Some tornadoes also produce so-called suction vortices, or mini-tornadoes, that swirl and rotate within the main funnel. Like baby dinosaurs engaged in a feeding frenzy at the foot of a parent, suction vortices can greatly exacerbate the tornado’s destruction as they crisscross the main damage path. With wind speeds equal to or greater than those of the main funnel, suction vortices are hyperdestructive and explain why one house within the damage path may emerge relatively unscathed while the one beside it is flattened.
Some or all of these mechanisms are, to a greater or lesser extent, at work as the tornado moves forward at ground speeds ranging from 10 to 65 miles per hour. The tornado’s forward motion adds yet another variable to the complex formula of destruction. The winds on the right, leading edge of the twister are accelerated by the combined effect of the tornado’s lateral progress and the counterclockwise rotation of the funnel.
The tornado’s destructive abilities have been well described, thanks in large part to Tetsuya (Theodore) “Ted” Fujita. In 1971, Fujita, then a University of Chicago meteorology professor, developed the now-famous Fujita Scale. Because tornadic winds are too dangerous to measure in real time, Fujita focused instead on the physical evidence left in the twister’s wake. He was influenced in this by the unusual work he’d done as a young physics professor in Japan immediately after World War II. Fujita and a group of students were asked in late 1945 to conduct a survey of the damage caused by the atomic bombs dropped on Hiroshima and Nagasaki.2
The Fujita Scale was introduced in 1971 and assigned one of six progressively more intense designations to tornadoes based on the damage they caused. In 2007, the F-Scale was modified to provide greater consistency in damage assessments, and today is known as the EF-Scale (EF stands for Enhanced Fujita). An EF-0 tornado, with wind speeds not exceeding 85 miles per hour, causes light damage: Sign boards are pushed over, shallow-rooted trees are knocked down and branches are snapped from larger trees. EF-0s account for about 43 percent of the 883 tornadoes that strike the United States, on average, each year.3
The EF-1 tornado is a little stouter. With winds of between 86 and 110 miles per hour, an EF-1 will peel off roofing material, push mobile homes off foundations and shove cars from the road. The EF-1 accounts for 33 percent of all U.S. tornadoes.4
Wind speeds and resulting damage increase dramatically with higher Fujita ratings. Fortunately, the likelihood of these monsters occurring also falls off as the intensity level climbs. An EF-3, for instance — with winds of between 136 and 165 miles per hour — will uproot whole forests, lift roofs and derail trains. Yet it will occur in only about 4 percent of tornadoes.5 Similarly, EF-4 tornadoes, with wind speeds of between 166 and 200 miles per hour, happen just 1 percent of the time.6 EF-4s can level well-constructed houses, blow structures with weak foundations some distance and generate large missiles from debris.
At the top of the EF-Scale is the EF-5. Although these storms represent only about one-tenth of 1 percent of all tornadoes, that statistic is no comfort to the unfortunate souls caught in the path of the one or two EF-5s that form, on average, each year in the United States.7 EF-5s pack winds of more than 200 miles per hour, have the potential to exceed 300 miles per hour and cause often inconceivable damage: Strong frame houses are lifted off foundations and carried considerable distances before disintegrating; trees are debarked; automobile-sized missiles fly through the air in excess of 100 yards; and steel, reinforced concrete structures are badly damaged. According to Fujita, with EF-5 tornadoes, “Incredible phenomena will occur.’’
Like tornadoes themselves, the storms that spawn them have been the subject of scrutiny and speculation for centuries. In the broadest sense, severe thunderstorms are a leveling mechanism that temporarily restores stability to the atmosphere in response to a buildup of heat and moisture. Weather is an enormously complex system with a single driving force — the energy of the sun — at its core. Differences between higher and lower temperatures compel air masses to move, collide, mix and eventually achieve a period of equilibrium, before new forces impinge upon them and the cycle begins anew.
Wind, clouds, rain, snow, hurricanes and tornadoes all represent various permutations in nature’s never-ending, but ultimately doomed, pursuit of long-term stability. The factors that affect this quest are many: Earth’s annual journey around the sun; its daily rotation on its axis and the resulting phenomenon called the Coriolis effect, or the bending of wind as it moves between air masses; the heating and cooling of land and water; the friction that develops as wind moves across terrain; the physics of energy transfer from water to vapor and back again; and gravity itself — all play a role in shaping a system of staggering size, intricacy and possibility. And while the atmosphere can never completely achieve the constancy it seeks due to the steady flow of energy from the sun, it is remarkably successful at balancing competing forces over time and thus maintaining a livable climate on Earth.
Behind the weather’s almost mystical complexity lies a cause-and-effect chain often described as the “butterfly effect,’’ which presupposes that small variations of an initial condition (such as a butterfly flapping its wings) can, under the right circumstances, work their way through the system to produce huge changes or outcomes (such as a hurricane or tornado). Whether a butterfly’s wings can actually trigger a cataclysm is a matter of conjecture. But the metaphor is useful in illustrating the progressive, connected nature of all weather phenomena, from brief summer squall to arctic cold snap.
With tornadoes, that chain depends on just the right mix of circumstances and conditions. Most tornadoes — and nearly all the strongest and most violent ones — are spawned by supercell thunderstorms. Across the Great Plains and the Midwest, supercells frequently emerge as large, isolated and enigmatic beasts that feed on warm, moist air. Like their less-severe cousins, typical thunderstorms, supercells are born when moisture-laden air begins to rise rapidly into areas of lower pressure and cooler air. The subsequent updrafts create cumulus clouds, with their distinctive, cotton-like appearance. Once the water vapor cools to the appropriate temperature, the water condenses into rain.
This cycle is the basic mechanism for all precipitation. What makes the supercell different is the strength and rotation of the updraft. The updraft’s velocity is a reflection of the differences in temperature and pressure between the warm, heavy, damp air near the ground and the cool, lighter, drier air above. The greater the differential, the greater the atmospheric instability and the more explosively the air rises. In storms that form as powerful low-pressure systems collide with pools of warm, moist air, it is not unusual for the updraft to shoot up 50,000 feet to the tropopause, or the edge of the lower atmosphere. Here the energy-laden vapor slams into a ceiling of stable air at the base of the stratosphere and spreads out like wet concrete to form the ominous, overhanging cliff of the anvil cloud.
Updraft rotation, or its meteorological term, vorticity, is the other key ingredient in the creation of the supercell and its progeny, the tornado. Circulation within the storm can begin in a benign fashion. For example, a southwesterly breeze may be blowing along the surface on a sunny, humid spring day. But 1,000 or 2,000 feet aloft, the wind is blowing from a different direction at an equal or greater speed. Like two hands rolling a pencil, countervailing winds — called shear — begin to shape a horizontal tube of rotating air. As the updraft begins to strengthen, the rising air grabs this invisible tube, not unlike someone lifting the middle of a Slinky, and raises it to the vertical position.
With the rotating updraft in place, a high-speed conveyor has been established to feed vast quantities of fuel in the form of warm, moist air directly into the storm.
Fully organized, a supercell is a highly efficient, self-sustaining heat engine designed to gather excessive quantities of warm, moist vapor near the surface and convert it to rain and cooler air. With boiling dark clouds and a towering anvil, blinding rain, pelting hail, cataclysmic lightning, shattering thunder and howling win
ds, the supercell — quite apart from its role as parent to the tornado — can be one of the most awesome and powerful spectacles on Earth.
Its killer offspring, the tornado, is conceived deep within the storm’s turbulent womb. Around the updraft, a broader, counterclockwise circulation can sometimes begin as the updraft gathers strength. This rotation, called a mesocyclone, spins faster and tighter and eventually can elongate to nearly the full height of the storm, from the anvil nearly to the ground. At the same time, winds aloft on the back side of the storm — sometimes originating in the jet stream, sometimes coming in at lower altitudes — plow into the updraft column. Because of the extreme intensity of the updraft, the flow of the lateral winds is blocked. As a result, these currents are diverted downward to form a cascading downdraft.
It is along the volatile boundary between the rear-flank downdraft and the mesocyclone rotation that the tornado is formed. Typically, the bottom of the spinning mesocyclone will drop from beneath the low cloud deck on the back side of the storm like a massive, rotating freight elevator. This formation is called the wall cloud, one of the many tornadic features identified by Fujita. The funnel, a twisting column of water vapor, then emerges and, just as often as not, drops to the ground. Once touchdown takes place, the earth impedes the free flow of air back up into the funnel. The tornado pulls harder and faster, like a vacuum hanging up in thick carpet, in an attempt to feed the voracious low pressure above.
Many aspects of tornado formation, or tornadogenesis, remain only dimly understood. Despite scientific advances through the last half of the 20th century, specific cause-and-effect equations encompassing fluid dynamics and atmospheric physics have yet to be worked out. The fact is, scientists still don’t know everything that occurs behind the supercell’s mysterious curtain or why some of these storms spawn tornadoes but most do not.
Yet if the details remain sketchy, nature has repeatedly demonstrated the general conditions in which tornadoes are likely to form. And at no place on Earth do these circumstances present themselves with more vigor or frequency than on the plains and prairies of the central United States. Unbounded by natural barriers, advancing air masses turn the vast amphitheater of the plains into a battlefield in the spring and summer months. Cold, dry air plunging south from Canada collides with warm, humid air rising up from the Gulf of Mexico to create the potent, unstable mix required for severe thunderstorm formation. In many cases, rotating low-pressure systems moving out of the Southwest work like giant waterwheels, extracting humid air from the Gulf and cooler air from the north and pulling both together at the center of the low.
Other geographic features contribute to the prevalence of tornadoes in the country’s midsection. Warm, dry air from the desert Southwest can move in aloft and form what amounts to a temporary ceiling or cap, several thousand feet above the warm, humid air near the ground. This inversion creates a lid that prevents the high-octane, moist air from naturally rising. As a result, the air near the surface continues to heat and expand like boiling water in a pressure cooker until it finally blows through the inversion to reach the cooler air above, thus creating the violent updraft of a nascent supercell.
In another scenario, winds whipping east from the Great Basin and the Northwest are diverted upward as they pass over the Rocky Mountains. This cools the air and wrings out its moisture. The colder, drier air moves over the plains on top of warm, moist air at lower levels, thus setting up the all-important temperature gradient and cocking the hammer for a severe weather outbreak.
Tornadoes, of course, are by no means limited to the Great Plains. They can and do occur in every state, and for that matter, in nearly every region of the world. Killer storms are common in the Deep and mid-South. One of the worst in U.S. history moved straight up the Mississippi River near Natchez, Mississippi, on May 7, 1840. More than 300 were killed, including many caught on the open water in boats.8 Another killer ripped through the Worcester, Massachusetts, area in June of 1953, claiming 94 lives.9 Florida, for its part, is regularly bedeviled by tornadoes. That state ranked behind only Texas and Oklahoma in the average number of tornadoes per year (44) between 1953 and 1989, according to Storm Prediction Center statistics.10
Yet for frequency and intensity, a diagonal swath of the central United States known as Tornado Alley remains the most prolific tornado breeding ground in the world. The region’s boundaries are amorphous. But most meteorologists generally consider Tornado Alley to encompass portions of six states, stretching from north-central Texas northeast through central Oklahoma, into the eastern third of Kansas, across the southeast corner of Nebraska, through the northwest corner of Missouri and into central Iowa. It’s along this corridor that all the ingredients for tornado-producing storms habitually converge. Texas, Oklahoma, Kansas and Nebraska alone accounted for more than 9,300 tornadoes during the 36-year period ending in 1989.11 That’s about 260 per year. In Texas, Oklahoma and Kansas, the combined death toll from tornadoes through the same period was 872, or about 24 per year.12
In terms of intensity, no state has faced more EF-5 tornadoes than Kansas, with 16 recorded between 1880 and 2008. Iowa and Texas are next with 10 each, followed by Oklahoma with nine and Nebraska with six.13
Native Americans always contended with the sudden terror of the tornado. But as settlers poured into the West after the Civil War, tornadoes became a mysterious, horrific new fact of life for a growing number of Americans. Fortunately, science was searching for answers. One individual in particular worked relentlessly in the late 19th century to better understand and predict tornadoes. Today, we can only wonder what might have been had his efforts been taken more seriously in his day. For if ever there was a man ahead of his time, it was John Park Finley.
The son of a farmer from Ypsilanti, Michigan, Finley was born in Ann Arbor on April 11, 1854. He enrolled in Michigan State Agricultural and Mechanical College, now Michigan State; studied meteorology and agriculture; and in 1873, earned a Bachelor of Science degree.14 With letters of recommendation from his college professors, Finley enlisted in the Army and won assignment to the prestigious U.S. Army Signal Corps, the predecessor of today’s National Weather Service. Finley was detailed to Fort Whipple, Virginia, and trained in multiple disciplines, including telegraphy, signaling, electricity and meteorology.
In May of 1879, the 25-year-old was ordered west to conduct a survey of damage resulting from a tornado outbreak that had struck portions of Kansas, Missouri, Nebraska and Iowa. Over a 20-day period, Finley traveled more than 500 miles by horse and buggy throughout the damaged area. He compiled a wealth of data, including eyewitness accounts, calculations of tornadoes’ speed, grim observations of some of the 42 killed and damage assessments very much like those Fujita would base his tornado scale on nearly 100 years later.15
Private Finley’s hardbound, 116-page report included numerous drawings and maps. Finley even speculated on the cause of tornadoes, accurately suggesting that “marked contrasts of temperature and moisture invariably foretell an atmospheric disturbance of unusual violence . . .’’ 16
His superiors were impressed with the young man’s work and Finley was allowed to continue his tornado studies. He next gathered old records on tornadoes, some dating to the late 1700s, and put together a seminal report titled “The Character of 600 Tornadoes.” The report represented by far the most comprehensive survey of tornadoes up until that time and included tables, maps and a list of rules for forecasting tornadoes.17 Finley subsequently convinced his superiors that an entire tornado season, not just individual storms, should be studied in detail with an eye toward predicting the storms. They agreed, and in 1882, Finley set up shop in Kansas City, Missouri, to oversee the effort. Through the spring of ’82, Finley traveled extensively across the country’s midsection, enlisting a network of field spotters to report severe weather information back to the Kansas City office.18
After a major tornado outbreak killed hundreds across the South in February 1884, Finley began making expe
rimental and — given the tools available at the time — relatively successful predictions regarding the potential for tornado-producing storms over large sections of the country. In so doing, he became the first meteorologist to accurately predict conditions favorable to the formation of tornadoes, with 28 of his 100 predictions verified.19 Through 1884, Finley continued to refine his prediction models and increase his tornado spotter network, which eventually totaled more than 2,000 volunteer reporters.20
At least one supporter, astronomer Edward S. Holden, urged action regarding Finley’s groundbreaking efforts. Holden suggested that the Signal Corps use telegraph lines to establish a warning system capable of reaching towns and households in tornado-prone areas. He proposed that wires be strung throughout communities and bells installed in every house. An alert could be centrally activated to warn of heightened tornado risk. Holden even recommended that cannons be fired to warn people outdoors.21
Though perhaps less than practical, Holden’s ideas nonetheless underscored the need for some type of comprehensive tornado forecasting and warning system. But the Signal Corps would have none of it. Officials were worried that tornado predictions would spark widespread panic. As a result, the organization in 1885 banned the use of the word “tornado’’ in weather forecasts. In an official report two years later, the chief signal officer justified the decision by claiming that “the harm done by such a prediction would eventually be greater than that which results from the tornado itself.’’22
With the ban, Finley and his efforts quickly fell from favor. He was pulled from tornado research and shunted to a bureaucratic siding within the corps. He nonetheless managed to produce a second comprehensive book on tornadoes in 1887 that included a section on what the public could do to protect themselves from the threat.23
And Hell Followed With It: Life and Death in a Kansas Tornado Page 6
