While some may guess lightning, tornadoes or flooding, experts have found that it's actually heat.
As we enter another day of this early summer heat wave, it is something to keep in mind.
In a study done by the National Weather Service, experts looked at the 30-year average of weather fatalities. They found that heat was the highest killer, followed by flooding.
In 2018, 108 American lives were cut short but hot weather -- 98 more than casualties caused by tornadoes, which are the third highest weather killer in the U.S.
Why isn't heat the first thing to pop into our minds when it comes to killer weather?
"Maybe it's because we all have experienced hot days, but many have never seen a tornado," Big Weather said. "Or, perhaps, it's because they don't make movies about heat waves, but some people might've seen 'Twister' or 'The Wizard of Oz.' But the fact is heat kills."
So who is most at risk for a heat-related death or illness?
Children up to 4 years old and people over the age of 65.
There are also a few other factors that can contribute like existing medical conditions such as diabetes or heart disease, obesity and socially isolated people.
However, even young and healthy folks can put themselves at a higher risk by taking part in strenuous activities, especially during the warmest time of day.
So what should you do? Here are a few tips on these hot days: