Tornados

Ted Spitzmiller

Understanding severe weather is critical for pilots and people who live in tornado prone areas

Tornados (twisters)• A violent, dangerous, rotating column of air

in contact with both the surface of the earth and a cumulonimbus cloud

• Come in many shapes and sizes– Typically a form of visible condensation funnel – Narrow end touches the earth and encircled

by debris cloud and dust • Most tornadoes wind speeds <110 mph

– Approximately 250 feet across– Travel a few miles before dissipating

• Some attain wind speeds of 300 mph – Stretch more than two miles across – Stay on the ground for dozens of miles

Tornados Winds Destroy

Size

• 500 feet across on average• On ground for 5 miles • Weak tornadoes, can be narrow,

sometimes only a few feet • One reported a damage path only 7 feet

wide• Wedge tornadoes can have a damage

path a mile wide or more. • Hallam, Nebraska 2004, was 2.5 wide at

the ground• Path length—Tri-State Tornado,

(Missouri, Illinois, and Indiana) 1925, on ground 219 miles

• Many which appear to have path lengths of 100 miles or longer are composed of a family of tornadoes formed in quick succession

Types of Tornadoes• Landspout, Multiple vortex ,

and Waterspout. • Waterspouts are a spiraling

funnel-shaped wind current develop over bodies of water

• Frequently in tropical areas close to equator, less common at high latitudes

• Other tornado-like phenomena include the gustnado, dust devil, fire whirls, and steam devil.

Tornadoes On Every Continent Except Antarctica.

• Vast majority occur in Tornado Alley region of US

• Can be detected before or as they occur using Pulse-Doppler radar

• Recognized patterns in velocity and reflectivity data

• Hook echoes

Scales for Rating Strength • Fujita scale rates tornadoes by damage

caused • Some countries use Enhanced Fujita Scale • F0 or EF0 tornado, weakest category,

damages trees, but not substantial structures

• F5 or EF5 tornado, strongest category, rips buildings off foundations and can deform large skyscrapers.

• Doppler radar data, photogrammetry, and ground swirl patterns (cycloidal marks) may also be analyzed to determine intensity and assign a rating

Etymology (Origin of words)

• Word tornado altered form of Spanish word tronada—"thunderstorm"

• Taken from Latin tonare, meaning "to thunder“• Commonly a "twister", or old-fashioned colloquial term cyclone• “Cyclone" a synonym for "tornado" in 1939 film, The Wizard of Oz.

Definitions• For a vortex to be classified as a tornado,

it must be in contact with both the ground and the cloud base

• Tornado refers to the vortex of wind, not the condensation cloud

Funnel Cloud• A tornado is not necessarily visible

– intense low pressure causes high wind speeds (Bernoulli's principle) and rapid rotation (due to cyclostrophic balance)

– Water vapor in air to become visible as a funnel cloud or condensation funnel

• Funnel cloud term defines any rotating cloud below a cumuliform cloud

• Tornadoes often begin as funnel clouds

• Difficult to discern the difference between a funnel cloud and a tornado from a distance

Outbreaks and Families• Occasionally, a single

storm will produce more than one tornado

• Multiple tornadoes produced by same storm cell are a "tornado family"

• Tornadoes look different depending on viewing angle

Life cycle

• A sequence of images showing the birth of a tornado.

• First, the rotating cloud base lowers.

• Lowering becomes a funnel, which continues descending while winds build near the surface, kicking up dust and other debris.

• Finally, visible funnel extends to the ground, and the tornado begins causing major damage.

Often Develop From Supercells • Most intense tornadoes (EF3 to EF5)

develop from supercells. • Very heavy rain, frequent lightning,

strong wind gusts, and hail are common

• Most tornadoes from supercells follow a recognizable life cycle.

• Begins when increasing rainfall drags an area of quickly descending air known as the rear flank downdraft (RFD)

• Downdraft accelerates as it approaches the ground

• Drags the supercell's rotating mesocyclone towards the ground with it

Formation• As mesocyclone approaches

the ground, a visible condensation funnel appears to descend from the base of storm, often from a rotating wall cloud

• As the funnel descends, the RFD also reaches the ground, creating a gust front that can cause damage a good distance from the tornado

• Usually, funnel cloud becomes a tornado within minutes of the RFD reaching the ground

Maturity• Initially, tornado has a good source

of warm, moist inflow to power it• It grows until it reaches the "mature

stage“• Can last anywhere from a few

minutes to more than an hour• During that time a tornado often

causes the most damage, • Meanwhile, the RFD, now an area of

cool surface winds, begins to wrap around the tornado, cutting off inflow of warm air which feeds the tornado

Demise• As RFD completely wraps around and chokes off

tornado's air supply, vortex weakens, becomes thin and rope-like

• “Dissipating stage"; no more than a few minutes, tornado fizzles.

• During this stage the shape can be blown into fantastic patterns

• Even though dissipating, it is still causing damage. • Storm contracts into a rope-like tube and, like the

ice skater who pulls her arms in to spin faster, winds can increase at this point.

• Its associated mesocyclone weakens, as the rear flank downdraft cuts off inflow powering it.

• If a new mesocyclone develops, the cycle may start again, producing one or more new tornadoes.

Keeping Safe

How to Avoid Injury

• Be aware of approaching bad weather (radio/TV)

• Evacuate the warning area• Go to local public shelters• Take shelter in a basement or first floor

bathroom.• Crouch in the bathtub with a blanket over you