Here’s how we find out how strong tornadoes were
The degree of damage table for a small barn shows the wind speeds needed to do certain damage. DOD stands for Degree of Damage. EXP stands for Expected. LB stands for Lower Bound — the lowest wind speed that could inflict the damage. UP stands for Upper Bound — the highest wind speed that could inflict the damage.
(CNN) — Around 1,200 tornadoes occur on average in the United States each year and every one of them gets its own rating on the Enhanced Fujita Scale.
More commonly known as the EF Scale, it organizes tornadoes into strength categories ranging from EF0 to EF5. Where a twister stacks up on the scale depends on its maximum wind speed, which meteorologists estimate based on the type of damage it left behind.
This after-the-fact rating system is needed because tornadoes are small and short-lived compared to other types of weather — like hurricanes — making them very difficult to accurately measure with typical weather radar and observation networks.
Without the EF Scale, specialized equipment like mobile weather radars would need to be set up at the right place and time to catch every tornado — and that’s simply not possible.
Enter damage survey teams from the National Weather Service. These meteorologists deploy to impacted areas and document what they find.
They determine if a tornado occurred, its exact track, how strong it became and several other factors. These surveys are vital and are the basis for tracking tornado behavior and trends over time in the US.
Here’s a step-by-step guide to how they work.
Step 1: Arriving on scene
NWS survey teams work with local emergency managers, police and fire crews to get a sense of where damage occurred in order to build an initial snapshot of the tornado and its track. They can look back at radar data when these kinds of reports aren’t available.
Typically, they will start the survey in the area with the worst damage before investigating the rest of the path.
Time is critical in these surveys: Teams need to document everything before communities start to clean up and try to resume normal life. Every damaged building or broken tree is essentially forensic evidence for a case where the tornado is the suspect.
Step 2: Identifying damage indicators
Once teams locate the worst damage, they start to select damage indicators outlined in the EF Scale. A damage indicator is basically the type of building or object the tornado hit— for example, a three-story house or power pole.
There are 28 distinct damage indicators to choose from. Crews are able to analyze the damage to each indicator in a standardized way based on their construction or the materials the object was made from.
The types of damage indicators chosen for the EF Scale better account for modern structural engineering and allow less room for subjectivity than the original Fujita Scale it replaced in 2007. Vehicle damage is not factored in.
For example, the survey team finds that a small barn, a nearby single-family home and several hardwood trees have tornado damage. These are indicators 1, 2 and 27 on the list the teams consult.
Step 3: Deciding degree of damage
Identifying a damage indicator activates the next step in the survey flow chart: Degree of damage. Some indicators have as few as three possible degrees for crews to choose from, while others have up to 12.
The more intense the damage, the higher the degree. Each degree of damage is tied to three wind speed estimates: an expected speed, a lower estimate and a higher estimate. Determining the correct degree of damage is the only way for crews to pinpoint a tornado’s winds and therefore its strength.
Perhaps the small barn our example survey team found was completely destroyed by the tornado. That would warrant a maximum degree of damage — an 8 out of 8 — in the chart below.
In this case, the expected wind speed needed to do that kind of destruction would be 112 mph, assuming the barn was built to code. If the barn was built poorly, 94 mph winds could deal the same damage, while a well-built barn would need 131 mph winds.
Step 4: Rating the tornado
One damage indicator isn’t enough to tell the full story. Crews repeat steps two and three until an average maximum wind estimate for the tornado becomes apparent.
Say the example barn was built very well: That means the tornado’s winds could have reached up to 131 mph.
And suppose the crews also found some hardwood trees — damage indicator 27 — snapped in half near the barn. A snapped trunk warrants a degree of damage of 4 out of 5, meaning the tornado’s winds could be anywhere from 93 to 134 mph. Let’s say the trees were quite thick, making them harder to damage — that means the winds were likely somewhere closer to the top end of that range.
Given all this damage, a wind range around the 130 mph mark starts to emerge.
Once the team agrees on a maximum wind estimate, they see where it falls on the EF Scale. The example tornado would be an EF2.
But sometimes a tornado is spotted in a remote, open area and leaves no damage behind. Since crews have no damage indicators, the twister is documented but rated an EF Unknown or EFU.
Step 5: Finishing the survey
Tornadoes are rated by their maximum damage, but it might not have caused that level of damage along its whole path. It’s still critical for teams to survey the entire track to get the full picture.
They must determine exactly where the twister began and ended — which will give them its path length — and also how wide it became. Surveys also record the number of people injured or killed by a tornado.
The NWS typically releases preliminary information on a tornado the day after it occurs via a Public Information Statement, though complex surveys of long-tracked or very destructive tornadoes can take more time to complete.
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