Tornadoes

OutlineMotivationDefinitionWhen and WhereIngredientsForecastingHistorical Events

Year DeathsIn mobile

homes#Injuries

Estimateddamage

1985 94 30%

1986 15 47%

1987 59 41%

1989 50 24% 2490 $1088M

1990 53 13% 1150 $668M

1991 39 51% 854 $798M

1992 39 51% 1300 $764M

1993 33 39% 3990 $368M

1994 69 38% 1067 $518M

1995 30 647 $408M

Most tornado outbreaks result in less economic losses than other natural disasters

However, tornados account for a higher number of fatalities on average than hurricanes.

This is largely due to two reasons:1)Warning time2)Catastrophic nature of damage

Storm Structure (from the hurricane lectures)Tropical Cyclone

SizeTypical hurricane

strength tropical cyclones are about 300 miles (483 km) wide although they can vary considerably.

The relative sizes of the largest and smallest tropical cyclones on record as compared to the United States.

What about a tornado’s size????

Four reasons why people ignore tornado warningsMany victims (in tornado alley) live in mobile

homes, and do not have access to a safe evacuation shelter

People tend to ignore any warnings that fall outside of the ‘normal’ tornado season (tornadoes may, in fact, occur during any month of the year)

Optimism bias: ‘Bad things only happen to other people’

Warnings are often ignored, because there may be no planned evacuation plan, or because people think that the storms do not pose a significant threat

In general, the weakest tornados have wind speeds of hurricane force.

The overall number of fatalities associated with tornados continues to decrease

This decrease in fatalities is being realized even as the number of reported tornados is increasing

This decrease in fatalities is being realized even as the number of reported tornados is increasing

What is a tornado?A tornado is a violently rotating column of air extending between, and in contact with, a cloud and the surface of the earth.

The stronger tornadoes attain an awe-inspiring intensity, with wind speeds that exceed 200 mph and in extreme cases may approach 300 mph.

How do we know how strong the wind speeds in a tornado are?The most common way to identify the strength of a tornado

is through an examination of the damage caused. This concept led to the development of the Fujita and Enhanced Fujita Scales (http://www.spc.noaa.gov/efscale/).

EF Scale DamageEF1 – 31.6% of all reports

EF2 – 10.7% of all reports

EF Scale DamageEF3 – 3.4% of all reports EF4 – 0.7% of all reports

EF Scale DamageEF5 – .1% of all reports

How do we know how strong the wind speeds in a tornado are?Another way is to remotely measure the wind

speeds by using a mobile doppler radar.

How do we know how strong the wind speeds in a tornado are?Doppler radars can measure the speed of wind because

the beam that is received by the radar will have a different frequency depending on the motion of the water drops

How do we know how strong the wind speeds in a tornado are?This is an

image of the wind velocities from a doppler radar scan.

The circle indicates a possible tornado.

Map of World-Wide Tornado Fromation

Over 85% of world-wide tornado reports occur in North America

The geography of North America is uniquely favorable for the formation of tornados.Warm-moist air

sourceCold air sourceAll significant

mountain ranges are oriented north-south.

The Gulf of Mexico and the western North Atlantic Ocean are important sources of water vapour forNorth American thunderstorms

States with the largest number of tornadoes are located in ‘tornado alley’Tornado alley corresponds to a northeast-

southwest orientation that corresponds to the orientation of strong fronts traveling across the central United States in the spring and early summer, and to the orientation of the upper-level ‘jet stream’

These states include Texas, Oklahoma, Nebraska, Kansas, Iowa, Missouri, Illinois, Indiana, Mississippi, and Florida

States with the largest number of tornadoes are located in ‘tornado alley’However, the

traditional tornado alley isn’t where deaths are most frequent.

Instead, most deaths occur in the deep south.

Why?

Tornadoes (when?)Most frequent in the spring and summer

Tornadoes (when?)Most frequent in the spring and summer

Tornadoes (when?)Most frequent in the spring and summer

Tornadoes (when?)Most common in the evenings

All tornados form within thunderstormsThese thunderstorms can be found in:Landfalling hurricanesWithin supercell thunderstorms

In squall lines, often located ahead of cold fronts

ThunderstormsDefined by its

scale (short and short-lived)

Typically 5-10 km horizontally and vertically

An individual cell lasts only 30-60 minutes

How do thunderstorms form?Require three elements:1. A source of moisture2. A conditionally unstable atmosphere3. A mechanism to trigger a thunderstorm

updraft, either through forced lifting or heating

Try this: In order to get a thunderstorm you have to go out on a “lim”. l=lift, i=instability, m=moisture

How do thunderstorms form?Typically, the source

of moisture is the Gulf of Mexico.

Cool dry air aloft often comes from the Plains or the Rockies.This combination of

warm moist air at the surface and cool dry air aloft leads to instability.

So what do we mean by instability?

What is a lapse rate?A lapse rate is

defined as the rate of change in temperature observed while moving upwards through the Earth's atmosphere.

Key pointsRate at which a temperature decreases with

height.

Units: generally C/km, sometimes K/km

Stability of Atmosphere This describes the tendency for the atmosphere

to either resist or enhance vertical motions. The stability of the atmosphere is directly related to the changes of temperature with height.

First we need to consider the temperature changes experienced by rising air. As a parcel of air rises it moves into regions of lower pressure. This means that the surrounding air is pushing on the parcel with less force. So the air in the parcel will expand, and the volume will become larger.

When the air expands, the molecules must now cover a larger volume. This means that the air in the parcel must perform work to inhabit the increased volume. The work done by the parcel will result in lower kinetic energy, and the temperature must fall.

Now, what happens when air becomes saturated?Consider: condensation causes a release of

latent heat.So the expansion of the air will induce a

cooling, but this will be partially offset by heat release from the condensation of water.

As a result, the rate of temperature change of rising air that is saturated is smaller than for dry air. This is called the Moist Adiabatic Lapse Rate, and it is not a constant value. This is because the rate of condensation changes with height.

Stability and Saturated Air Recall that when rising air becomes saturated, latent heat is released, and slows the rate of cooling. The moist adiabatic rate is variable, but always less negative than the dry adiabatic rate.

Conditional Instability Air is stable

to a certain

height, however, if a

“lifting mechanism” can cause air to rise, to a

level where

condensation is reached the air is now

saturated.

What is a “trigger” or lifting mechanism?

Any mechanism which lifts a parcel of air up to the point where it becomes buoyant is can trigger a thunderstorm.

Examples of lifting mechanisms

Tornadoes migrate northward from spring to summer:

Triggering in the form of forced lift occurs along the front range of the Rocky Mountains and along the sea-breeze convergence zone in Florida

Airmass thunderstorms occur in three stages1. Cumulus (warm, buoyant plume

with updrafts)2. Mature (combination of both

updrafts and downdrafts)3. Dissipating (falling precipitation

shuts off the updrafts…all downdrafts and system self destructs

Stages of a thunderstorm

Severe thunderstormsHowever, airmass thunderstorms don’t

typically produce tornados and almost never produce strong tornados.

The reasons that airmass thunderstorms don’t produce thunderstorms are two fold:Updrafts are suppressed by the weight of the

waterThere is no inherent rotation to work with

Both liabilities can be overcome by shearSpeed shear Directional Shear

The updraft then tilts the rotation from the horizontal axis to the vertical axis

Also the updraft is tilted by the shear, removing the updraft from the precipitation area. At this point the thunderstorm is called a “supercell”.

Eventually, this rotating updraft produces a small scale low pressure system called a mesocyclone

About 10% of the time, this rotation becomes “stretched” by the rear flank downdraft and concentrated into a tornado

About 10% of the time, this rotation becomes “stretched” by the rear flank downdraft and concentrated into a tornado

Overhead view of a supercell

T

Tornado forecastingTornado

forecasting is based largely on the recognition of the patterns which are conducive to tornado formation.

Tornado forecastingHowever, we can

not pinpoint actual locations of tornado formation because the process is somewhat stochastic.

Consequently, forecasts are for generalized regions with probabilities of events.

•88 tornadoes touched down in areas from Alabama to Pennsylvania•36 fatalities•one of the most extensive tornado outbreaks in several years•the outbreak is especially unusual during the month of November•Much of the activity occurred between the hours of 3PM and 9PMEST during the afternoon and evening of November 10, 2002

A map summarizing the severe weather reports for Nov. 10, 2002

An image of a tornado near West Mansfield, Ohio on Nov. 10, 2002:

Vehicles having been thrown across Main Street in Clark, Pennsylvania:

An evacuation of patrons at a movie theatre just minutes prior to the tornado saved lives in Van Wert, Ohio:

There was very little left of a mobile home in Fruitvale, Tennessee:

Canada’s most life-threatening tornadoes:Regina, Saskatchewan; June 30, 1912; 28 dead, hundreds

injuredEdmonton, Alberta; July 31, 1987; 27 dead, hundreds injuredWindsor, Ontario; June 17, 1946; 17 dead, hundreds injuredPine Lake, Alberta; July 14, 2000; 12 dead, 140 injuredValleyfield, Quebec; August 16, 1888; 9 dead, 14 injuredWindsor, Ontario; April 3, 1974; 9 dead, 30 injuredBarrie, Ontario; May 31, 1985; 8 dead, 155 injuredSudbury, Ontario; August 20, 1970; 6 dead, 200 injuredSt-Rose, Quebec; June 14, 1892; 6 dead, 26 injuredBuctouche, New Brunswick; August 6, 1879; 5 dead, 10

injured

The Edmonton tornado of July 31, 1987Wind speeds up to 420 km per hour (a

minimal F5) and stayed on the ground for an hour

27 people were killed with several hundred injuries

300 homes destroyed$330 million in total property damageOne of the worst natural disasters in

Canadian history

The tornado droppped down from a supercell thunderstorm at approximately 1458 and stayed on the ground for about an hour; the path was about 40 km long.

An aerial damage survey of several of the industrial sites in listed in the previous slide:

A view of the Edmonton tornado from the shores of the North Saskatchewan River:

A view of a residential area being affected:

And just after the damage was done:

MitigationResidents accessing either radio or television were warned as much as 15 minutes prior to the tornado’s impact

What about other more recent cases?

First, what is a tornado warning?A tornado warning means that a tornado is

believed to be present in the warning areaThe warning may be based upon a Doppler

radar signature, or on public spotter reportsStorm spotters can be the first line of defence

against tornadoes, particularly in regions outside of Doppler radar coverage

Warning times for tornadoes:Longest warning times are based upon

mesoscale vortex signatures seen only one hour or more in advance

The best way to be informed is to listen to a Weather radio (20 dollars and up)

The theatre manager in van Wert, Ohio evacuated his customers after hearing the warnings being broadcast on the weather radio

This alert action saved many lives

Tornado watches provide more potential lead time:A tornado watch states that conditions are favorable for tornadoes to form

These watches may be given as much as 12 hours in advance of an anticipated event

Tornado safety measures:When a tornado watch is issued, listen

to a weather radioIf at home, move to a designated

shelter, such as a basementIf there is no basement, move to an

interior room and avoid windows (put as many walls as possible between you and the tornado)

If outside, move to a low elevation away from any possible airborne object (including vehicles and mobile homes!)

Tornado myths:The

skyscrapers in cities prevent tornados from forming in or striking downtown areas.

Tornado myths:Highway

overpasses are a safe place to shelter if you are on the road when you see a tornado.

Tornado myths:It is

commonly and mistakenly thought that if the condensation funnel of a tornado does not reach the ground, then the tornado can not cause substantial damage.

Tornado myths:It has been

thought in the distant past that tornadoes moved almost exclusively in a northeasterly direction.

Readings:http://www.mb.ec.gc.ca/air/

summersevere/tornadoes.en.htmlhttp://www.nssl.noaa.gov/eduhttp://www.fema.gov/library/tornado.htmBluestein, H. B., 1999: Tornado alley:

Monster storms of the Great Plains. Oxford University Press.

Zebrowski, Chapter 8.Rauber, Walsh, and Charlevoix, chapters

17-18.