TORNADOS
A tornado is a narrow, violently rotating column of air that extends from the base of cumulonimbus thunderstorm clouds to the ground. Because wind is invisible, you can't always see a tornado. A visible sign of the tornado, a condensation funnel made up of water droplets, sometimes forms and may or may not touch the ground during the tornado lifecycle. Dust and debris in the rotating column also make a tornado visible and confirm its presence.
Tornadoes are the most violent of all atmospheric storms. They vary a great deal in size and strength and dynamic. Powerful ones are vortexes of swirling winds that often exceed 200mph, making them the highest know winds on earth — and powerful enough to lift tractor trailers off the grounds, rip the asphalt off roads and drive splinters of wood into trees. Tornados sound like powerful jets taking off and are destructive enough to destroy entire towns and leave scores dead.
A cow was transported 10 miles by a twister in Iowa in 1878 and a tornado in Minnesota moved a headstone three miles in 1886. An insurance policy was found more than 40 miles from its original residence in Oklahoma in 1957 and a 210-mile trip was taken by a canceled check in Nebraska in 1915, according to a study several years ago by researchers at the University of Oklahoma and St. Louis University.
Tornados mostly occur in the United States, particularly the great Plains area and the South. They also occur on other part of the world. You get them in eastern China, Bangladesh, occasionally in Europe and with some frequency in southern Brazil and Uruguay, They occur mostly in places where warm, humid winds blows low over continental land masses and clashes with high, cool, dry air coming from another direction.
Much about tornadoes remains a mystery or is only partially understood. It is believed that winds speed a the center of powerful tornado can reach 600mph but no one has yet measured these speeds. But scientists are learning a great deal about tornados by producing artificial ones in the laboratory, models on computers and chasing reals one in vans.
Types of Tornadoes
Tornados form at the boundary or air masses with different temperatures and moisture content. Shearing winds — wind of different speeds and directs at different altitudes — set the air masses spinning. The result is a mesocyclone — a massive column of swirling air — that gives birth to most tornados. The mesocyclone in turn is part if a larger system called a super cell — a thunderstorm with a low cloud base and powerful updrafts. A typical supercell can to be 10 to 20 miles across and 60,000 feet high, reaching into the stratosphere. Only one in a thousand thunderstorms develop into a supercell and one in six super cells produce a tornado.
There are two types of tornadoes: those that come from a supercell thunderstorm, and those that do not. Tornadoes that form from a supercell thunderstorm are the most common, and often the most dangerous. A supercell is a long-lived (greater than 1 hour) and highly organized storm feeding off an updraft (a rising current of air) that is tilted and rotating. This rotating updraft - as large as 10 miles in diameter and up to 50,000 feet tall - can be present as much as 20 to 60 minutes before a tornado forms. Scientists call this rotation a mesocyclone when it is detected by Doppler radar. The tornado is a very small extension of this larger rotation. Most large and violent tornadoes come from supercells.
Non-supercell tornadoes are circulations that form without a rotating updraft. One non-supercell tornado is the gustnado, a whirl of dust or debris at or near the ground with no condensation funnel, which forms along the gust front of a storm. Another non-supercell tornado is a landspout. A landspout is a tornado with a narrow, rope-like condensation funnel that forms when the thunderstorm cloud is still growing and there is no rotating updraft - the spinning motion originates near the ground. Waterspouts are similar to landspouts, except they occur over water. Damage from these types of tornadoes tends to be minimal.
Tornado Formation
Scientists have learned a lot about tornadogenesis from theoretical studies, field projects and physical models but tornadogenesis — the way tornadoes form — has vexed researchers for decades.
Supercell Tornadogenesis. A rotating updraft is a key to the development of a supercell, and eventually a tornado. There are many ideas about how this rotation begins. One way a column of air can begin to rotate is from wind shear — when winds at two different levels above the ground blow at different speeds or in different directions.
An example of wind shear that can eventually create a tornado is when winds at ground level, often slowed down by friction with the Earth's surface, come from the southwest at 5 mph. But higher up, at 5000 feet above the same location, the winds are blowing from the southeast at 25 mph! An invisible "tube" of air begins to rotate horizontally. Rising air within the thunderstorm tilts the rotating air from horizontal to vertical — now the area of rotation extends through much of the storm.Once the updraft is rotating and being fed by warm, moist air flowing in at ground level, a tornado can form. There are many ideas about this too.
In the United States, tornadoes typically spawn in the clash between warm, moist air from the Gulf of Mexico and cooler, dry air from the north and west — conditions that mark Tornado Alley in the Midwest and South, the most common breeding grounds for twisters. Factors that may influence tornados include La Nina, a periodic cooling of the tropical Pacific Ocean which can affect weather worldwide. In a La Nina year there tend to be more tornadoes than average.
We still have many questions. Scientists know from field studies that perhaps as few as 20 percent of all supercell thunderstorms actually produce tornadoes. Why does one supercell thunderstorm produce a tornado and another nearby storm does not? What are some of the causes of winds moving at different speeds or directions that create the rotation? What are other circulation sources for tornadoes? What is the role of downdrafts (a sinking current of air) and the distribution of temperature and moisture (both horizontally and vertically) in tornadogenesis? Scientists hope to learn more about the processes that create wind shear and rotation, tilt it vertically, and concentrate the rotation into a tornado when they participate in a large field experiment in 2007. And, since not all tornadoes come from supercells, what about tornadogenesis in non-supercell thunderstorms?
Non- Supercell Tornadogenesis. A non-supercell tornado does not form from organized storm-scale rotation. These tornadoes form from a vertically spinning parcel of air already occurring near the ground, about 1-10 km in diameter, that is caused by wind shear from a warm, cold, or sea breeze front, or a dryline. When an updraft moves over the spinning, and stretches it, a tornado can form. Eastern Colorado experiences non-supercell tornadoes when cool air rushes down off the Rocky Mountains and collides with the hot dry air of the plains. Since these types of tornadoes happen mostly over scarcely populated land, scientists are not sure how strong they are, but they tend to be small. Waterspouts and gustnadoes are formed in this way too.
Surface winds seem to play an important role in tornado development. When a mesocyclone forms surface winds are often sucked in as updrafts. Along with the surface winds are present the updraft slows smoothly. But if the winds are cut off — by a cold down draft for example — the spinning air contracts and gains speed, producing a tornado. The funnel contracts the faster the tornado spins.
Deadly Tornadoes in the United States in 2011
Randolph E. Schmid of AP wrote: “Storm science has greatly improved tornado warnings in recent years. But if that's led anyone into a sense of security, that feeling has taken a beating in recent weeks. Super Outbreak 2011, on April 25-28, killed more than 300 people in the South and Midwest. Less than a month later, a devastating tornado took more than 120 lives around Joplin, Mo. The year 2011 was the deadliest year for tornadoes since 1950, based on an assessment of National Weather Service figures. This despite warnings of as much as 20 minutes, thanks to improved weather radar installed across the country in the 1990s. Before that, tornado warnings often weren't issued until a twister was sighted on the ground.[Source: Randolph E. Schmid, AP, May 28, 2011]
The death toll reported Saturday by the city of Joplin stands at 139, which if correct puts this year's tornado death toll at 520 — exceeding the previous highest recorded death toll in a single year of 519 in 1953. But Missouri state officials counted 126 dead, a discrepancy that left unclear whether 2011 has yet set the modern record for tornado fatalities. There were deadlier storms before 1950, but those counts were based on estimates and not on precise figures
The National Weather Service said 58 tornadoes touched down in Alabama on April 27, killing 238 people in that state alone and injuring thousands. Scores died in other states from twisters spawned by the same storm system. Put together, emergency management officials say the twisters left a path of destruction 10 miles wide and 610 miles long, or about as far as a drive from Birmingham to Columbus, Ohio. Statewide, Alabama officials estimate there was enough debris to stack a football field a mile high with rubble.
Viewing pictures of the tornado aftermath it's hard to overestimate the power of such storms, and records bear out how strong they can be. "You see pictures of World War II, the devastation and all that with the bombing. That's really what it looked like," said Kerry Sachetta, the principal of a flattened Joplin High School. "I couldn't even make out the side of the building. It was total devastation in my view. I just couldn't believe what I saw."
Why there have been so many tornado threats this year is harder to say. The meandering jet stream high in the atmosphere that directs the movements of weather was in a pattern that encourages warm Gulf air to move in and clash with drier air masses. While studies of global warming have suggested it could cause more and stronger storms, National Weather Service Director Jack Hayes isn't ready to blame climate change — at least not yet — saying it's too soon to link individual events with the ongoing warming.
Tornado researcher Howard B. Bluestein of the University of Oklahoma says his best guess is this unusual outburst of twisters is due to natural variability of the weather. "Sometimes you get a weather pattern in which the ingredients for a tornado are there over a wide area and persist for a long time. That's what we're having this year," he said. "If we see this happen next year and the following year and the following year," then maybe climate change could be to blame, he said.
Why Tornadoes Remain So Deadly
Scientists see a variety of factors that helped make this year's twisters deadlier — from La Nina to public complacency, from global warming to urban sprawl. "We thought for the longest time physical science could get us by ... that we could design out of disaster," said meteorology professor Walker Ashley of Northern Illinois University. Now scientists are finding they need to take human nature into account. [Source: Randolph E. Schmid, AP, May 28, 2011]
What is clear is that certain factors add to the risk of death. The most vulnerable folks are those living in mobile homes and houses without basements. For a variety of reasons, a lot of homes don't have basements. Twisters occurring on weekends — like the Joplin tornado — and at night tend to be greater killers because they catch people at home. At night, twisters are harder to see and sleeping people may not hear a warning.
Those less likely to be killed in a storm tend to be more educated and to have a plan in place beforehand. In Sedalia, Mo., 30-year-old Sean McCabe had the right idea when the tornado struck, heading to the basement. He said the storm shoved him down the final flight of steps. He had scrapes and cuts on his hands, wrists, back and feet. Blood was visible in the house, and much of the roof of the house was gone. "I saw little debris and then I saw big debris, and I'm like OK, let's go," said McCabe. One Joplin resident said a picture that was sucked off his house's wall was found in Springfield, 70 miles away.
Contributing to the massive loss of life is the growth of urban areas, suggested Marshall Shepherd, a professor of atmospheric science at the University of Georgia. "Historically, the central business districts of cities have not been hit that frequently," he explained. But as you increase the land area covered by homes and businesses, he said, "you're increasing the size of the dartboard."
"A lot of it is complacency," Ashley said. "The population seems to be becoming desensitized to nature. I don't know why." Studies have shown that 15 to 20 minutes is the most effective amount of warning time, and longer warning times can increase deaths. Weather experts aren't sure why, but worry that people think that if a twister hasn't appeared in a certain amount of time, it must have been a false alarm.
Jerry Brotzge, a research scientist at the Center for Analysis & Prediction of Storms, University of Oklahoma, said many people who hear warnings will look outside to see if they can see the tornado — "they need some kind of confirmation, they want to see it." But the Joplin tornado was at least partly rain-wrapped, meaning that a powerful rainstorm obscured it from some directions and "they wouldn't have seen it coming." "Even when people are sheltered in their homes, if they are not underground they can die," Brotzge added.
But asking people to evacuate an area is also a difficult decision, he said, "what if you have a traffic jam and the tornado hits that." Ashley concluded: "The take-home is, people have to take personal responsibility for their lives."
Image Sources: World Meteorological Organization; National Oceanic and Atmospheric Administration (NOAA), Wikimedia Commons
Text Sources: World Meteorological Organization; National Oceanic and Atmospheric Administration (NOAA), New York Times, Washington Post, Los Angeles Times, Times of London, Yomiuri Shimbun, The Guardian, National Geographic, The New Yorker, Time, Newsweek, Reuters, AP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.
Last updated January 2012