The massive twister of three-quarters of a mile wide struck Joplin, Mo., just weeks after several tornadoes struck southern states mainly hitting Alabama.

And again, just a week after Missouri, seven tornadoes struck Massachusetts on Wednesday, June 1.

The death toll of 314 in Alabama was the worst since 1925. In Missouri, 134 victims have been identified from the twister on May 22, and there is no one left on the missing list, officials said. Massachusetts' tornado has so far confirmed 4 deaths, and police and firefighters are still searching for victims door to door.

Over 1,200 tornadoes have swept across the United States in 2011, according to the preliminary numbers from the National Oceanic and Atmospheric Administration(NOAA). In April, there were 875 confirmed tornadoes, tripling the previous April high of 267, in 1974.

 

According to ThinkQuest,

Tornado conditions are caused when different temperatures and humidity meet to form thunderclouds. In the United States, warm, wet winds from the Gulf of Mexico move northward in spring and summer, meeting colder, dry Canadian winds moving southward. The place where these two winds meet is called a dry line. High, dry air coming from the north piles on top of low-moving, moist Gulf air at a height of over 10,000 feet. The warm southern winds try to rise, but the cold northern air blocks them. This clash causes the warm, trapped air to rotate horizontally between the two air masses. At the same time, the sun heats the earth below, warming more air that continues to try and rise. Finally, the rising warm wind become strong enough to force itself up through the colder air layer.

When this occurs, the cold air on top begins to sink, sending the rising warm wind spinning upward. The warm winds rotate faster and faster in a high column. When the updraft is strong, the column can rise to heights of 10 miles or more, twisting at speeds of up to 100 miles an hour. The rotating winds produce strong storm clouds about 70,000 feet high, sometimes spreading 10 miles wide.

This storm system may stay intact for several hours, at which point its thunderclouds are known as supercells. These storm clouds can send down an inch of rain in a mere ten minutes or shower the ground with baseball-sized hailstones. Supercells can accumulate into huge clusters, forming a line almost 100 miles long, which can then develop into mesocyclones.

The vast majority of tornadoes in the world occur in the Tornado Alley region of the United States, though they can occur nearly anywhere in North America. 

 

Why has this year been met with so many deadly tornadoes? 

Some claim that the tornadoes are a harbinger of climate change. Large-scale climate factors are said to contribute to the increase of twisters in the States.

In a blog post on Reuters, Gregg Easterbrook said climate change, not global warming, is the threat causing more tornadoes. While greenhouse gas levels in the atmosphere are rising, weather variations are also rising, including not only tornadoes but droughts and deluge rains as well.

The impact of global warming on trends in tornado activity is not yet identifiable. Easterbrook said, the mild warming of the past 100 years - about 1 degree Fahrenheit globally averaged - was good for crop yields, and moderated demand for energy. (Power use for warming on cold days exceeds power use for cooling on hot days). If all that happens is continued mildly rising temperatures, that might be beneficial.

On the other hand, the climate change can bring more tornadoes, increase droughts in some places while increasing floods in other places, as observed.

An increase in the sea surface temperature of a source region increases atmospheric moisture content, fueling an increase in severe weather and tornado activity, especially in the cool season.

 

Some claim the exit of La Niña, a coupled ocean-atmosphere phenomenon that is the counterpart of El Niño, as the cause of the wild tornado streak.

La Niña cools the waters of the equatorial Eastern Central Pacific Ocean, lowering the temperature by 3-5°C.

One of the most powerful moments of La Niña was observed last year, and it made a sudden exit around 3 months ago.

La Niña would have been beneficial for all these people that have been so clobbered, said Bill Patzert, a climatologist at NASA's Jet Propulsion Laboratory in California. If La Niña had maintained its strength, perhaps we wouldn't have seen so many tornadoes.

The jet stream, a high-speed air current which acts as an atmospheric fence where cool dry air meets with warm moist air enables the conditions for tornadoes.

Without La Niña's stabilizing effect on the jet stream, which should have pushed it to higher latitudes, the jet stream has traveled south in the last couple of months, with ample chance to mix cool and dry northern air with warm and moist southern air .

The northern air was kept especially cold last winter, and the southwest air was in unusually hot conditions.

In addition, according to Christian Science Monitor, the sea surface temperature of the Gulf of Mexico is between 1.8 and 2.7 °F warmer than average, said Jake Crouch, a climatologist at the National Climatic Data Center in Asheville, N.C. The warm, moist air is the perfect fuel for severe weather. The extreme conditions pave paths for more storms, and more powerful ones.

While the number of tornadoes have risen, overall tornado deaths have been slowly declining over the past 30 years, dipping sharply after the 1970s, most likely as a result of improved forecasting and construction technology. 

An article at BrainPosts.com provides these totals:

                Decade                 Total Deaths                       Deaths per Million

                1950s                     1419                                       8.6

                1960s                     942                                         4.9

                1970s                     998                                         4.7

                1980s                     522                                         2.2

                1990s                     579                                         2.2

                2000s                     556                                         1.9

The studies of tornadoes is relatively new in meteorology, the interdisciplinary scientific study of the atmosphere. Researched for about 140 years and intensively studied for about 60 years, many aspects of tornadoes stll remain a mystery.

Scientists still have not grasped exactly how most tornadoes form, and tornadoes occasionally strike without time to issue, let alone realize, a tornado worning.