The definition of a severe thunderstorm is a storm that produces hail of 3/4 of an inch diameter or greater, or wind gusts of 58 mph or greater. Ever wonder how these criteria were chosen? Engineering studies show that hail less than one inch in diameter rarely causes roof damage, and we can get 58 mph winds on a sunny day in Oklahoma. A severe thunderstorm with the bare minimum criteria will not cause much more damage than a few downed tree limbs.
To find out why these levels were chosen for a severe thunderstorm, we must look to the aviation industry. Back in the 1950s and 1960s when severe thunderstorm watches were first issued, there were actually two different types of watches. The Aviation Severe Thunderstorm Watch had a wind criteria of 50 mph, while the Public Severe Thunderstorm Watch had a wind criteria of 75 mph. To reduce confusion, these two were combined in 1970, and the wind criteria was compromised to 58 mph (50 knots).
The hail criteria is also linked to aviation. A study in 1952 concluded that the smallest hail that causes significant damage to an aircraft at plane speeds between 200-300 mph is 3/4 of an inch diameter hail. The 3/4 inch criteria has stuck ever since.
There has been an attempt recently to change the hail size criteria to one inch, but 3/4 inch remains.