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On this day 40 years ago, 148 tornadoes tore through the U.S. and Ontario killing hundreds.

Tornadoes are clustering in the US, and climate change may be to blame

Scott Sutherland
Meteorologist/Science Writer

Tuesday, April 7, 2015, 12:30 - If you've been keeping track of the news about tornado outbreaks in the US over the past few decades, you may have noticed a trend - fewer outbreaks, but at the same time each one appears to be worse than the last. According to a new report, it's probably not your imagination. In fact, it could be due to climate change.

One of the basic predictions coming out of climate science for years now is that we will be seeing an overall increase in the number of extreme weather events. This is not to say that the overall number of weather events is going to increase, just that the percentage of extreme weather events, compared to more 'normal' weather events, is going to get higher. 

RELATED: United States in tornado drought since start of 2015

The reason for this is due to the effects of adding more energy (in the form of heat) to our climate, and thus our weather systems. Among other things, this causes an overall decrease in the amount of wind shear - the difference in speed and direction of the winds over a short distance in the atmosphere. Tornadoes depend on wind shear, since this is what typically causes the initial 'roll' in the air that is the first stage of tornado development. So, reducing wind shear overall will cause fewer days when tornadoes spawn, but when wind shear is still high enough on a particular day that tornadoes can form, the amount of energy in the system should make those twisters that do spawn more powerful, or it could result in more tornadoes forming on that day, or both. Thus, due to climate change, we should see fewer tornado outbreaks, but those that do occur should be worse.

Now, a study by scientists from NOAA - the US National Oceanic and Atmospheric Administration - is showing evidence of this trend.

"When people ask, 'Are we getting more tornadoes, are we getting fewer tornadoes, are they later, are they earlier?' - the answer to everything is yes," said Harold Brooks, the lead study author from NOAA's Severe Storms Laboratory, according to

Looking back at US weather records from 1964 to 2013, they found that while the average number of tornadoes per year reported across the country (excluding the weakest ones) wasn't changing by much, the number of days that these tornadoes occurred on did change. It dropped, from 187 tornado days in 1970 to just 110 tornado days in 2011. This means that the tornadoes are being concentrated in fewer days, clustering together. According to the records, nine of those days in 2011 saw 30 tornadoes spawn each. Apparently, that's the same number of 30+ tornado days that occurred in the 20 years between 1961-1981.

"In effect, there is a low probability of a day having a tornado, but if a day does have a tornado, there is a much higher chance of having many tornadoes," the study said. "Concentrating tornado damage on fewer days, but increasing the total damage on those days, has implications for people who respond, such as emergency managers and insurance interests. More resources will be needed to respond, but they won't be used as often."

This study isn't the only one to make these same observations. Another, published in August, came to the same conclusions using different methods, thus strengthening the results of both studies.

While Brooks and his colleagues hint at the effects of climate change, due to the increased loss of sea ice in the Arctic, the authors of the previous study are a bit more direct.

"The greater heat and moisture in the atmosphere is a direct result of a warming planet, and the warming is greater at the poles than at lower latitudes, amplifying and slowing the jet stream," said James Elsner of Florida State University, the lead author of the study, according to The Smithsonian. "Shear will decrease on average across the globe as the warming in the Arctic outpaces warming elsewhere, but sufficient shear persists regionally when the jet stream waves amplify and stall."

That provides not only the localized shear to produce tornadoes, but the environment for concentrated tornado clusters to form.

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