Around the world at any given moment, approximately 1,800 thunderstorms are occurring. That's equal to 16 million per year. On average, there are 100,000 thunderstorms in the United States annually. Approximately 10% of those are rated severe. The most frequent storm activity happens during seasonal changes; from fall to winter and from winter to spring.
All thunderstorms pose hazards. Thunderstorms can produce:
Tornadoes, which cause an average of 70 fatalities and 1,500 injuries per year
Lightning, which all storms produce, causes on average 80 fatalities and 300 injuries annually
Flash floods, which are the leading cause of weather-related fatalities
Hail, which damages more than $1 billion in property and crops annually
Severe Thunderstorm Characteristics
Severe thunderstorms are especially likely to have characteristics that present a danger to people and the threat of property damage. A severe storm is defined as a storm that produces:
Hail greater than 1 inch (quarter-sized) hail
Winds of 58 mph or greater
For thunderstorms to develop, they need moisture to form clouds and rain, unstable air with relatively warm air that can rise rapidly, and lift from fronts, sea breezes, or mountains.