Tornado swarms have become increasingly common since the 1970s, however the total number of tornadoes seen each year has stayed about the same.
According to Scientific American, the number of days with at least 20 tornadoes has doubled over the last few decades to roughly five days per year. Days with 30 or more have risen to three.
The new findings were detailed in the Friday issue of the
journal Science.
Study author Harold Brooks of the U.S. National Oceanic and Atmospheric Administration's National Severe Storms Laboratory in Norman, Oklahoma, said it is unclear if the clustering of tornado occurrences can be attributed to climate change, man-made or otherwise.
In the new report, his team makes it clear that the total number of tornadoes has stayed steady, averaging 495 per year. The only thing that's changed is their distribution across the calendar. The clustering has resulted in fewer days with tornadoes overall, however on days when there are tornadoes, there are more of them.
In addition to discovering shifts in the distribution of tornadoes, the scientists also took a stab at defining a start date for tornado season. Unlike hurricane season, which is usually six months long, tornadoes happen throughout the year, so the team tied the start to a metric. When 50 F1/EF1-strength tornadoes had been observed, that would be the start date.
Predictably, the average start date fell in March, on the 22nd. Notably, the variability of the start dates has increased in recent years.
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