Originally posted 5-25-2011
Over the past few days two incredibly powerful storms developed and marched across the midsection of the USA. On Sunday, May 22nd, a category 5 tornado devastated Joplin, Missouri. That twister left a path of destruction 1 mile wide and 4 miles long. It destroyed homes, businesses, and damaged a hospital and a school. Loss of life was high, with 122 confirmed deaths at last report. Tuesday evening the second storm system developed and swept through the eastern Great Plains and into the midwest. Multiple tornados were reported on the ground around Oklahoma City, Kansas City, and up into Illinois and Indiana. The death toll for this second storm is currently at 14, including deaths in Oklahoma, Kansas, Arkansas, and possibly Texas.
This photo from the on-line Chicago Magazine shows the path of destruction through Joplin. As you can see, the destruction is widespread.
The link below will take you to an NPR page where you can see a before and after comparison of the devastation in Joplin.
http://www.npr.org/2011/05/26/136655052/before-and-after-a-birds-eye-view-of-joplin?sc=fb&cc=fp
My prayers and heart go out to the communities, families, and friends who lost loved ones, were injured, or were otherwise impacted by these powerful forces of nature. These are terrible tragedies.
Many people have questions about why tornados develop, and why some people and some property is hit while others are not. I can't help with the latter question, but a good deal is known about why tornados develop where they do, as well as why some storms are stronger than others.
While I have never been hit or impacted directly by a tornado, I lived in "Tornado Alley" for about 16 years - 6 years in Wichita, KS, and 10 years in north-central Indiana. I've seen only one tornado, but my family and I have ducked for cover with everyone else many times when the tornado sirens blared.
If you have never lived in the Great Plains or midwest, tornados are probably foreign to you, and you may not know why they typically form where they do.
The geography and associated meteorology of the central USA makes it a spawning ground for tornados. This map from the Oklahoma Climatological Survey shows the average number of tornados per year per 10,000 square miles (i.e., 100 mile x 100 mile area). As this map shows, tornados are rare west of the Rocky Mountains, but are formed just about everywhere else in the lower 48 states above the Gulf of Mexico. So, why is that?
It takes powerful storms to produce tornados. Generally speaking, a storm powerful enough to produce a tornado is formed only when a moving mass of warm, moisture-rich air collides with another mass of cooler, drier air. Here's why the central USA is prime tornado country.
The map below shows some of the conditions that can produce tornados. First of all, weather fronts generally move from west to east across the USA. This means that masses of moist air move across the west coast and onshore. Once there they are pushed up and over the Sierra Nevada Mountains and the Rocky Mountains. When this happens the air cools. Cool air cannot hold as much moisture as warm air, so the moist cool air drops its moisture on the Sierra Nevadas first. That cool dry air then warms again when it moves across the Great Basin in Nevada and Utah, or across the arid states of Arizona and New Mexico. As the air warms it picks up additional moisture which it then drops as it crosses the Rockies. So the air is now cool and dry as it descends the eastern slope of the Rockies and enters the western edge of the Great Plains.
At the same time as the cooler drier air is moving west, driven by prevailing surface winds at lower altitudes and by the high-level jet stream, warm, moist air moves north from the Gulf of Mexico. When the warm moist air from the Gulf collides with the cool, dry air from the west, storm fronts develop. And, the greater the humidity and temperature difference, the stronger the storm system. For those of you who live in America's midsection, you know that these systems can extend all the way from Texas up into the Dakotas and across into Illinois and Indiana.
This satellite photo from NOAA shows the storm system that spawned the tornado that struck Joplin. As you can see, this photo shows the system extending from eastern Texas all the way up into Minnesota and on eastward toward the Great Lakes.
This closer image, also from NOAA, shows the location of Joplin as the storm front moved through. This bumpier clouds are thunderheads that can extend miles into the atmosphere. The collision of the warm and cool air causes these thunderheads to form, and the air inside of them moves in powerful convection cycles, with extremely strong updrafts and downdrafts. The moisture in thunderheads can be pulled up so high that droplets of water freeze and form hail. The more powerful the air movement, the longer it can hold the hailstones in the air, and the larger they can become before they can't be held in the cloud and they fall.
Most of the time these storm systems produce strong rainstorms, but when conditions are met, they produce hail and tornados. This is a photo of an isolated thunderhead "supercell" photographed in Minnesota by a storm chaser, Tadd Harris.
The storm system that hit Longdale, OK, was a large and powerful system. In fact, it produced a state record hailstone for Missouri. This hailstone fell in Meadville, MO, a town in north-central Missouri. Here's a photo of that large hailstone.
This hailstone may give you some idea of the air circulation and wind forces that exist inside of thunderheads. The air movement had to be strong enough to repeatedly fling the hailstone into the upper region of the thunderhead so that more ice could accumulate, increasing its size. Eventually the hailstone got massive enough that gravity overcame internal wind forces, and it fell to the ground.
Why was the Joplin tornado so powerful? Meteorologists reported an unusual set of circumstances that led to this event. For one thing, northern North America is experiencing cooler than usual weather conditions this spring. This observation is shown in the map below. This map from NOAA shows temperature anomalies around the Arctic Ocean. This map centers on the North Pole. To the left is northern Russia. To the right is Greenland and Hudson Bay. At the lower part of the map is Alaska and the Bering Sea. The darker blue/purple, the cooler the current conditions compared to historical records. So, our north is, this spring, cooler than usual.
At the same time, the air moving north from the Gulf of Mexico was warmer than usual for this time of year. So when that cooler than usual and warmer than usual air collided, it produced the more powerful than usual storms of 5/22 and 5/24.
All right, what should you do in case of a tornado? If you are indoors, you will want to get below ground level if at all possible. A basement is best, and if you can get in a small room in a basement, like a bathroom, that's even better. When we lived in Indiana we had a split-level home with a walk-out basement, but the front half of the basement was below grade. There was a small bathroom on that level, and that's where we'd wedge ourselves when the tornado sirens went off. When I lived in Kansas, we had a full basement. Regardless, you are best off if you are protected from windows and open areas. If you live out in the country, it's often a good idea to have a storm cellar or root cellar, etc., where you can go when there is a storm.
If you are in a mobile home, have a plan of a nearby place you can go ASAP if there is a tornado watch/warning. Mobile homes are notorious for suffering the worst wind and tornado damage of any type of home. They can be blown over, twisted, etc. Oh, a "Tornado Watch" means that conditions exist that could produce a tornado. A "Tornado Warning" means that a tornado has been spotted. To pay attention to these signals and act and plan accordingly.
If you are out in the open, what should you do? If possible - find cover. If no cover is available look for some kind of protection, such as an overpass where you can take shelter, or even a ditch or other area. Get below ground level. You should NOT take cover under trees. These powerful storms often produce a lot of lightning, and trees act like lightning rods. The main reason you want to be below ground level is that tornados on the ground pick up a huge amount of debris that gets flung around. If you can get below ground level, even a little bit, you will minimize your exposure to that debris. This photo from Time.com shows what some of that debris can do.
This is getting long, but I'll just mention one last thing. It's about why tornados cause buildings and structures to explode. The high wind velocities in tornados cause air to be sucked into them and out the top of the tornado. This produces an extremely powerful localized low pressure zone around the base of the tornado. The air pressure inside of an enclosed structure nearby or in the tornado's path is higher than that of the tornado. And, when the tornado gets close enough, that pressure difference can cause windows to blow out, walls to blow out, and roofs to be pulled off. The effects of this pressure differential can be reduced by opening windows, ideally on the side of the house away form a tornado's most likely track. Of course, other damage is caused by debris that the tornado has accumulated while it has been on the ground, and those projectiles can reach incredible velocities and cause damage to anything they strike.
I hope this was helpful and useful. Keep an eye on the sky and stay safe!
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