A damage indicator (DI) is one of the twenty-eight standardized building, structure, or tree categories that damage surveyors use to translate observed destruction into an estimated tornado wind speed under the Enhanced Fujita Scale. Each indicator carries its own set of failure modes, called degrees of damage, and each degree of damage has an expected wind speed with a defensible range.
Why indicators exist
The original Fujita Scale asked surveyors to compare damage to a small number of general descriptions, which produced inconsistent ratings and made it hard to defend any single number. The Texas Tech Wind Science and Engineering Center rebuilt the scale around specific, identifiable objects that a survey team could reliably recognize on the ground.
The twenty-eight indicators run from small barns and one- and two-family residences through large shopping malls, motels, transmission line towers, and hardwood and softwood trees. Choosing an object as the reference removed much of the ambiguity of judging damage in the abstract.
How a DI is used in the field
For each candidate structure or tree, the surveyor identifies the correct DI, then walks through its list of degrees of damage. The degrees are ordered from the first visible sign of failure to complete destruction, and each degree has been assigned an expected wind speed by expert consensus. Alongside the expected value, each degree carries a lower and upper bound that lets the surveyor account for real-world variability.
Construction quality matters. A given degree of damage on a house built with hurricane clips and continuous anchor bolts implies a higher wind speed than the same visible damage on a house with only cut nails and no wall-to-foundation connection. The lower and upper bounds exist so the surveyor can push the estimate in the direction the evidence supports rather than being forced to a single number.
How the estimates come together
A tornado's final rating is the strongest well-supported wind estimate along its path, drawn from the mix of damage indicators available. Surveyors typically look for multiple indicators of similar damage in the same neighborhood, because a single anomalous failure can be misleading. Where indicators disagree, the strongest one that survives scrutiny sets the rating.
This is why survey teams document as many indicators as they can, even in the same block. A single house is a data point; a subdivision is a case.
Important limits
Damage indicators only speak to what the storm actually hit. A tornado that crosses open range, wheatfields, or scrub may leave few or none of the twenty-eight indicators, in which case the tornado is often rated by tree damage alone or given a lower rating than it may have deserved. The system also depends on the surveyor reading construction quality correctly, which is not always obvious from the outside of a building.
The DI list is also not exhaustive. Fences, agricultural equipment, and vehicles have their own failure modes that fall outside the twenty-eight standard indicators and can only inform an estimate indirectly. The framework is a large step forward in consistency, but it is still an engineering estimate rather than a wind measurement.