Tornadoes are among the most powerful and localized weather events, making their severity difficult to measure in real-time. Since a direct measurement of wind speeds inside a funnel cloud is impossible, meteorologists rely on a formal system that classifies intensity after the event occurs. This classification is based on the damage left behind, which provides an estimate of the wind speeds involved. The National Weather Service (NWS) uses specific procedures for classifying and recording tornado existence for historical tracking and analysis.
The Enhanced Fujita (EF) Scale
The standard method used by the NWS to classify a tornado’s intensity is the Enhanced Fujita (EF) Scale. This system rates tornadoes on a six-category scale, ranging from EF0, the weakest, to EF5, the strongest. Implemented in 2007, the EF Scale replaced the original Fujita Scale to create a more accurate correlation between wind speed and the damage observed.
The scale uses the severity of destruction to estimate wind speed, rather than relying on an actual measurement of the wind inside the tornado. The wind speeds associated with each category are estimates based on engineering studies and expert judgment. This system was designed to standardize assessments and account for differences in construction quality.
A tornado classified as EF0 has estimated three-second wind gusts between 65 and 85 miles per hour and typically causes light damage, such as peeling surface material off roofs or breaking branches off trees. In contrast, an EF5 tornado, the highest rating, involves estimated wind speeds exceeding 200 miles per hour. An EF5 tornado causes incredible damage, often leveling strong frame houses and sweeping them clean off their foundations.
EF0 and EF1 tornadoes, though considered “weak,” account for roughly 80% of all tornadoes in the United States. The occurrence rate drops quickly as intensity increases; violent tornadoes, rated EF4 or EF5, make up less than one percent of all confirmed reports. The EF Scale also includes an “EF-Unknown” category for tornadoes that cannot be rated due to a lack of damage evidence.
Applying the EF Rating
Assigning the official EF rating occurs only after the event has concluded. The National Weather Service provides these official ratings, aiming to assign a rating based on the highest estimated wind speed that occurred anywhere along the damage path.
Trained NWS personnel conduct post-storm damage surveys to assess the destruction. Surveyors use a system involving 28 different types of structures and vegetation, known as Damage Indicators (DI). These indicators include residential buildings, mobile homes, large trees, and transmission line towers.
For each Damage Indicator, there are multiple levels of destruction, referred to as Degrees of Damage (DOD). These levels range from the first visible signs of damage to the complete destruction of the indicator. The surveyor matches the observed damage to the appropriate DI and DOD, which corresponds to a range of estimated wind speeds.
The final rating is an estimate derived from a judgment of the damage, considering factors like the quality of construction. The surveyor determines if the wind speed was higher or lower than the expected value for that specific level of damage. This assessment is performed on multiple structures along the path before the final EF rating is determined.
Official Identification of Tornado Events
Tornadoes are identified and tracked using a systematic reference method rather than being given personal names like hurricanes or tropical storms. The formal identification of a tornado event includes several pieces of information for record-keeping and historical data. This tracking method is utilized by NWS and NOAA databases to maintain a comprehensive history of severe weather.
An event is typically identified by the date, location, and its sequence number within that area or period. For example, an event might be logged as the “May 10th 2023 Tornado, Oklahoma County, Event #1.” This approach ensures that every recorded event has a unique, non-subjective identifier.
The tornado itself is not given a name, although the rotating parent storm may have meteorological identifiers. Official records focus on documenting the time, location, and intensity rating assigned after the damage survey. This standardized documentation is foundational for long-term climatology studies and improving warning systems.