What makes a thunderstorm severe?

Often people will report a thunderstorm as being severe if it is loud, has a lot of lightning or heavy rain. However, what makes a thunderstorm severe is none of the above. A severe thunderstorm is a thunderstorm that has to meet one of three criteria:
1) Hail that is 1 inch, roughly the size of a quarter, or larger.
2) 58 mph or greater wind speed.
3) A tornado
Hail is a dangerous product that is created in a thunderstorm. Hail is formed by small ice particles and super-cooled water droplets that combine in the updraft portion of the thunderstorm. The hail can fall down in the down draft or get recycled through the updraft. This recycling process allows the hailstone to grow and the stronger the updraft of the thunderstorm the larger the hailstone can become. The hail will then fall out of the storm when it gets to large for the updraft to support it. The safest place to be during a thunderstorm is inside of a building.

Winds in a severe thunderstorm can come in to forms: a tornado or straight line winds Straight line winds are formed when cool air in the thunderstorm falls out of the bottom of the storm and then hits the ground going in all directions. Tornadoes are when winds spiral in to an updraft of a storm. A myth is that a tornado is needed for a storm to be dangerous, but here in Central Pennsylvania, more damage, injuries and fatalities often come from straight line winds. The safest place to be protected from wind is in a basement, or the center of a small room in the middle of the lowest level of a house. 

And while neither are a criteria for a severe thunderstorm, lightning and flooding often kill more people each year than hurricanes or tornadoes. Remember to head indoors to a house or a car when you hear thunder and to stay away from flooded areas, especially on roads when in a car. 


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