Every tornado in the United States is officially rated by the National Weather Service using the Enhanced Fujita (EF) Scale. The rating is based on the observed damage, not on wind measurements directly. NWS survey teams examine the tornado's path within days of the event and assign a rating from EF0 (weakest) to EF5 (strongest).
A tornado occurs and produces damage. Local storm spotters, chasers, and TV news often provide first observations. Emergency managers document initial casualties and destruction.
Within 24 hours (usually less), NWS survey teams from the relevant local Weather Forecast Office deploy to the damage path. For major events, additional teams from neighboring offices may join.
Survey teams typically include:
Surveyors walk (or drive/fly over) the tornado's path from start to end. They identify specific damage locations, photograph them, and note:
The EF Scale uses 28 official damage indicators. Common examples:
Each DI has multiple degrees of damage. For example, a single-family residence has 10 DoDs ranging from "loss of shingles" (DoD 1) to "slab swept clean" (DoD 10). Each DoD is mapped to an expected wind speed range.
Surveyors identify the highest DoD documented at any point along the path. That DoD maps to a wind speed range with:
The tornado's official EF rating is based on the highest wind speed indicated:
Within a few days to a few weeks (depending on complexity), the NWS issues the official Storm Damage Assessment. The tornado is entered into the national database with:
You might wonder: if we can measure winds directly with radar, why not use that? Because:
Some meteorologists argue for adding radar-measured winds to the rating system. Ongoing debate continues.
A violent tornado that never strikes a structure can only be rated based on trees, terrain, and vehicles. The El Reno 2013 tornado had radar-measured 296 mph winds but was rated EF3 because it didn't hit engineered structures.
Multi-vortex tornadoes have sub-vortices that produce extreme localized damage. Surveyors have to identify whether damage came from the main tornado or a sub-vortex.
Debris from one nearby damage indicator can strike another, causing damage that's harder to attribute to wind alone.
A weak home built to poor standards may fail at 90 mph - much lower than the EF Scale assumes. Surveyors must judge construction quality.
Tornadoes from before the EF Scale (Feb 1, 2007) are rated on the original Fujita Scale (F0-F5). NOAA has NOT retrospectively converted these to EF ratings. When you see a "1974 F5" in historical records, it remains F5 - not EF5.
Some historical tornadoes have been re-evaluated by researchers using modern damage-survey methods. Estimates of the Tri-State 1925 tornado's rating have shifted between F4 and F5 depending on which review is cited.
No US tornado has been officially rated EF5 since Moore, OK on May 20, 2013. Several recent tornadoes (Bassfield 2020, Mayfield 2021, Rolling Fork 2023) have shown damage consistent with EF5 but were rated EF4. Meteorologists debate whether the EF Scale's damage-indicator requirements have become too strict. Full EF5 list →
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