Chris Vagasky, University of Wisconsin-Madison
As the weather warms, people spend more time outdoors, going to barbecues, beaches and ballgames. But summer isn’t just the season of baseball and outdoor festivals – it’s also lightning season.
Each year in the United States, lightning strikes around 37 million times. It kills 21 people a year in the U.S. on average.
For as often as lightning occurs – there are only a few days each year nationwide without lightning – there are still a lot of misunderstandings about nature’s largest spark. Because of this, a lot of people take unnecessary risks when thunderstorms are nearby.
I am a meteorologist who studies lightning and lightning safety, and a member of the National Lightning Safety Council. Here are some fast facts to keep your family and friends safe this summer.
What is lightning, and where does it come from?
Lightning is a giant electric spark in the atmosphere and is classified based on whether it hits the ground or not.
In-cloud lightning is any lightning that doesn’t hit ground, while cloud-to-ground – or, less commonly, ground-to-cloud – is any lightning that hits an object on the ground. Cloud-to-ground lightning accounts for only 10% to 50% of the lightning in a thunderstorm, but it can cause damage, including fires, injuries and fatalities, so it is important to know where it is striking.
Lightning occurs when rain, ice crystals and a type of hail called graupel collide in a thunderstorm cloud.
When these precipitation particles collide, they exchange electrons, which creates an electric charge in the cloud. Because most of the electric charge exists in the clouds, most lightning happens in the clouds. When the electric charge in the cloud is strong, it can cause an opposite charge to build up on the ground, making cloud-to-ground lightning possible. Exactly what initiates a strike is still an open question.
When and where does lightning happen?
Lightning can happen any time the conditions for thunderstorms – moisture, atmospheric instability, and a way for air to rise – are present.
There is a seasonality to lightning: Most lightning in the United States strikes in June, July or August. In just those three months, more than 60% of the year’s lightning typically occurs. Lightning is least common in winter, but it can still happen. About 2% of yearly lightning occurs during winter.