International Fujita scale

The International Fujita scale (abbreviated as IF-Scale) rates the intensity of tornadoes and other wind events based on the severity of the damage they cause. It is used by the European Severe Storms Laboratory (ESSL) and various other organizations including Deutscher Wetterdienst (DWD) and State Meteorological Agency (AEMET). The scale is intended to be analogous to the Fujita and Enhanced Fujita scales, while being more applicable internationally by accounting for factors such as differences in building codes.

In 2018, the first draft version of the IF-scale, version 0.10 was published. This version was based on a 12-step rating scale. Over the next few years, dozens of tornadoes would be rated on this version of the scale. Most notably, the 2021 South Moravia tornado received a rating (IF4) and full damage survey on the IF-scale conducted by ESSL, the Czech Hydrometeorological Institute and four other organizations. On May 6, 2023, version 0.99.9d was published, which changed it to a 9-step rating scale. In late July 2023, the first official version of the IF scale was published.

Parameters
The 12 categories for the International Fujita scale are listed below, in order of increasing intensity. Although the wind speeds and photographic damage examples are updated, which are more or less still accurate. However, for the actual IF-scale in practice, damage indicators (the type of structure which has been damaged) are predominantly used in determining the tornado intensity. The IF-scale steps are defined by a central value and an error. The errors have been estimated to be 30% of the central value, resulting in overlapping speed ranges. The distances between the central values of the steps have been so chosen that the upper bound exceeds the central value of the next step, ensuring a balance between the resolution of the scale and the expected errors. Since ESSL required that the steps be consistent with the original Fujita scale, they introduced steps with – and + suffixes indicating steps one third higher or lower than the central value of the original scale, e.g. F1- equals "F2 - 1⁄3F2" and F2+ equals "F2 + 1⁄3F2". Above F2, such a subdivision was not introduced and only full steps are used.

2023 version
On May 6, 2023, version 0.99.9d was published, which changed it to a 9-step rating scale. In this version, the wind speed damage indicator was introduced, which made it the first tornado intensity and damage scale to use measured wind speeds and Doppler weather radar measured wind speeds. When the first official publication of the IF scale, the 9-step rating scale was kept. It was noted that each scale's wind speed is to be taken with a 20% error margin on each side of the central value. This was done to ensure the lower or upper bound of the overlapping rating came close to the central value of the other rating.

Damage indicators, subclasses, and degrees of damage
The IF scale currently has 23 damage indicators (DI), each with a varying number of subclasses and degrees of damage (DoD).

DI: Wind Speed Measurement
A unique feature of the International Fujita scale compared to the Fujita or Enhanced Fujita scale is a new damage indicator based on measured wind speeds. For the IF scale, only wind speeds measured at or below 10 m can be used to determine a rating. Doppler weather radar measurements are also able to be used to determine a rating if they are measured within damaging distance. For radar measurements, any readings below 60 m can be used to determine a rating.

Three second measurement
For three-second wind speed measurements, it is assumed to be an average of 88.8% of the three-second measurement.

Two second measurement
For two-second wind speed measurements, it is assumed to be an average of 90.9% of the two-second measurement.

One second measurement
For one-second wind speed measurements, it is assumed to be an average of 92.5% of the one-second measurement.

Zero second measurement
For zero-second wind speed measurements, it is assumed to be an instantaneous wind speed measurement. This can only be used if it was 10Hz or higher sample rate.