In the United States, more than 90 people died across six midwestern and southern states when strong winds known as tornadoes destroyed homes, work buildings and religious centers on Friday night.

Such weather events do happen in December, but they do not happen often this time of year because the temperature on the ground is not so different from air high in the sky.

However, people in Kentucky said the air was warm and wet on Friday.

Weather scientists said the mixing of the warm and cold air can create "instability" which produces storms and the fast, spinning winds of tornadoes.

Climate change

The question is whether the deadly storm came because of climate change. The weather scientists, known as meteorologists, are not completely sure.

Victor Gensini is a meteorologist at Northern Illinois University. He said strong winds are common in the winter, but they do not create tornadoes because the air close to the Earth is also cold. He called the tornado that traveled hundreds of miles on Friday "remarkable."

The weather scientists say a warming planet has something to do with strong winter storms.

The National Weather Service (NWS) noted that since 1950 there had not been a November tornado recorded in the northeastern U.S. state of Connecticut. There was one last month, however.

John Gordon of the NWS in Louisville, Kentucky called what happened on Friday "the worst-case scenario."

Gensini said the tornado traveled about 300 kilometers. Normally, tornadoes die out in minutes, he said, but this one lasted for hours. He said the storm moved as a fast as a car going down a large roadway.

Harold Brooks studies tornadoes at the National Severe Storms Laboratory. While the U.S. has more tornadoes than any other country, tornadoes only form once out of every 10 strong storms. As a result, he said, it is hard to say if the winter tornadoes are closely related to climate change.

Gensini, however, said the conditions for strong storms are becoming more common in the winter as the planet warms up.

Patterns are changing

In the past, Brooks said, tornadoes happened more often during warm days in the summer. However, as the winter becomes warmer, especially in the southern U.S., tornado season will shift. More will happen in the winter and fewer will happen in the summer.

Jason Furtado is a meteorologist at the University of Oklahoma. He said people who live in places like Kentucky, which suffered the most from Friday's storms, are now living in an area where tornadoes are common. That is a shift south and east from the past.

He said the people in that area of the U.S. are "becoming increasingly vulnerable."

I'm Jill Robbins.

Dan Friedell adapted this story for Learning English based on reporting by the Associated Press. Susan Shand was the editor.