Ask the Meteorologist: How are ratings assigned to tornadoes?: news

Ask the Meteorologist: How Are Ratings Assigned to Tornadoes?

Tornadoes are not rated by real-time direct wind speed measurements during the storm; instead, meteorologists determine tornado intensity through post-storm forensic surveys using the Enhanced Fujita (EF) Scale.

Because meteorological instruments rarely survive inside a tornado vortex, the National Weather Service (NWS) and atmospheric scientists evaluate the damage left in the storm’s path to reverse-engineer estimated 3-second wind gusts.

The Forensic Survey Process

When a tornado touches down, NWS survey teams conduct an on-the-ground and aerial investigation across the entire damage track. They evaluate several key factors:

• Damage Indicators (DIs): Meteorologists identify specific objects or structures affected by the tornado. The EF Scale recognizes 28 official damage indicators, including single-family residences, mobile homes, schools, commercial brick buildings, transmission towers, and various species of trees (hardwood vs. softwood).

  National Weather Service+ 1

• Degrees of Damage (DoD): For each indicator, surveyors evaluate the precise level of destruction on a standardized scale. For example, damage to a residential home is categorized along stages ranging from broken windows and missing shingles to collapsed walls or total foundation sweeping.

  National Weather Service

• Construction Quality & Structural Integrity: A crucial aspect of assigning accurate tornado ratings is examining how well a structure was built. Teams check anchoring systems (such as anchor bolts versus simple nails), framing methods, and load path connections. If poor construction caused a building to fail easily, the estimated wind speed is adjusted downward toward the lower bound; conversely, exceptional construction that resisted intense forces can push wind speed estimates upward.

  National Weather Service

The Enhanced Fujita (EF) Scale Wind Categories

Once survey teams establish the expected wind speed from multiple damage indicators, the tornado receives a preliminary rating corresponding to the highest level of damage found anywhere along its path:

• EF0 (Light Damage): 65 to 85 mph (105 to 137 km/h)

  Wikipedia

  • Minor roof surface damage, snapped tree branches, and downed shallow-rooted trees.

• EF1 (Moderate Damage): 86 to 110 mph (138 to 177 km/h)

  • Roofs stripped, broken windows, overturned mobile homes, and exterior doors lost.

    Wikipedia

• EF2 (Considerable Damage): 111 to 135 mph (178 to 217 km/h)

  • Roofs torn off well-constructed houses, exterior wall collapse, large trees snapped or uprooted.

    Tornadoes & Tornado events Wiki – Fandom