Tornado ratings and storm surveys, explained | 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