Sixty-one years of tornadoes, in one map

John Nelson—the data visualization designer responsible for that global map of earthquakes I posted last week—has also made a strangely beautiful map showing every tornado to hit the U.S. between 1950 and 2011.

Part of what makes this map interesting is that it shows not only the touchdown location, but also the path of the tornado as it moved. Better yet, Nelson has several other related maps that break the data down in different ways. For instance, if you look at the tornado map broken down by seasonality, you can see a really amazing pattern, where what constitutes "Tornado Alley" appears to move northward over the course of the year. In December, January, and February, the bulk of tornadoes have been centered on south and south-central states like Mississippi, Texas and Kentucky. In peak tornado season—March, April, and May—the southern states are still affected, but the reach of the tornadoes has extended north and west. By June, July, and August, most of the tornado activity is happening in states like Michigan and Minnesota.

Another interesting thing I spotted on these maps: There's a hole in tornado activity centered on West Virginia. All around the state, there's a history of tornadoes. In the Mountain State, though, the number of tornadoes drops off precipitously. I'm really curious what's causing that, or whether it's a flaw in the data.

Compare tornado habits throughout the seasons

Compare tornado numbers by F-scale

Compare tornado history before and after the historically devastating 2011 season.

Watch an animation of tornadoes by year


  1. >i n the Mountain State, though,

    >Mountain state

    There’s your answer.  The appalachians are there, and Tornados can’t go up or down hills or mountains well, they tend to collapse.

    1. I heard that when I was growing up in Indiana. Then on April 5, 1974 I rode past where one of the tornados had crossed Indiana Highway 60 near Borden two days before.  You could clearly see the path (flattening trees in a swath 30-40 (possibly more) wide) it made as it came down the southwestern hillside, crossed the stream and road, and then went up the opposite hill. Steeper and higher mountains might have the effect you describe, but hills didn’t that day.

      (Perhaps even more impressive was the big, full size 60’s sedan ‘pinned’ to the ground by a 2×4 that had speared down through the trunk lid, through the car and into the blacktop beneath the car. This was outside the tornado’s actual path of destruction by 50 to 100 meters. The house the car was parked at didn’t have any obvious damage that I can recall.)

      If you look at the higher resolution versions of the map above, look just west of the end of the Maryland panhandle. You will see the gash of an F-3 tornado running from near the PA border over into MD. That is across the ridgelines, not with them.

      Mountains have fewer tornados because the terrain disrupts the kind of storm cell activity that spawns them, not because the tornados can’t handle elevation change. (Note the thinning over the Ozarks as well.)

  2. Supercool map. Going to add my voice to the comments about West Virginia… it’s an Appalachian hole, not a WV hole. Living here (in southern Appalachia, northeast Tennessee) means that when we have gotten tornados it’s been an extraordinary event. 

  3. Anyone care to enlighten me as to why there are so few tornadoes west of the Rockies? The difference is dramatic!

    1. The Rockies themselves are why. The mixing of the cold dry air from the north and warm damp air from the south is what loves to spawn tornadoes. Those fronts (generally) can’t meet in the mountain ranges, but instead have to go around the mountains. Which is why the midwest gets pounded so heavily.

    2. Anyone care to enlighten me as to why there are so few tornadoes west of the Rockies?


  4. uniquely mountainous terrain with virtually no flat spots in West Virginia, that is why there are no tornadoes. If you ever travel there, there is hardly a breeze due to the terrian. It’s weird.

    1. HUH hardly a breeze?? Maybe in a valley but we have tons of windmills and they are building them at an incredible rate. 

  5. Tornadoes are illegal in Canada. Clearly that’s why they all stopped dead at the 49th Parallel. They also appear to be hydrophobic and won’t cross the great lakes. God it’s good to live in Tornado free Canada.

    1. NOAA is an office of the US government.  Get your own danged OAA if you want maps of tornados in Canada. 

  6. An interesting thing about that map for me is that I can clearly make out the freak tornado in New England that killed my grandfather in the 50s. While we do occasionally get tornados in this part of the country, a F4/F5 tornado is rare enough to stick out pretty brightly. I cant imagine living in one of those places just west of the Mississippi that frequently see tornados (for lots of reasons, but the tornado frequency would be number 1).

  7. Interestingly,  I live in the one place where there has never been any really violent weather.  The easternmost tip of South America.    My town, João Pessoa, Brazil is a 7ºS 34ºW.  

    In the over 450 years that the city has been here, there has never been a hurricane, tornado, nor earthquake or volcanic eruption.  In fact, thunderstorms are even rare.  I have seen two or three years go by without even hearing thunder.  Even then, it is often from an off-shore storm several miles away.

  8. I was curious if the tornado that took the back corner off my high school (Los Altos High, in Los Altos, California) in the late 1990s would show up… Oddly, there’s a little December to May hotspot right over Los Altos.  That’s surprising…

    Good infographics surprise you.

  9. A majority of the trajectories seem to have a southwest to northeast (or vice-versa) orientation.   Can someone enlighten me as to why?

    1.  I would assume it has to do with the jetstream, and the fact that most storms move in that direction, at least in this part of the US. There’s definitely a better technical answer out there (having to do with the ways low and high pressure systems move), but i’m not quite knowledgeable enough to articulate it well. Basically though, tornadoes move in the direction that a storm moves.

  10. I want a heatmap of where tornadoes have the lowest probability of landing based on this data. Basically a rendering of that mid-east hole there, and the west.

  11. >Another interesting thing I spotted on these maps: There’s a hole in tornado activity centered on West Virginia. All around the state, there’s a history of tornadoes. In the Mountain State, though, the number of tornadoes drops off precipitously. I’m really curious what’s causing that, or whether it’s a flaw in the data.

    I am disappoint.

    1. Are you disappointed in the author who didn’t get the connection between topology and tornadoes,  or are you disappointed the West Virginia isn’t getting regularly smacked by tornadoes? 

      I guess either position is understandable.

      1.  with the author, but I like how you left an option I hadn’t considered open to my disappointment.  nice.

        Maggie lives in glaciated midwestern flatland and seems to have no concept of how mountains disrupt ALL weather patterns, though ironically, she answered her own question by calling WV by its nickname “the mountain state.”

        but anyway, she is a nice person and posts awesome stuff.  I just figured a smart like her would intuit the mountain thing or maybe she was being hyperbolic?

  12. Maggie, I adore your science posts (as a science editor myself), but this is my FAVORITE EVER!!

  13. I’m also interested in how the range of tornadoes seems to increase as the years pass. Possibly due to warming of the atmosphere, that allows the tornadoes to spread north more frequently?

  14. The increase is mainly due to Doppler radar.  Not the cause, but the ability to count. Over the last twenty-five years, the creation and expansion of the Doppler radar network has enabled us to detect tornadic signatures that would previously have been missed, especially out in the middle of nowhere. Doppler has also enabled us to more accurately rate tornadoes, layering wind velocity information on top of the how-much-damage-was-there scale.  This has led to a higher count of higher-intensity tornadoes; again, because of detection, not raw numbers.

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