How Tornadoes Are Rated: The EF Scale Measures Damage, Not Wind

Direct answer: the Enhanced Fujita (EF) scale rates a tornado from EF0 to EF5 based on the damage it caused, from which a wind speed is then estimated. No instrument measures the wind inside most tornadoes. The rating is assigned by damage survey teams after the event, which is why a tornado's rating often appears or changes days later.

The scale

• EF0 — 105 to 137 km/h. Peeled roof surfaces, broken branches, damaged gutters and signs.
• EF1 — 138 to 178 km/h. Roofs badly stripped, mobile homes overturned, exterior doors lost, windows broken.
• EF2 — 179 to 218 km/h. Roofs torn from well-built houses, mobile homes destroyed, large trees snapped or uprooted, cars lifted.
• EF3 — 219 to 266 km/h. Entire storeys of well-built houses destroyed, trains overturned, heavy cars thrown.
• EF4 — 267 to 322 km/h. Well-built houses levelled; structures with weak foundations thrown some distance.
• EF5 — above 322 km/h. Strong framed houses swept from their foundations and disintegrated; car-sized objects thrown more than 100 metres.

Why it is a damage scale

Tornadoes are small, short-lived and violent. Weather stations almost never sit inside one, and portable instruments deployed by researchers score direct hits only rarely. So in 1971 Ted Fujita built a scale that worked backwards: observe what broke, infer what wind it took.

The original F scale over-estimated wind speeds because it assumed buildings were stronger than they are. The Enhanced Fujita scale replaced it in the United States in 2007. It uses 28 damage indicators — one-family homes, schools, transmission towers, hardwood trees and so on — each with a set of degrees of damage calibrated by structural engineers to a most-likely wind speed. Survey teams match what they see to the indicator, and the worst damage anywhere along the track sets the rating.

The consequence: ratings run conservative

This method has a structural bias built in. A violent tornado crossing open farmland with nothing to destroy cannot be rated highly, because there is no damage indicator to score. Several of the most intense tornadoes ever measured by mobile radar carry modest EF ratings simply because they passed over fields.

The top of the scale is stricter still. An EF5 usually requires a well-constructed, well-anchored house to be swept clean off its slab. Poor anchoring — a house merely nailed to its foundation rather than bolted — caps the rating at EF4 no matter how total the destruction, because engineers cannot rule out that a lower wind did it. EF5 ratings are therefore rare and occasionally disputed.

Reading tornado news correctly

• A rating is not available during the event. Surveys take one to several days. A tornado described as "EF3" an hour afterwards is a guess.
• Ratings get revised as surveys extend along the track.
• A low rating does not mean a weak tornado — it may mean there was little in the way to break.
• Track length and width matter as much as the rating for understanding impact.

Watch versus warning

• A tornado watch means conditions are favourable for tornadoes across a broad region, often for several hours. Stay alert and review your plan.
• A tornado warning means a tornado has been spotted or is indicated by radar in your area right now. Take shelter immediately.

Warning lead times in the United States average roughly ten to fifteen minutes. That is enough to reach a basement or an interior room. It is not enough to drive away from a storm — attempting to outrun a tornado by car is a recognised cause of death.

Outside the United States

The EF scale is a US standard. Much of Europe uses the original F scale or the International Fujita scale developed by the European Severe Storms Laboratory, which broadens the damage indicators to cover European construction. Tornadoes occur on every continent except Antarctica — Bangladesh, eastern India and Argentina all suffer deadly ones — but only a handful of countries run systematic damage surveys, so global tornado statistics are heavily skewed toward the countries that count carefully.

Know what to do before the siren with the tornado safety guide, and watch current activity on the live tornado page.

Frequently asked questions

Who decides a tornado's EF rating?

A damage survey team, usually from the national weather service, walks or flies the damage path after the tornado and matches what it finds to standard damage indicators. The worst damage along the track sets the rating.

Why are EF5 tornadoes so rare?

Partly because they genuinely are, and partly because the rating requires a well-anchored, well-built structure to be swept away. Where construction quality cannot be verified, engineers assign a lower rating.

Is there an EF6?

No. The scale ends at EF5, which is open-ended above 322 km/h. Damage at that level is already total, so a higher category would add nothing.

What is the difference between a tornado watch and a tornado warning?

A watch means conditions are favourable for tornadoes over a broad area for the next several hours. A warning means a tornado is happening or imminent in your specific area — take shelter now.

How much warning do you get for a tornado?

In the United States, average lead time is roughly ten to fifteen minutes — enough to reach a basement or interior room on the lowest floor, not enough to outrun the storm by car.

Sources

• NOAA Storm Prediction Center — The Enhanced Fujita Scale
• NOAA National Weather Service — Tornado Watch vs Warning
• NOAA National Severe Storms Laboratory — Tornado Basics
• European Severe Storms Laboratory — International Fujita Scale

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