When severe weather strikes, the public often hears about an “EF0 tornado.” EF0 refers to the lowest intensity rating on the Enhanced Fujita Scale (EF Scale), the standard system used in the United States to classify tornado strength. The term “EFO” stands for Enhanced Fujita Observation, representing the systematic methodology used by meteorologists and engineers to assess the damage left by a storm. The resulting EF classification is derived from this scientific observation process, which provides a standardized, post-event analysis of wind speed based on the destruction witnessed on the ground.
From F-Scale to Enhanced Fujita
The Enhanced Fujita Scale replaced the original Fujita Scale (F-Scale), developed in 1971 by Dr. T. Theodore Fujita. The original F-Scale relied on damage descriptions that were often subjective and lacked a strong correlation between damage and estimated wind speeds. Wind speed estimates on the old scale were frequently inflated, leading to potential inaccuracies in historical records.
Recognizing these limitations, the meteorological community implemented the Enhanced Fujita Scale in the United States on February 1, 2007. The primary goal of this revision was to improve the accuracy of wind speed estimations by incorporating engineering principles and utilizing a more rigorous set of damage indicators. This shift allowed for a more consistent assessment across different types of structures and terrain.
Defining the EF Rating Tiers
The Enhanced Fujita Scale consists of six categories, ranging from EF0 through EF5. The EF0 rating, the most common, corresponds to estimated three-second wind gusts between 65 and 85 miles per hour. Damage at this level is minor, involving the removal of some roof shingles, damage to gutters or siding, and snapping of small tree branches.
An EF1 tornado features estimated winds between 86 and 110 miles per hour, capable of causing moderate damage, such as stripping roofs or overturning mobile homes. An EF2 rating signifies considerable damage with winds from 111 to 135 miles per hour, often tearing roofs off well-constructed houses and uprooting large trees. EF3 storms have estimated wind speeds ranging from 136 to 165 miles per hour, leading to severe damage where entire stories of well-built homes may be destroyed. When wind speeds reach 166 to 200 miles per hour, the resulting EF4 damage is devastating, capable of leveling well-constructed houses and throwing cars considerable distances. The highest classification, EF5, is reserved for damage caused by winds exceeding 200 miles per hour. At this intensity, strong frame houses are often swept clean off their foundations, and significant structural deformation occurs in large buildings.
The Survey and Observation Process
The “O” in EFO—Observation—is the technical process by which the National Weather Service (NWS) and structural engineers determine the final EF rating. This systematic approach relies on identifying specific Damage Indicators (DIs) found in the tornado’s path. DIs are standardized categories of structures or objects, such as a one- or two-family residence, a mobile home, a small barn, or different types of trees.
The EF scale incorporates 28 different DIs, ensuring the assessment is standardized regardless of the type of object damaged. For each observed Damage Indicator, surveyors assign a Degree of Damage (DOD) based on the observed destruction. The DODs are incremental severity levels, typically ranging from the beginning of visible damage to total destruction, tailored to the vulnerability of that particular structure type.
Each Degree of Damage corresponds to a specific, statistically derived range of wind speeds, which helps to mitigate the subjectivity of the old F-Scale. The surveyor uses professional judgment and the observed DOD to select the best-fit wind speed within that range, called the Estimated Peak Wind Speed. The wind speeds estimated are based on three-second gusts at the point of damage, not sustained wind measurements.
The final EF rating assigned to the tornado is determined by the highest degree of damage found anywhere along the storm’s track. For example, if a tornado causes scattered EF0 damage for miles but one isolated, well-built structure shows clear EF3 damage, the entire event is officially rated as an EF3. The NWS is the only federal agency with the authority to provide these official EF Scale ratings.
How EFO Data Is Used
The data collected through the Enhanced Fujita Observation process serves multiple functions beyond immediate public reporting. This standardized data is compiled into long-term climate records, allowing meteorologists to study the frequency, intensity, and geographic distribution of severe weather events. This information is fundamental for understanding tornado climatology and improving long-range forecasting models.
The data is also applied to public safety and infrastructure planning. Accurate EF ratings influence the development and revision of modern building codes, ensuring that new construction can withstand certain wind loads. The official rating derived from the EFO process is utilized by insurance companies to process claims and by government agencies to allocate disaster relief funds.
By providing a consistent, engineering-based measurement of intensity, the EFO process ensures that historical data is accurate and comparable. This standardized approach is integral for mitigating future risks and informing preparedness efforts.