes occur in many places of the world, but no country
experiences more tornadoes than the United States, which averages
more than 1000 annually and experienced a record 1817 tornadoes
during 2004. There has been a general increase in the number of reported tornadoes
per year in the United States in recent decades. However, this probably has
much to do with the fact that a higher percentage of tornadoes that do form are detected
and reported compared with historical tornado counts prior to sophisticated RADARs,
population growth, and better awareness by the public, including tornado chasing.

Roughly 75% of all tornadoes reported worldwide occur
in the United States. Surprisingly, central Florida experiences the greatest
number of tornadoes. However, most of the tornadoes that occur
in Florida are weak and cause relatively minor damage and few deaths.

While tornadoes have occurred in every state the greatest
number of strong tornadoes, those that cause extensive damage and are
responsible for the largest share of deaths, occur in the tornado belt or tornado alley of the
Central Plains, which streches from central Texas northward
through Oklahoma and Kansas to Nebraska and includes parts of
Colorado, Iowa, Illinois, Missouri, and Arkansas.
In this region warm, humid tropical surface air from
the Gulf of Mexico interacts with much colder and drier air masses moving
down from Canada. This area is unique on Earth. A warm ocean surface lies
to the south and a large continental land mass to the north with no
mountainous barriers to prevent the clash of warm, humid air masses from the south
and very cold, dry air masses from the north. This also explains why this
region experiences high numbers of severe thunderstorms as well.

About three-fourths of all tornadoes in the United States
develop from March to July. Although tornadoes have occurred at
all times of the day and night, they are most frequent in the late
afternoon, when the surface air is most unstable.

The three figures on below display where tornadoes of differing strengths
are most common in the United States. Click on the thumbnails to
see larger image. The EF scale is defined in the next section.

Tornado days per year (All tornadoes)
Significant (EF-2 or stronger) Tornado days
per century
Violent (EF-4 or larger) Tornado days
per millennium

The three figures on above display where tornadoes of differing strengths
are most common in the United States. Click on the thumbnails to
see larger image.

Tornado Winds

The strong winds of a tornado can destroy buildings, uproot
trees, and hurl all sorts of lethal missiles into the air. Our
earlier knowledge of the furious winds of a tornado came mainly
from observations of the damage done and the analysis of motion
pictures.

In the late 1960s, the late Dr. T. Theodore Fujita, a noted
authority on tornadoes at the University of Chicago, proposed a
scale (called the Fujita scale for
classifying tornadoes according to their rotational wind speed
based on the damage done by the storm. The way this scale has been
used is that a tornado’s windspeed is estimated based on
the damage caused by the tornado (after the fact). The Fujita has
six categories for tornadoes, labeled F0, F1, F2, F3, F4, and F5, where
F0 is the weakest category and F5 the strongest category. Later
research has shown that the Fujita scale overestimates the
actual windspeeds in tornadoes. For example, it was once believed
that the strongest tornadoes produced winds of 300 mph or greater.
New research estimates that the strongest tornadoes produce winds
of up to 225 mph.

Starting February 1, 2007, the National Weather Service revised
the original Fujita scale linking tornado damage to windspeed. The
new scale is called the enhanced Fujita scale (EF scale). There are
still 6 categories for tornadoes (now EF-0 through EF-5). The new
EF scale
was developed to improve the estimation of windspeed based on the damage
by better considering the structural integrity of different building types.

Tornado Characteristics

Summary of Tornado Characteristics

Characteristic
Most Common
Extreme / Possible

Location on Earth
United States
Nearly Anywhere

Time of year (USA)
March – July
Any month

Time of day
4 – 6 PM
Any time

Size (diameter)
50 yards
> 1 mile

Movement-speed
30 mph
0 – 70 mph

Movement-direction (USA)
Toward Northeast
Any Direction

Length of Ground Path
< 2 miles
> 300 miles

Time on Ground
< 5 minutes
> 6 hours

Wind Speed
< 100 mph (EF0,EF1)
> 200 mph (EF5)

Tornado Safety

Most tornado-related deaths and injuries are caused by flying debris, so
the most important consideration is to shelter yourself from flying debris.
If possible go to a sturdy structure. Your best bet is to move into a
basement or underground storm shelter. If these are not available go to an
interior closet away from windows. Cars and mobile homes are not sturdy
structures and are dangerous locations to wait out a tornado. If you are
caught outside, you want to get as low to the ground as possible. Your best bet
is to lie flat in a ditch or depression in the ground.

Interesting and
Easy to Understand article on tornado characteristics and safety from the
Storm Prediction Center in Norman, OK.

You Tube video of cars tossed around by a tornado in Alabama

Tornado Formation

Ingredients necessary for tornado formation:

1. An unstable atmosphere that is capable of producing strong thunderstorm updrafts and downdrafts.

2. A lifting force. The most common lifting forces are heating of air near the
surface and weather fronts.

3. Vertical wind shear to provide rotation.

The exact sequence of events that lead to tornadoes are not fully understood.
Although meteorologists can locate regions where the
conditions are more favorable for tornado development (based on the ingredients above),
the exact location and time cannot be predicted accurately. When favorable conditions
are present over a particular region, a tornado watch may be issued.

There seem to be several mechanisms that can lead to tornado formation, none of
which are fully understood.
One proposed mechanism of tornado formation involves rotating thunderstorm updrafts. A simple
description is provied in the two liked pages:
( formation page 1, formation page 2).
The mechanism described on the pages above is probably not the mechanism typically responsible
for tornado formation in supercell thunderstorms, though. The proposed explanations for tornado formation
in supercells is too complex to cover in this class.

Tornado Warning

Tornadoes come and go so quickly and are so small that predicting where and when one will hit more than 15 to 30 minutes ahead of time is not possible. The average warning time for a tornado with winds of 158 miles an hour or faster – the type that accounts for most deaths – is 18 minutes.
Of course if the tornado forms right on top of you, then in some cases
no warning can be given. Tornadoes can be detected with
Doppler Radar and when they are,
tornado warnings are issued. All weather radars work by measuring the reflected (or more correctly backscattered)
radiation coming from large particles such as raindrops and hail. Doppler radars can also detect
whether the reflecting particles are moving toward or away from the radar site. Thus, the rotating
winds around a tornado can show up very well on Doppler Radars. Sometimes the rotation can
be observed at cloud level,and a tornado warning issued, before the tornado circulation touches down on the ground.

Tornado Alley

Why is the favored region for strong tornado formation in the
central plains states (often referred to as TORNADO ALLELY)?
Severe tornadoes often form when three very different types of air come together in a particular way.
Near the ground surface southerly or southeasterly transport warm, humid air from the Gulf of Mexico into the Plains.

As you go up to the middle layers of the atmosphere above the humid air,
the winds veer to the southwest and transport hot, dry air from the Mexico highlands
and deserts over the Plains. This forms a layer of hot, dry air in which the
temperature often increases with height. The hot, dry layer is very stable and
inhibits any convection that tries to develop. The winds continue to veer as you go
up into the higher layers of the atmosphere. At the high layers of the atmosphere,
westerly winds transport cool, moist air from the Pacific Ocean over the Rocky Mountains
and into the Plains above the convective cap. This sets up a scenario that has warm,
humid air near the surface with hot dry air above it and cool, moist air at the higher levels
above the dry air. These three air streams also arrive from different directions, which
provides the necessary wind shear. This scenario highly favors severe weather, including
supercells and tornadoes.

The existence of the mid-level convective cap, which will be referred to as an inversion layer,
is a key ingredient. An inversion layer is a layer in the atmosphere
where the temperature increases with increasing height. Recall that typically temperature decreases with
increasing height. An inversion layer is extremely stable for rising motion. At first an inversion layer
will inhibit thunderstorm formation because rising parcels from the surface will not be able to penetrate
through this layer. However, as the day progresses and the
atmosphere gradually warms from below the inversion
layer erodes away, and surface
parcels become warm enough to “break through” the old inversion layer and reach the unstable
atmosphere above. All day the sun’s energy was used to heat the lower atmosphere and increase the water
vapor content by evaporation essentially adding more energy
to the lower atmosphere. When parcels are finally able to “break through”, it is like blowing the lid
off of a pressure cooker.
This is depicted graphically on this
convection cap page from USA Today
An illustrative diagram of a convective cap will be discussed
in lecture. Click here to view the diagram.

Other Tornado Links

A comprehensive explanation behind the formation of
tornadoes is given in the NOAA web page: tornadoes… Nature’s
Most Violent Storms A PREPAREDNESS GUIDE Including Safety
Information for Schools U.S. DEPARTMENT OF COMMERCE National
Oceanic and Atmospheric Administration National Weather Service
September 1992 (NOAA, FEMA, The American Red Cross).

Another informative guide (updated in Jan 2009), which includes links to historical tornadoes,
is hosted by the US STORM PREDICTION
CENTER is also available The Online Tornado FAQ.