The National Weather Service dispatched investigators to New York City on Friday to determine whether a tornado had touched down in Gotham the previous night. Within 24 hours, they concluded that two tornadoes had struck Queens and Brooklyn. How can you tell the difference between a tornado and any other big storm hours after the winds have died down?
By looking at the debris. All severe storms leave behind a big mess, but each mess is a little different. In the absence of golden evidence like smartphone video of a funnel cloud, tornado investigators comb through the debris for indicators of unique twister traits like rotating winds and a narrow path of destruction. The presence of damaged roofing tiles and nailed-in shingles can be helpful for determining wind speeds after the fact, because these standard construction features are known to peel off at wind speeds of around 80 to 100 miles per hour.
If aerial photos can trace a path of bare roofs through a series of neighborhoods, with no effect on houses outside the storm course, it's a pretty good indication that a tornado passed through. Tornadoes also cause trees to fall in a signature swirl pattern, indicating rotating winds. Projectiles embedded in objects on either side of the storm's path are helpful as well, as tornadoes are more likely than straight winds to throw things up in the air.
Out in the country's breadbasket, the amber waves of grain make tornado verification a lot simpler. Crops are often pushed down in a curved path along an easily traced route, indicating that rotational winds passed through. But even Midwestern tornadoes are difficult to identify sometimes, because other weather systems can produce similar results. Downbursts—blasts of wind that descend from the atmosphere and spread in different directions after hitting the ground—can cause debris to spread in a scattershot pattern. (Queens experienced a downburst on Thursday that caused more damage than the tornadoes.) Gustnadoes, ground-based rotational winds that are disconnected from a tornado in the atmosphere, are even better at imitating tornadoes. They produce all the same debris effects, falsely suggesting that the associated tornado touched down rather than just whirled by. A gustnado's path is quite short, though, since they rarely last more than a few seconds.
So why does it matter whether the storm that hit New York was a tornado? It helps with forecasting and climate research. Meteorologists base their tornado warnings on historical patterns—certain storms seem to give rise to tornadoes, while others don't. They constantly update and refine their models based on real-world storm formation. In addition, since tornadoes represent a confluence of atmospheric events, some climatologists believe that changes in tornado frequency or the conditions that give rise to twisters may be leading indicators of global warming.
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Explainer thanks John T. Snow of the University of Oklahoma. Thanks also to reader Chris Dutton for asking the question.