Tornado vs. straight-line wind damage

Why the difference matters for a building

Most commercial property policies treat both as windstorm, so the label rarely changes coverage by itself. What the survey record helps with is showing when the wind hit, roughly how strong it was and that wind, not age or wear, did the damage. That record is easier to use when you know which kind of event to look for.

Reading the damage

Surveyors walk the damage and read it the way an investigator reads a scene. Uprooted large trees are the best wind vane: their trunks fall in the direction the wind blew. Small branches and snapped trees say less, because they can fail from a weakness in the tree rather than the wind's direction.

• Tornado: trees fall generally pointing inward toward the center of the path, and large uprooted trees often cross each other in a herringbone pattern.^10,11
• Tornado: leaf litter, dirt or mud is often sprayed on the sides of objects, and projectiles are stuck in the ground or in nearby objects, though downbursts can throw projectiles too.⁵
• Straight-line wind: debris is blown downwind in one general direction, often fanning out from an impact point.^12,5
• Straight-line wind: from the air, downbursts often leave pockets of damage, like footprints, that may not connect.⁵

Storm Data's rule: damage counts as a tornado if it could only have been tornadic, or if it shows two of these: well-defined edges to the damage path; trees lying 30 degrees or more off the path's axis; suction vortex marks, ground striations or extreme missiles; or a converging debris pattern. Credible eyewitness reports, photos or video of a violent circulation from cloud base to ground also count, even with little damage.⁴

Surveyors also check radar, photos, video and eyewitness accounts. Without conclusive evidence of rotation reaching from the ground into the parent storm, the NWS classifies damage as non-tornadic.⁶

Why the two get mixed up

• Twisted trees: a tree with uneven limbs catches more wind on one side, so straight wind can twist it apart.
• The roar: winds of 75 mph or more often sound very loud, leading some people to believe they heard a tornado.¹²
• Fast tornadoes: forward speed adds to the winds on one side, so debris can lie in one direction and the convergent pattern may be missing. A 100 mph tornado moving east at 50 mph produces 150 mph winds on its south side and 50 mph on its north side.^11,4
• Terrain: hills and valleys can bend downburst debris into a convergent pattern, and wind funneled between them can do much more damage.
• Both at once: rear-flank downdraft winds can hit right next to a tornado, and when the two can be told apart, Storm Data logs that damage as thunderstorm wind, not as part of the tornado path.^1,4

One night shows how closely they mix. On July 14, 2024, a line of storms produced 9 tornadoes in the NWS Chicago area and two main corridors of straight-line winds of 70 mph or more.¹⁴

In St. Charles, a brief EF0 tornado spun up as a surge of straight-line winds arrived, damaged the roof of a self-storage building and tossed cart corrals at a store; past a shopping center, the damage appeared to change to straight-line wind damage.¹⁴

Same damage from either wind on commercial buildings

The EF scale estimates wind from damage to 28 indicators, including strip malls, big box stores, apartments of 3 stories or less, metal buildings and warehouses. Its speeds are 3-second gust estimates at the point of damage, not measurements.⁹

Most of those speeds are within reach of straight-line winds: microbursts can top 100 mph, and derecho gusts as high as 130 mph, equal to a strong EF2 tornado, have been recorded.^2,8

Storm Data's thunderstorm wind guide says roofs may be partially peeled off industrial, commercial and warehouse buildings at 75 to 89 mph. Damage alone does not automatically mean winds reached 58 mph, though: when estimating thunderstorm winds, Storm Data preparers weigh the damaged property's condition, age, construction, exposure, terrain and nearby objects that funnel wind.⁴

How each is rated and recorded

Tornadoes are rated on the EF scale, and a tornado's rating is the highest wind estimated anywhere along its path.

A tornado that damages none of the scale's qualified indicators, such as one that crosses only crops, can be rated EF-Unknown (EF-U). An EF rating can be off by one category either way.^5,4,14

Storm Data logs downbursts, microbursts, macrobursts and gustnadoes as Thunderstorm Wind events, with the peak wind in knots marked as measured or estimated. Tornado events carry an EF rating instead.^1,4

Thunderstorm wind is severe at 58 mph (50 knots), and Storm Data calls damage from winds over 74 mph (64 knots) extreme, equal to EF0-category winds.^5,4

Finding the survey for your storm

Not every storm gets a survey. Surveys usually follow significant events: widespread damage, injuries, deaths, heavy media interest, or doubt about the type of storm. Officials and insurance companies use the results, so check these sources for the date and place of your damage.⁵

• NWS office event pages: for July 14, 2024, NWS Chicago posted a summary of each tornado and straight-line wind corridor, with KMZ and shapefile downloads.¹⁴
• Damage Assessment Toolkit: the public viewer at apps.dat.noaa.gov/StormDamage/DamageViewer lets users view, download and analyze survey data for a chosen time range, anywhere in the country.¹⁶
• To find a storm in the viewer, set the Begin and End dates in the upper right corner; survey data download as KMZ files.¹⁷
• Toolkit data are quality controlled but still preliminary; official statistics are in Storm Data.¹⁸
• Storm Prediction Center (SPC) storm reports: preliminary reports listed as is, with each day running from 1200 UTC to 1159 UTC the next day.¹⁹
• NCEI Storm Events Database: searchable Storm Data, with tornado and thunderstorm wind events filed by county. Events take 3 months to appear.²⁰