The United States averages about 1,200 tornadoes each year, and each one receives a unique rating on the Enhanced Fujita Scale.
This scale, commonly known as the EF scale, categorizes tornadoes into intensity categories from EF0 to EF5. Where a twister falls on the scale is determined by the maximum wind speed. Meteorologists base their estimates on the type of damage a twister leaves behind.
This post-assessment system is necessary because tornadoes are small and short-lived compared to other types of weather such as hurricanes, making them very difficult to accurately measure with common weather radars and observation networks.
Without the EF scale, capturing all tornadoes would require specialized equipment, such as mobile weather radars, to be placed at the right place and time, which would simply be impossible.
A damage investigation team from the National Weather Service will participate. These meteorologists are dispatched to affected areas and document what they find.
They determine whether a tornado has occurred, its exact trajectory, the strength of the tornado, and several other factors. These studies are critical and provide the basis for tracking tornado behavior and trends in the United States over time.
Here we will explain step by step how they work.
NWS survey teams will work with local emergency managers, police, and fire personnel to understand where damage occurred to create an initial snapshot of the tornado and its trajectory. If this type of report is not available, you can still look back at the radar data.
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Typically, we begin with the most severely affected areas before investigating the rest of the route.
Time is of the essence in these investigations. The team must document everything before the community attempts to begin cleaning and resume normal life. Damaged buildings and broken trees all serve as forensic evidence if a tornado is a suspect.
Once the team has identified the worst injuries, they begin selecting injury indicators outlined in the EF scale. The damage index is basically the type of building or object that the tornado hit (for example, a three-story house or a utility pole).
You can choose from 28 different damage indicators. Workers can analyze damage to each indicator in a standardized way based on the structure of each indicator and the material of the object.
The types of damage indicators chosen for the EF scale better take modern structural engineering into account and allow for less subjectivity than the original Fujita scale, which it replaced in 2007. Damage to the vehicle is not taken into account.
For example, the survey team found that a small barn, a nearby single-family home, and several hardwood trees sustained tornado damage. These are indicators 1, 2, and 27 of the list that the team will refer to.
Identifying the damage indicators activates the next step in the investigation flowchart: Damage Extent. Some indicators have only three degrees for the crew to choose from, while others have up to 12 degrees.
The stronger the damage, the higher the level. The extent of damage is associated with three wind speed estimates: expected wind speed, lower estimate, and upper estimate. Determining the exact extent of damage is the only way crews can accurately determine the tornado’s winds and, therefore, its strength.
Most likely, the small barn that our research team found was completely destroyed by the tornado. This guarantees the maximum damage rating (8 out of 8) in the chart below.
In this case, assuming the barn was built for encryption, the expected wind speed needed to do that type of destruction would be 112 miles per hour. If the barn is poorly built, winds of 94 miles per hour can cause the same damage, but a well-built barn will require winds of 131 miles per hour.
One damage metric alone is not enough to tell the whole story. The crew repeats steps 2 and 3 until an estimate of the tornado’s average maximum winds is determined.
Suppose the barn in question was very well built. This means the tornado’s wind speeds could have reached up to 131 miles per hour.
The crew also finds a hardwood tree (damage index 27) snapped in half near the barn. If the trunk breaks, the damage is rated 4 out of 5. This means that tornado wind speeds can range from 93 mph to 134 mph. Let’s say the trees are quite thick and are not susceptible to damage. That means the wind was likely near the upper end of the range.
Given all this damage, wind speed ranges start to hover around 130 mph.
Once the team agrees on a maximum wind estimate, we look to see where it falls on the EF scale. An example of a tornado is EF2.
However, sometimes tornadoes are found in remote, open areas and leave no damage behind. The twister is documented but rated as EF Unknown or EFU as the crew has no damage indicators.
Although tornadoes are rated by their maximum damage, they may not have caused that level of damage along their entire path. It is still important for teams to survey the entire track and get the big picture.
They need to determine exactly where the twister begins and ends. This determines the length of its path and also tells us how wide the twister is. The study also recorded the number of people injured or killed by the tornado.
NWS typically releases preliminary information about tornadoes through Freedom of Information statements the day after they occur, but complex investigations of long-tracked tornadoes or highly destructive tornadoes may take longer to complete.
