A quirky aspect of the way we measure twisters helps explain why there hasn't been a top-tier-rated tornado in 12 years.

Researches found little evidence that tornadoes were getting weaker. Instead, it appears that a subjective tornado grading system is to blame.

The Enhanced Fujita Scale is used to determine the severity of tornadoes: from an EF-0, the weakest, to an EF-5, the strongest. Wind speeds and damage are the main components to rating a storm.

A recent study explores the puzzling decline in EF5 tornadoes, contributing it to more strict criteria in the Enhanced Fujita scale introduced in 2007.