How Are Tornadoes Rated?

The Rating Process, Step by Step

Step 1: The Event

A tornado occurs and produces damage. Local storm spotters, chasers, and TV news often provide first observations. Emergency managers document initial casualties and destruction.

Step 2: NWS Survey Teams Deploy

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:

• NWS meteorologists with damage survey training
• Structural engineers (for major events)
• Emergency management representatives
• Sometimes academic tornado researchers

Step 3: Damage Assessment

Surveyors walk (or drive/fly over) the tornado's path from start to end. They identify specific damage locations, photograph them, and note:

• The type of structure or feature damaged
• The exact damage sustained
• Whether the damage occurred at that location
• Track direction and continuity

Step 4: Damage Indicators (DIs)

The EF Scale uses 28 official damage indicators. Common examples:

• Single-family residences (DI 2)
• Manufactured homes (DI 3)
• Schools (DI 11)
• Metal buildings (DI 14)
• Free-standing towers (DI 20)
• Softwood trees (DI 27)
• Hardwood trees (DI 28)

Step 5: Degrees of Damage (DoDs)

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.

Step 6: Wind Speed Assignment

Surveyors identify the highest DoD documented at any point along the path. That DoD maps to a wind speed range with:

• Lower bound (LB) - minimum wind to produce the damage
• Expected wind (EXP) - typical wind speed
• Upper bound (UB) - maximum wind that could produce this damage

Step 7: Rating Assignment

The tornado's official EF rating is based on the highest wind speed indicated:

• 65-85 mph → EF0
• 86-110 mph → EF1
• 111-135 mph → EF2
• 136-165 mph → EF3
• 166-200 mph → EF4
• 200+ mph → EF5

Step 8: Official Report

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:

• Date and time
• EF rating
• Peak wind estimate
• Path length and width
• Fatalities and injuries
• Damage estimate

Why Rating Is Based on Damage, Not Winds

You might wonder: if we can measure winds directly with radar, why not use that? Because:

• Ground-level winds are what actually cause damage - radar measures winds above ground
• Not every tornado is captured by close-range radar
• Historical tornadoes (pre-1990s) can't be re-measured with modern radar
• Damage-based rating provides consistency across decades of records

Some meteorologists argue for adding radar-measured winds to the rating system. Ongoing debate continues.

Rating Challenges

Tornadoes Over Open Country

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.

Multiple Vortices

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 Contamination

Debris from one nearby damage indicator can strike another, causing damage that's harder to attribute to wind alone.

Pre-Existing Conditions

A weak home built to poor standards may fail at 90 mph - much lower than the EF Scale assumes. Surveyors must judge construction quality.

Historical Ratings and Reevaluations

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.

The Modern EF5 Drought

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 →