From Tricky Twisters by Jacqueline Adams

Extreme Weather

(The numbers you see within the text represent lines numbers you will need to reference in several of the questions.)

from Tricky Twisters by Jacqueline Adams

Last November 6, a tornado tore through Indiana, killing 23 people. It was the deadliest tornado to hit the state in more than 30 years. But that was just the beginning. Six days later, nine twisters whirled through Iowa, followed on November 15 by a line of thunderstorms that spawned 35 tornadoes across five

states.

The twist: Spring is considered peak tornado season in the U.S., so people weren't expecting a funnel-filled fall. Plus, some of these twisters sprang up out of unusual storm systems. New research suggests that tornadoes may arise from unsuspected storms more often than once thought.

10 Recipe for Disaster

Whether it's springtime or fall, a tornado doesn't just strike out of the clear blue sky. These whirlwinds form inside of a thunderstorm. Three main ingredients are needed to cook up a thunderstorm: moist air near the ground, cold air above, and a trigger to make the moist air rise. “Where the air is forced to rise, that's where thunderstorms are triggered,” says Paul Markowski, an atmospheric scientist at the Pennsylvania State University.

The rising air forms a cloud. The condensation, or water that changed from vapor to a liquid, within the cloud produces warming that is critical to sustain the thunderstorm.

20 If a fourth ingredient—wind shear—gets added to the mix, trouble really brews. That's because these wild winds, which blow at different speeds or directions at different altitudes, can turn an ordinary thunderstorm into a supercell—a swirling thunderstorm most likely to produce a twister. Wind shear causes the air to spin like a rolling pin. Then, the storm's updraft, or rising warm air, tugs the rolling column of air upward. Scientists think a tug-of-war between downdrafts, or cool, sinking air, and warm updrafts stretch the

rotating air column into a tornado.

Tornado Target

The Great Plains area, which extends from Texas to Nebraska, often 30 experiences the perfect mix of ingredients for tornadoes, earning the region the nickname “Tornado Alley” (see map). In this region, warm, moist air blowing north from the Gulf of Mexico collides with drier air from the high plateaus of Mexico. This collision, called a dryline, causes the moist air to rise and form thunderstorms. The gradually sloping terrain of the Great Plains region helps produce strong wind shear that's ideal for tornado formation. “Tornado Alley has more tornadoes than any other part of the world,” says Markowski.

Most of Tornado Alley's twisters form from supercells that develop on spring afternoons-when the sun's heat has had plenty of time to warm the surface air. This warmer, less dense air rises, helping to trigger the

40 thunderstorm.

Extra Ordinary

Most of what scientists know about tornado formation comes from studying supercells in Tornado Alley. But last fall's tornadoes show that terrible twisters can form anywhere-sometimes even developing from a

completely different type of thunderstorm. Some of last November's disasters began in a squall line. Unlike a

supercell, this line made up of individual thunderstorms can stretch for hundreds of miles.

But how much of a threat do squall lines actually pose? To find out, Robert 50 J. Trapp, an atmospheric scientist at Purdue University in Indiana, and his colleagues studied storm records and radar images of 3,828 tornadoes that formed in the U.S. from 1998 to 2000. Their findings: Squall lines spin out tornadoes more often than previously thought, especially in regions outside of Tornado Alley. Overall, only 18 percent of the tornadoes they studied were spawned by squall lines. But when the team zeroed in on specific areas, some

of the percentages were much higher. For example, they found that half of the tornadoes in Indiana-one of the states hit hardest last fall-spun out of squall lines.

Answer the following questions about the excerpt from “Tricky Twisters.”

1. Which statement is the main idea of the second paragraph?

a. / Most tornadoes in the United States happen in spring.
b. / People must be prepared for tornadoes every day.
c. / Tornadoes are sometimes called “twisters.”
d. / Scientists have new information about tornadoes.

2. Which text feature helps you find definitions for weather terms?

a. / subheadings
b. / the title
c. / a list
d. / italicized words

3. Why is “Recipe for Disaster” an appropriate subheading for lines 11–27?

a. / The air temperatures during a tornado are similar to temperatures used in baking.
b. / The author likens the conditions that produce a tornado to ingredients in cooking.
c. / Scientists use measures and directions when they study tornadoes’ occurrences.
d. / You can use kitchen utensils and ingredients to make a tornado model.

4. Which statement best summarizes lines 12–19?

a. / Thunderstorms occur when moist air near the ground rises to meet cold air above.
b. / Tornados never form as a result of the creation of a thunderstorm.
c. / The most important ingredient in a thunderstorm is moist air.
d. / Thunderstorms are complicated and difficult to trigger.

5. Which statement best summarizes lines 37–40?

a. / The warm air of spring creates supercells that form most tornadoes in Tornado Alley.
b. / The sun triggers thunderstorms that, in turn, cause tornadoes.
c. / Spring tornadoes are warmer than those that form in the fall.
d. / Tornado Alley’s twisters form only in the spring.

6. The graphic aid that appears in this article is a

a. / diagram
b. / map
c. / chart
d. / timeline

7. The author’s purpose in including the graphic aid is to

a. / make the article longer
b. / hold readers’ attention
c. / highlight an important concept
d. / persuade readers to avoid tornadoes

8. The author’s purpose in including dates, percentages, and other data in lines 49–58 is to

a. / entertain
b. / show feeling
c. / persuade
d. / inform

9. To support the main idea in the sentence that begins on line 20, the author provides details about

a. / how wind shear differs from updrafts
b. / what wind shear looks like
c. / how wind shear is measured
d. / how wind shear affects a storm

10. The subheading “Tornado Target” refers to

a. / a dryline
b. / the Great Plains
c. / the Gulf of Mexico
d. / high plateaus in Mexico

11. The author’s purpose in including the “Tornado Alley” map as a graphic aid is to

a. / make the article longer
b. / show a tornado-shaped region
c. / visually support details in the text
d. / persuade readers to avoid tornadoes

12. Which idea from the text does the map help you understand?

a. / “Tornado Alley” refers to the Great Plains, an area from Texas to Nebraska.
b. / Warm, moist air blowing north collides with drier air and creates tornadoes.
c. / We learn about tornadoes mostly by studying supercells in Tornado Alley.
d. / A squall line consists of separate thunderstorms and can extend hundreds of miles.

13. The detail “But when the team zeroed in on specific areas, some of the percentages were much higher” (lines 55–56) supports the main idea that

a. / Trapp and his colleagues studied records from thousands of tornadoes
b. / squall lines pose more of a threat in some regions than in others
c. / a small percentage of tornadoes are spawned from squall lines
d. / devastating tornadoes can form outside the boundaries of Tornado Alley

14. Which main idea does the detail about storms in Indiana in lines 56–58 support?

a. / The Great Plains region is also called “Tornado Alley.”
b. / Squall lines produce more tornadoes in some areas than in others.
c. / Most tornadoes in Tornado Alley form from supercells.
d. / Unexpected storms can hit the United States in the fall.

Understanding Vocabulary

15. What is the meaning of the word unsuspected in line 9 of the excerpt from “Tricky Twisters”?

a. / not known to exist
b. / frequently occurred
c. / fully researched
d. / contrary to human reasoning

16. What is the meaning of the word atmospheric as it is used in line 16 of the excerpt from “Tricky Twisters”?

a. / causing galaxies
b. / creating poisonous gasses
c. / relating to air
d. / tracking storm trends

17. What is the meaning of the word formation in line 35 of the excerpt from “Tricky Twisters”?

a. / creation
b. / study
c. / movement
d. / power

In this article, author Peter Tyson ponders why the United States seems to host three-quarters of all the tornadoes in the world.

from Tornado Country

Have you ever stopped to wonder why the Great Plains, and by extension the country as a whole, gets the lion's share of our annual planetary quota of tornadoes? I hadn't—mostly, I suspect, because I'm an East Coaster, and for us tornadoes lie in the realm of the freak occurrence. As we'll see, most people elsewhere in the world appear to feel similarly about tornadoes.

The answer, I found, is two-fold. It has to do with what you might expect (climatological conditions in the Great Plains are unparalleled for spawning tornadoes), but also with what might come as a surprise (very few nations even bother to record tornadoes). One expert I spoke with believes that even 10 countries that report their worst windstorms may be underreporting by a factor of seven. And "tornadic events" that get reported as a single tornado in a country with a nonexistent damage-assessment system might, with a better such system, be found to have been 10 separate tornadoes—this makes assigning twister numbers by country even trickier.

The truth is, the U.S. very well may not get three out of every four tornadoes that occur on Earth; it may just look that way.

Accident of Geography

The Great Plains has been likened to a funnel factory. It possesses all the ingredients needed to produce, as one expert put it to me, “some hellacious 20 thunderstorms”-the parents of tornadoes. In spring and early summer, warm, moist air blowing in from the Gulf of Mexico can become trapped beneath a "lid" of hot, dry air gusting from the high desert region of the Southwest and, above that, cold, dry air sweeping over the Rockies. Like a lid on a pot of boiling water, this “convection cap” keeps the warm air from rising. The

pressure builds, until a cold front or other boundary between air masses moves in and weakens the cap. Quite suddenly, the warm, humid air can burst forth, billowing upwards at up to 100 miles per hour and swelling into 50,000-foot-tall thunderstorms in minutes.

Some of these thunderstorms begin rotating through most of their depth. 30 (This happens because of wind shear, a dramatic change in wind speed or direction over a very short distance.) Called “supercells,” these storms serve as ideal generators of tornadoes, from those that scrape off a few shingles to those rare, mile-wide monsters that leave nothing in their wake but cleared concrete foundations. “No other place on the planet has the source of warm, moist air on the equatorward side and a wide, high range of mountains extending from north to south on the west side,” says Harold Brooks, a tornado expert at the National

Severe Storms Laboratory in Norman, Oklahoma. “The Andes aren't as wide as the Rockies, and the Himalayas don't extend very far from north to south.”

While the Great Plains gets the bulk of American tornadoes, other parts of 40 the country witness them as well. Florida sees more twisters than Oklahoma, though they're far weaker. Cyclones also strike Colorado, and occasionally a ripsnorter will touch down in other states. In 1979, I was living in Hartford, Connecticut, when a tornado raked through nearby Windsor Locks. That tornado is fifth on a list of tornadoes that have caused at least $200 million in damage (in inflation-adjusted 1999 dollars). All 50 states, in fact, have experienced twisters.

All told, about 1,200 tornadoes occur annually in the United States. The entire rest of the world collectively reports just 200 to 300 every year. Yet only in this country is the number of reported tornadoes roughly equal to the 50 number of actual tornadoes in any given year. The U.S. began officially collecting tornado reports back in 1953 and rating tornadoes using the Fujita Scale 20 years later. No other nation has such a robust or longstanding system.

As a sign of how lackluster tornado reporting is elsewhere, can you name a single country outside the U.S. where tornadoes regularly occur? I couldn't before starting this article. In fact, I couldn't remember hearing of a single tornado that ever struck anywhere else in the world. I'm sure I've heard of some, but they didn't stick in my mind.

Tornado Countries

Not surprisingly, the planet does have other tornado seedbeds, and some 60 occasionally germinate twisters to rival the nastiest the U.S. has to dole out. In raw numbers, Canada probably comes in second to the U.S. The same climatological regime that brings tornadoes to the southern Great Plains in early spring moves north through the year to unleash more of the same on western Canada in July.

After the U.S. and Canada, Bangladesh and East India probably get the most violent tornadoes; they certainly suffer the deadliest. On April 26, 1989, the most lethal tornado on record swept Bangladesh, killing about 1,300 people, injuring 12,000, and leaving 80,000 homeless. High population density, flimsy housing, and a nonexistent tornado warning system mean killer tornadoes are 70 all too common there, says Jonathan Finch, a meteorologist at the National Weather Service in Dodge City, Kansas, who is an expert on that region's tornado climatology.