Tornadoes

Lightning

Thunder

A strong electrical field

occurs prior to lightning Cloud-to-cloud

lightning (80%)

Cloud-to-ground

lightning

Lecture #20

April 14, 2010, Wednesday

“Thunderstorms & Tornadoes”

Part 2

The word tornado is an altered form of the Spanish

word tronada, which means "thunderstorm." This in turn

was taken from the Latin tonare, meaning "to thunder". It

most likely reached its present form through a

combination of the Spanish tronada and tornar ("to

turn"); however, this may be a folk etymology.

A tornado is also commonly referred to as a twister,

and is also sometimes referred to by the old-fashioned

colloquial term cyclone.

The term "cyclone" is used as a synonym for "tornado"

in the often-aired 1939 film, The Wizard of Oz. The term

"twister" is also used in that film, along with being the

title of the 1996 film Twister.

A tornado in

Anadarko, OK,

is the thin tube

from the cloud

to the ground.

Its lower part

is surrounded

by a

translucent

dust cloud,

kicked up by

the tornado's

strong winds

at the surface.

In Glossary of Meteorology by the American Meteorological

Society (AMS), Tornado is defined as “A violently rotating

column of air, in contact with the ground, either pendant

from a cumuliform cloud or underneath a cumuliform cloud,

and often (but not always) visible as a funnel cloud”.

When tornadoes do occur without visible funnel cloud,

debris at the surface is usually the indication of an intense

circulation in contact with the ground.

"Tornado" refers to the vortex

of wind, not the condensation

cloud.

There is some disagreement over the definition of

"funnel cloud" and "condensation funnel". According to

the Glossary of Meteorology, a funnel cloud is any rotating

cloud pendant from a cumulus or cumulonimbus, and thus

including most tornadoes under this definition. Among

many meteorologists, a funnel cloud is strictly defined as

a rotating cloud which is not associated with strong winds

at the surface, and a "condensation funnel" is a broad

term for any rotating cloud below a cumuliform cloud.

Tornadoes often begin as funnel clouds with no

associated strong winds at the surface, but not all of these

evolve into a tornado. However, many tornadoes are

preceded by a funnel cloud. Most tornadoes produce

strong winds at the surface while the visible funnel is still

above the ground, so it is difficult to discern the difference

between a funnel cloud and a tornado from a distance.

Funnel Clouds Discontinuous

Funnel Clouds

Wall Clouds

Tornado

Tornadoes

But air sucked in

the vortex cools

and condense to

form cloud, and

can be darken by

debris sucked in

from the surface

"Tornado" refers

to the vortex of

wind, not the

condensation

cloud.

Types

Multiple vortex tornado: a type of tornado in which two or

more columns of spinning air rotate around a common

center. Multi-vortex structure can occur in almost any

circulation, but is very often observed in intense

tornadoes. These vortices often create small areas of

heavier damage along the main tornado path.

Satellite tornado: a term for a weaker tornado which

forms very near a large, strong tornado contained within

the same mesocyclone. It may appear to orbit the larger

tornado (hence the name), giving the appearance of one,

large multi-vortex tornado. However, a satellite tornado is

a distinct funnel, and is much smaller than the main

funnel.

Some most violent tornadoes have multiple suction

vortices that account for rather selective damage patterns

A multiple-vortex tornado outside of Dallas, Texas on April 2, 1957

Types

Waterspout: defined by the National Weather Service

simply as a tornado over water. However, researchers

typically distinguish "fair weather" waterspouts from

tornadic waterspouts.

Fair weather waterspouts: less severe but far more common. They

form at the bases of cumulus congestus cloud towers in tropical and

semitropical waters. They have relatively weak winds, smooth

laminar walls, and typically travel very slowly, if at all. They occur

most commonly in the Florida Keys and in the northern Adriatic Sea.

Tornadic waterspouts: more literally "tornadoes over water“, either

form over water like mesocyclonic tornadoes, or be a land tornado

crossing onto water. Since they form from severe thunderstorms and

can be far more intense, faster, and longer-lived than fair weather

waterspouts, they are considered far more dangerous.

Water spout

Types

Landspout: officially known as a dust-tube tornado by

NWS, and is defined as a tornado not associated with a

mesocyclone by AMS.

The name stems from their characterization as essentially a "fair

weather waterspout on land".

Waterspouts and landspouts share many defining characteristics,

including relative weakness, short lifespan, and a small, smooth

condensation funnel which often does not reach the ground.

Landspouts also create a distinctively laminar cloud of dust when

they make contact with the ground, due to their differing mechanics

from true mesoform tornadoes.

Though usually weaker than classic tornadoes, they still produce

strong winds and may cause serious damage.

Landspout

Shape & Size

Most appear as a narrow funnel with a small cloud of debris near the

ground. However, tornadoes can appear in many shapes and sizes.

Small, relatively weak landspouts may only be visible as a small swirl

of dust on the ground. Although the condensation funnel may not

extend all the way to the ground, if associated surface winds are

greater than 40 mph (64 km/h), the circulation is considered a

tornado.

Multiple-vortex tornadoes can appear as a family of swirls circling a

common center, or may be completely obscured by condensation,

dust, and debris, appearing to be a single funnel.

Tornadoes may also be obscured completely by rain or dust. These

tornadoes are especially dangerous, as even experienced

meteorologists might not spot them.

Stovepipe tornado

Wedge tornado

A tornado with a nearly

cylindrical profile and relative

low height is sometimes

referred to as a stovepipe

tornado.

Large single-vortex

tornadoes can look like large

wedges stuck into the

ground, and so are known as

wedge tornadoes or wedges.

The stovepipe classification

is also used for this type of

tornado, if it otherwise fits

that profile.

A wedge tornado, nearly a mile wide.

This tornado hit Binger, Oklahoma.

A wedge can be so wide that it appears to be a block of dark clouds, wider

than the distance from the cloud base to the ground. Even experienced

storm observers may not be able to tell the difference between a low-

hanging cloud and a wedge tornado from a distance.

A rope tornado in its

dissipating stage in

Tecumseh, OK

Tornadoes in the

dissipating stage can

resemble narrow

tubes or ropes, and

often curl or twist into

complex shapes.

These tornadoes are

said to be roping out,

or becoming a rope

tornado.

Shape & Size

On the local scale, the tornado is the most intense of all

atmospheric circulations due to its small size compared

to mid-latitude cyclones, tropical cyclones or hurricanes,

and much larger pressure gradients over smaller spatial

scale, 100mb drop in pressure to the center low over a

few hundred meters.

The tornado vortex usually rotates cyclonically

(counterclockwise in the northern hemisphere, clockwise

in the southern). On rare occasions (1%) anticyclonically.

There is an extremely wide range of tornado sizes, even

for typical tornadoes.

In the US, on average, tornadoes are around 500 feet (150

m) across, and stay on the ground for 5 miles (8 km).

Shape & Size

Weak tornadoes, or strong but dissipating tornadoes, can

be exceedingly narrow, sometimes only a few feet across.

A tornado was once reported to have a damage path only

7 feet (2 m) long.

On the other end of the spectrum, wedge tornadoes can

have a damage path a mile (1.6 km) wide or more. A

tornado that affected Hallam, Nebraska on May 22, 2004

was at one point 2.5 miles (4 km) wide at the ground.

In terms of path length, the Tri-State Tornado, which

affected parts of Missouri, Illinois, and Indiana on March

18, 1925, was officially on the ground continuously for

219 miles (352 km).

Shape & Size

Many tornadoes which appear to have path lengths of 100

miles (160 km) or longer are actually a family of tornadoes

which have formed in quick succession.

But there is no substantial evidence that this occurred in

the case of the Tri-State Tornado. Modern reanalysis of

the path suggests that the tornado began 15 miles (24

km) further west than previously thought.

The wind speeds in a tornado can be as low as 18 m s-1

(40 mph, 177 km/h) to over 135 m s-1 (300 mph, 480 km/h).

Wind speeds are sometimes estimated on the basis of

wind damage.

Appearance

Tornadoes can have a wide range of colors, depending on the

environment in which they form.

Those form in a dry environment can be nearly invisible, marked only

by swirling debris at the base of the funnel.

Condensation funnels which pick up little or no debris can be gray to

white.

While traveling over a body of water as a waterspout, they can turn

very white or even blue.

Funnels which move slowly, ingesting a lot of debris and dirt, are

usually darker, taking on the color of debris.

Tornadoes in the Great Plains can turn red because of the reddish tint

of the soil, and tornadoes in mountainous areas can travel over

snow-covered ground, turning brilliantly white.

Appearance

Lighting conditions are a major factor in the appearance of a tornado.

A tornado which is "back-lit" (viewed with the sun behind it) appears

very dark. The same tornado, viewed with the sun at the observer's

back, may appear gray or brilliant white.

Tornadoes which occur near the time of sunset can be many different

colors, appearing in hues of yellow, orange, and pink.

Dust kicked up by the winds of the parent thunderstorm, heavy rain

and hail, and the darkness of night are all factors which can reduce

the visibility of tornadoes. Tornadoes occurring in these conditions

are especially dangerous, since only weather radar observations, or

possibly the sound of an approaching tornado, serve as any warning

to those in the storm's path. Fortunately most significant tornadoes

form under the storm's rain-free base, or the area under the

thunderstorm's updraft, where there is little or no rain.

Appearance

Most tornadoes occur in the late afternoon, when the bright

sun can penetrate even the thickest clouds.

Night-time tornadoes are often illuminated by frequent

lightning.

There is mounting evidence, including Doppler On Wheels

(DOW) mobile radar images and eyewitness accounts, that

most tornadoes have a clear, calm center with extremely low

pressure, akin to the eye of tropical cyclones. This area would

be clear (possibly full of dust), have relatively light winds, and

be very dark, since the light would be blocked by swirling

debris on the outside of the tornado.

Lightning is said to be the source of illumination for those who

claim to have seen the interior of a tornado.

Supercell Tornado Development

Most violent tornadoes arise from supercells.

It begins with the development of a mesocyclone (~30 minutes before formation of actual tornado) a large rotation region (6 mile diameter) within the cloud interior forms due to vertical wind shear as winds shift typically from southerly at the surface to westerly aloft while speed increases with altitude.

Strong updrafts tilt the rotation region to a vertical position while the diameter decreases a spatial decrease comes an increase in speed (rotation intensify) as dictated by the conservation of angular momentum.

Can develop

in any

severe

weather:

Frontal

boundary,

Squall lines,

MCCs,

Supercell,

hurricane

Tornadoes

Strong

Updrafts

Mesocyclone forms from horizontal

vortex becoming vertical vortex

A tornado producing

supercell

The rotating air column penetrates cloud base forming a wall cloud.

From the wall cloud, a narrow rotating vortex emerges as the funnel

clouds with the characteristics and intensity of a tornado except

touching ground.

Doppler radar detects mesocyclone development leading to

increased warning times (~30 minutes).

Only about 1/2 of all mesocyclones actually spawn a tornado.

A Doppler radar image indicating the likely

presence of a tornado over DeLand, Florida. Green colors indicate areas

where the precipitation is

moving towards the radar

dish, while red areas are

moving away.

In this case the radar is in

the bottom right corner of

the image. Strong

mesocyclones show up as

adjacent areas of bright

green and bright red, and

usually indicate an

imminent or occurring

tornado.

When these bright colors

are one against the other

on a radar display when in

association with rotation, it

is called a Tornado vortex

signature (TVS).

Tornado Research

Doppler On Wheel (DOW)

Meteorology is a relatively young science and the study

of tornadoes even more so.

Although studied for about 140 years and intensively for

around 60 years, there are still aspects of tornadoes

which remain a mystery.

Scientists do have a fairly good idea of the development

of thunderstorms and mesocyclones, and the

meteorological conditions conducive to their formation.

However, the step from supercell (or other respective

formative processes) to tornadogenesis and predicting

tornadic vs. non-tornadic mesocyclones is not yet well

understood and is the focus of much research.

Tornado Research

Intensity Scale (Damage)

Tornadoes vary in intensity regardless of shape, size, and location.

Strong tornadoes are typically larger than weak tornadoes. The

association with track length and duration also varies, although longer

track tornadoes tend to be stronger. In the case of violent tornadoes,

only a small portion of the path is of violent intensity.

In the United States, 80% of tornadoes are EF0 and EF1. The rate of

occurrence drops off quickly with increasing strength—less than 1%

are violent tornadoes (EF4 or stronger).

Outside the United States, areas in south-central Asia, and perhaps

portions of southeastern South America and southern Africa, violent

tornadoes are extremely rare. This is apparently mostly due to the

lesser number of tornadoes overall, as research shows that tornado

intensity distributions are fairly similar worldwide. A few significant

tornadoes occur annually in Europe, Asia, southern Africa, and

southeastern South America, respectively.

The original scale as derived by

Fujita was a 13-level scale (F0-

F12) designed to smoothly

connect the Beaufort scale and

the Mach number scale. The gap

between F0 and F1 corresponds

to the 11th and 12th levels of the

Beaufort scale, "violent storm"

and "hurricane" respectively.

On the original scale, the wind

speeds for F11 and F12

corresponded to Mach numbers

0.9 and 1.0 respectively. This

provided a smooth relationship

among the three scales. From

these wind speed numbers,

qualitative descriptions of

damage were made for each

category of the Fujita scale, and

then these descriptions were

used to classify tornadoes.

http://www.spc.noaa.gov/efscale/

F0 F1

F2 F3

F4 F5

EF0 EF1

EF2 EF3

EF4 EF5

Although tornadoes have been observed on every continent except

Antarctica, most occur in the United States. They also commonly occur

in southern Canada, south-central and eastern Asia, east-central South

America, Southern Africa, northwestern and southeast Europe, western

and southeastern Australia, and New Zealand.

Global Distribution of Tornadoes

An American phenomenon: US world’s leader in tornado production,

four times more than estimated in all of Europe, not including waterspouts.

The average number of reported tornadoes in the US is ~ 1000/year, with

the majority of them on the central plains and in the SE states.

Climatology

Due to the unique geography of North America which is a

relatively large continent that extends from the tropical south

into arctic areas, and has no major east-west mountain range to

block air flow between these two areas.

In the middle latitudes, where most tornadoes of the world

occur, the Rocky Mountains block moisture and atmospheric

flow, allowing drier air at mid-levels of the troposphere, and

causing cyclogenesis downstream to the east of the mountains.

The desert Southwest also feeds drier air and the dryline

(potential instability) , while the Gulf of Mexico fuels abundant

low-level moisture.

This unique topography allows for many collisions of warm and

cold air, the conditions that breed strong, long-lived storms

many times a year.

Tornado alley (belt): A term often used by the media to denote a zone

in the Great Plains region of the central US, often a north-south

oriented region centered on north TX, OK, KS, NE, where tornadoes

are most frequent. However, statistics are variable on all time

scales, therefore, this term has little scientific value.

Florida: many embedded in passing hurricanes (e.g. waterspout).

Probability of Any Tornado:

The map shows the average number

of days per year any tornado, no

matter how strong or weak, might

occur within 25 miles of a point. The

highest numbers indicate where at

least one tornado might occur

somewhere within 25 miles as often as

on 1.5 days per year.

Significant Tornado (F2 or greater):

Now we're looking at days per

century. In other words, central

Oklahomans can expect an F2 or

greater tornado within 25 miles about

every 3 years.

Violent Tornado (F4 or greater):

Now the scale is days per millennium,

meaning that southcentral Oklahoma

may have a violent tornado within 25

miles about once every 20 years.

Annual Cycle:

Residents of Norman, OK experience

a distinct tornado season, beginning

late February and peaking late May.

Even though we are in the heart of

tornado alley and can expect one- to

one-and one-half tornado days per

year, our chances on any particular

day peak at only about two percent.

A recent NSSL study, using data from 1921 to

1995, estimated the daily climatological probability

of an F2 or greater tornado occurring near any

location in the U.S. For this work developing

highly accurate and accessible estimates of the

long-term threat from thunderstorms, winds, and

large hail as well as tornadoes, an NSSL scientist

was awarded a Department of Commerce Silver

Medal.

Tornado season: usually refers to the time of year where

the U.S. sees the most tornadoes. The peak “tornado

season” for the southern plains -- often referred to as

Tornado Alley -- is during May into early June. On the Gulf

coast, it is earlier during the spring. In the northern plains

and upper Midwest, tornado season is in June or July.

But, remember, tornadoes can happen at any time of year.

Tornadoes can also happen at any time of day, but most

tornadoes occur between 4-9 p.m.

Tornado Climatology: location & time

Cold fronts

Drylines

HurricanesNumber of tornadoes

per 10,000 square miles

54% of all tornados take place in spring when airmass contrast is

strongest thus, peak tornado season arrives later northward

Southeast in

March

Mid-atlantic

in April

Southern

plains in

May

Northern

plains in

June

Late spring

is the time

of greatest

overall

activity.

Tornado Climatology: location & time

March April May June July

Fatalities

Due to their small spatial scales and short lived, tornadoes kill relatively few people.

From 1950-1994, an average 91 people killed in 760 tornadoes reported in the US ~ 88% of all tornadoes kill no one assuming one tornado kills 1 person.

Most fatalities occur in a few large tornadoes.

About 2% of all tornadoes are responsible for over 2/3 of all deaths (~ 70% by F4 & F5, nearly 30% by F2 & F3).

Mostly in mobile homes, trucks & autos.

The safest place to be during a tornado is in a building basement.

Safety Rules

Indoor, basement.

Interior, lowest floor, cover with mattress or other cover to avoid injury by flying debris.

Mobile home? Evacuate, no nearby shelter, lie flat in low ground.

Driving truck or car? Driving at right angles to its path (southwest to northeast in general), or pull off the road, seek shelter, go to low ground away from the road.

Highway overpass not the safest place as a shelter, especially in urban areas with flying debris.

Safety

One of the most persistent myths associated with tornadoes is that opening windows will lessen the damage caused by the tornado. While there is a large drop in atmospheric pressure inside a strong tornado, it is unlikely that the pressure drop would be enough to cause the house to explode. Some research indicates that opening windows may actually increase the severity of the tornado's damage. Regardless of the validity of the explosion claim, time would be better spent seeking shelter before a tornado than opening windows. A violent tornado can destroy a house whether its windows are open or closed.

Another commonly held belief is that highway overpasses provide adequate shelter from tornadoes. On the contrary, a highway overpass is a dangerous place during a tornado. In the 1999 Oklahoma tornado outbreak of May 3, 1999, three highway overpasses were directly struck by tornadoes, and at all three locations there was a fatality, along with many life-threatening injuries. The small area under the overpasses created a kind of wind tunnel, increasing the wind's speed, making matters worse. By comparison, during the same tornado outbreak, more than 2000 homes were completely destroyed, with another 7000 damaged, and yet only a few dozen people died in their homes.

Safety

An old belief is that the southwest corner of a basement provides the most protection during a tornado. The safest place is the side or corner of an underground room opposite the tornado's direction of approach (usually the northeast corner), or the central-most room on the lowest floor. Taking shelter under a sturdy table, in a basement, or under a staircase increases chances of survival even more.

Finally, there are areas which people believe to be protected from tornadoes, whether by a major river, a hill or mountain, or even protected by supernatural forces. Tornadoes have been known to cross major rivers, climb mountains, and affect valleys. As a general rule, no area is "safe" from tornadoes, though some areas are more susceptible than others.

Prediction

The Storm Prediction Center (SPC), located in Norman, Oklahoma, is part of the National Centers for Environmental Prediction (NCEP), operating under the control of the National Weather Service (NWS), which in turn is part of the National Oceanic and Atmospheric Administration (NOAA) of the United States Department of Commerce (DoC).

Until October 1995, the SPC was known as the National Severe Storms Forecast Center (NSSFC) and was located in Kansas City, Missouri.

From 1995 to 2006 it was housed in the same building as the National Severe Storms Laboratory, after which it moved to the National Weather Center.

It began in 1952 in Washington, D.C. as a special unit of forecasters in the Weather Bureau before moving to Kansas City in 1954; and with increased duties this unit became the NSSFC in 1966.

Watches and Warnings

A weather watch (severe thunderstorm, tornado) states that atmospheric conditions are favorable for the development of a particular severe weather event. Issued by SPC (Storm Prediction Center) of the US Weather Service in Norman, OK.

A warning states that severe weather is imminent and precautions should be taken immediately. Issued by local weather forecast offices.

Because tornadoes develop rapidly and are of short duration, many times a warning may be disseminated only after a tornado has touched down. However, warnings are still useful for in multiple events.

Salt Lake City Tornado, August 11, 1999. This tornado disproved

several myths, including the idea that tornadoes cannot occur in

areas like Utah.