Turnadoes

Purpose: To inform the audience about tornadoes.

Thesis: In order to better understand tornadoes, it is important to explore what causes tornadoes to develop, how researchers classify types of tornadoes, and odd occurrences that may be associated with tornadoes.

Organizational Pattern: Topical

I. Introduction

A. Attention Getter: What can hurdle automobiles through the air, rip ordinary homes

to shreds, defeather chickens, and travel at speeds over 60 mph?

B. Relevance: Illinois rests on the boundary of what tornado researchers call tornado

alley. This is the area of the country that receives the most tornadoes every year.

According to a 1995 brochure distributed by the National Oceanic and Atmospheric

Administration (NOAA), Illinois averages 27 tornadoes a year. Also, nearly 5 people die

every year in Illinois as a result of tornadoes [VISUAL AID]. In fact, according to

Tornado Project Online!, a website hosted by a company that gathers tornado

information for tornado researchers, the deadliest tornado in U.S. recorded history

occurred in Murphysboro, Illinois. In 1925 a violent tornado killed 234 people in this

Southern Illinois town.

C. Credibility: I grew up in the heart of tornado alley and have been interested in this

weather phenomenon for a very long time. Also, I am a trained weather spotter for the

Bloomington/Normal civil defense agency.

D. Thesis: In order to better understand tornadoes, it is important to explore what causes

tornadoes to develop, how researchers classify types of tornadoes, and odd occurrences

that may be associated with tornadoes.

E. Preview: So, let's crash through the causes of tornadoes, twist around the types of

tornadoes, and blow through some of the oddities associated with tornadoes.

Transition: Initially, I'll crash through the causes of tornadoes.

II. Body

A. What causes tornadoes?

1. According to the USA Today Tornado Information website, a tornado is a

"violently rotating column of air in contact with the ground and pendant from a

thunderstorm." Therefore, thunderstorms are the first step in the creation of a tornado.

2. The USA Today Tornado Information site also indicates that there are three

key conditions for thunderstorms to form.

a. First, moisture in the lower to mid levels of the atmosphere.

b. Second, unstable air. This is air that will continue rising once it begins

rising from near the ground.

c. The finial condition for the formation of tornado-producing

thunderstorms is a lifting force. A lifting force is a mechanism that cause the air to begin rising. The most common lifting force is heating of the air (which is why we experience so many thunderstorms in the spring as the air begins to warm).

3. The same source indicates that the strongest thunderstorms typically form in

warm, humid air that's east or south of advancing cold air.

4. I mentioned in the introduction that Illinois sees its fair share of tornadoes.

The following graph, adapted from the USA Today Tornado Information web site,

illustrates areas in the U.S. that receive the greatest number of tornadoes (tornado

alley). Thunderstorm-producing tornadoes are likely to form in this area as cold

air from the west and north clashes violently with warm air from the Gulf of

Mexico [VISUAL AID].

Transition: Now that we have crashed through the causes of tornadoes, let's twist around the types of tornadoes.

B. Types of tornadoes

1. According to renowned weather historian Dr. David Ludlum, author of the

1997 edition of the National Audubon Societies Field Guide to North American

Weather, tornado researchers use a scale, known as the Fujita-Pearson Tornado

Intensity Scale (named after its creators) to rate the intensity of tornadoes

[VISUAL AID].

2. Tornado statistics from NOAA (cited above) [VISUAL AID]

a. Weak tornadoes

1. Account for 69% of all tornadoes.

2. Winds are less than 110 mph.

b. Strong tornadoes

1. Account for 29% of all tornadoes.

2. Winds range from 110 to 205 mph.

c. Violent tornadoes

1. Represent only 2% of all tornadoes.

2. Winds exceed 205 mph.

3. According to Tornado Project Online!, although violent tornadoes account

for only 2% of all tornadoes, they are responsible for 67% of all deaths in

tornadoes [VISUAL AID].

4. In addition, astrogeophysicist Dr. Robert Davies-Jones notes in a 1995 edition

of Scientific American that most tornadoes have