Two reposts for your edification:

Tornadoes in Perspective

With all the interest in tornadoes, I thought it would be helpful to provide some contextual data (focusing on US tornadoes).

Damaged houses along Grace St. in St. Peter. (Photo courtesy of St. Peter Kiwanis) One thing that has to be said is that tornadoes are very powerful, but very small and short lived meteorological phenomena. They are like bullets being fired from cheap handguns by incompetent shooters. There are many stories of two gang bangers aiming their rods at each other at short distance, emptying them out, and failing to harm each other. In the mean time, a stray bullet from a drive by shooting can enter a home and kill an innocent young girl sitting at the table doing her homework.

Tornadoes are similar. The famous St. Peters tornado outbreak of Minnesota involved two major tornadoes from the same cell (I like to think of them as one phenomenon), which covered a great deal of ground over a very long time period, but only two people died. In 1925 a single tornado covered about 65 miles of land (same as the St. Peter tornado, roughly) in Missouri, Illinois and Indiana. It was probably an F5 (same as St. Peter). a total of about 695 people were killed, including 234 individuals in the town of Murphysboro, Illinois.

Here’s a description of the Minnesota event:

At 3:50 pm a tornado touched down 7 miles … east of Avoca, Minnesota…. As the tornado moved through Murray and Cottonwood counties, it damaged or destroyed 150 farmsteads, killed 500 dairy cattle…

At approximately 4:30 pm the twister… entered Comfrey, a town of 550 people located in both Cottonwood and Brown counties. … It moved through the center of town, destroying a grain elevator, the town hall and most of the main street businesses downtown. The town’s firehouse collapsed, and the school was heavily damaged. Fifty homes were destroyed and 100 people were left homeless. … Approximately 75% of the buildings in Comfrey were damaged or destroyed.

As the tornado moved through Brown County it achieved F4 strength where it damaged or destroyed 130 more farmsteads and killed 500 cattle. Approximately 15% of the 1000 farms in Brown County sustained damage … Northwest of Hanska a man was killed as a result his house collapsing. …. At one point the tornado had a width of 1.25 miles (2 km). After traveling across six counties for 1 hour and 25 minutes, the twister lifted back into the clouds at 5:15 pm …. With a path of 67 miles (108 km), this tornado is the fifth longest track tornado on record in Minnesota.

A few minutes later at 5:18 pm, the same supercell produced another large tornado two miles to the east of Nicollet. One mile to west of St. Peter, a 6 year old boy was killed when he was thrown from the vehicle he was riding in. At 5:30 pm the twister hit St. Peter, a town of about 10,000 people, at F3 strength. It inflicted severe damage on much of the town. … Gustavus Adolphus College sustained heavy damage after taking a direct hit from the twister, …. Officials estimated 500 homes in St. Peter were damaged or destroyed, and over 1000 trees were uprooted. Debris from St. Peter fell as far as Rice Lake, Wisconsin, 136 miles away….
[source]

A few years later, my wife showed me her dorm room, destroyed by this tornado. An iron radiator had broken loose and plowed into the very bed she may very well have been asleep in.

Except that it was spring break, the town was empty, the early warning system worked pretty well, Minnesotans tend to be cautious about tornadoes, and the rest of the tornado’s track was across sparsely inhabited farmland. So only two people (and a LOT of cattle) died in that disaster, even though there was an enormous amount of property damage.

All of the 25 deadliest US tornadoes seem to predate radar and early warning systems. Science can save your life.

So, how many tornadoes are there a year, and how deadly are they? Since tornadoes are small and short lived, counting them has been difficult and reliable data are only available for recent years. Even recent data are far from perfect. The death rates may be more accurate, but it is also possible that some earlier deaths attributed to tornadoes were actually due to other sorts of storms, given changes in the ability to verify tornadoes as opposed to, say, straight line wind storms.

Having said all that, the following chart (based on data from disastercenter.com is very interesting:

(In case you can’t make it out, the maximum on the Y-axis is 1,400.)

The steady increase in tornadoes is almost certainly due mainly to ability to spot them. This does not rule out the possibility that changes in climate (global warming) could related to tornado frequency, but this is not the first data set one might want to use to explore this issue. As I have suggested before, there is likely to be a link. What seems to be a qualitative shift in deaths is probably the implementation of warning systems and better science and safety education.

When is Tornado Season?

This story:

10 tornadoes confirmed in Ga., including one with winds topping 160 mph

Ten tornadoes, one packing winds of more than 160 mph, touched down in parts of Georgia on Wednesday, the National Weather Service said Friday.

The storms caused an estimated $25 million in insured losses, said John W. Oxendine, the state’s insurance commissioner.

“I spent some time surveying damage and talking to residents in Jasper, Putnam and Hancock Counties” on Friday, Oxendine said in statement. “I believe claims will easily reach $25 million. Actual losses are much higher when you consider things like infrastructure damage and uninsured losses.”

Reminds us that Tornado season is coming. Maybe it is already here in parts of the country, or maybe it is a bit early this year in the south. It is important to keep tornadoes in perspective. It would appear that for the last half century, the frequency of tornadoes in the US is rising, though this could be totally or in part because of increases in reporting. Warming climate should result in more tornadoes in areas where tornadoes already occur, or at least that is a reasonable assumption unless countervailing effects can be demonstrated.

So when is tornado season exactly?

Its complicated. The best way to answer this question is locally. Here in Minnesota the frequency of tornadoes starts to pick up in April, with the peak coming in June. In Georgia, where these reported tornadoes were, there is almost a bimodal season. They really can happen any time of the year in Georgia, but there is a major peak around mid february through about May, and another peak late in the year, around November, with this second peak being fairly weak.

The following is an animated GIF showing tornado activity from 1985 through 1989 over the course of a year.

(If this starts to annoy you, just hit the “escape” key. That will stop any animated GIF in most browser windows. File under “computer tricks.”)

This is a fairly complicated pattern, but it is a pattern. Throughout the year, tornado activity spreads from south to north, as does thunderstorm activity. This sort of storm is most common in the middle, flatter parts of the country, but also, the energy that feeds these storms comes mainly from the Gulf of Mexico. This is the same sea-surface temperature (and over-sea moisture) effect that results in hurricanes which strike land in the US having a similar pattern. And, this is why it is logical to expect a global warming – tornado link.

NOAA has a great web site with the data referred to here, and more cool graphics, here.

And if you live in any of the lit up areas of this GIF, make sure your Weather Radio is working!!!