What makes a thunderstorm severe?

We’re in the middle of severe weather awareness week for Alabama, so this week’s weather question is: “What makes a thunderstorm severe?”
The National Weather Service uses wind speed and hail size to qualify a thunderstorm as severe or not.
For winds – they have to have gusts over 57 mph (or 50 knots).
For hail – the diameter must be an inch or greater.
The size of hail is estimated by comparing it to known objects.
Here are a most of the objects the NWS compares hail size to:
balls
Pea, mothball and penny, dime or nickels are used to estimate hail size under 1 inch (below severe thunderstorm status).
Once the hail’s diameter reaches the size of a quarter then it is considered severe.
Some objects used to describe larger hail are:
Ping-pong, golf ball, tennis ball, baseball and even softballs.
marbles“Marble size” hail should not be used since marbles can be a range of different sizes, and unlike golf balls or pennies they do not have standard sizes.

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