EXPLAINER: Was tornado outbreak in the US related to climate change?

Washington, Dec 12 (AP) calendar said December but warm moist air screamed of springtime. d an eastbound storm front guided by a La Nina wear pattern into that mismatch and it spawned tornoes that killed dozens over five US states.

Tornoes in December are unusual, but not unheard of. But ferocity and path length of Friday night's tornoes likely put m in a category of ir own, meteorologists say. One of twisters if it is confirmed to have been just one likely broke a nearly 100-year-old record for how long a torno stayed on ground in a path of destruction, experts said.

One word: remarkable; unbelievable would be anor, said Norrn Illinois University meteorology professor Victor Gensini. It was really a late spring of setup in middle of December. Warm wear was a crucial ingredient in this torno outbreak, but wher climate change is a factor is not quite as clear, meteorologists say.

Scientists say figuring out how climate change is affecting frequency of tornoes is complicated and ir understanding is still evolving. But y do say atmospheric conditions that give rise to such outbreaks are intensifying in winter as planet warms. And torno alley is shifting farr east away from Kansas-Oklahoma area and into states where Friday's killers hit.

Here's a look at what's known about Friday's torno outbreak and role of climate change in such wear events.

What causes a torno?

Tornoes are whirling, vertical air columns that form from thunderstorms and stretch to ground. y travel with ferocious speed and lay waste to everything in ir path.

Thunderstorms occur when denser, drier cold air is pushed over warmer, humid air, conditions scientists call atmospheric instability. As that happens, an updraft is created when warm air rises. When winds vary in speed or direction at different altitudes a condition known as wind shear updraft will start to spin.

se changes in winds produce spin necessary for a torno. For especially strong tornoes, changes are needed in both wind's speed and direction.

When considerable variation in wind is found over lowest few thousand feet of atmosphere, torno-producing supercell thunderstorms' are possible, said Paul Markowski, professor of meteorology at Pennsylvania State University. That's what we h yesterday.

re's usually a lot of wind shear in winter because of big difference in temperature and air pressure between equator and Arctic, Gensini said.

But usually, re's not a lot of instability in winter that's needed for tornoes because air isn't as warm and humid, Gensini said. This time re was.

What conditions led to storms of this scale?

A few factors, which meteorologists will continue to study.

Spring-like temperatures across much of Midwest and South in December helped bring warm, moist air that helped form thunderstorms. Some of this is due to La Nina, which generally brings warmer than normal winter temperatures to Sourn U.S. But scientists also expect atypical, warm wear in winter to become more common as planet warms.

worst-case scenario happened. Warm air in cold season, middle of night, said John Gordon, a National Wear Service meteorologist in Louisville, Kentucky.

Once storm formed, exceptionally strong wind shear appears to have prevented tornoes from dissipating, experts say. Tornoes are thought to die off when thunderstorm updrafts lose energy.

Tornoes typically lose energy in a matter of minutes, but in this case, it was hours, Gensini said. That's partly reason for exceptionally long path of Friday's storm, going more than 200 miles (322 km) or so, he said.

record was 219 miles (352 km) and was set by a torno that struck four states in 1925. Gensini thinks this one will surpass it once meteorologists finish analysing it.

In order to get a really long path length, you have to have a really fast-moving storm. This storm was moving well over 50 miles (80 km) per hour for a majority of its life, Gensini said. That's not speed of winds, but of overall storm movement.

You're talking about highway-speed storm motions, Gensini said.

How related is climate change to torno outbreaks?

It's complicated. Scientists are still trying to sort out many conflicting factors about wher human-caused climate change is making tornoes more common or even more intense. About 1,200 twisters hit U.S. each year though that figure can vary according to NOAA National Severe Storms Laboratory. No or country sees as many.

Attributing a specific storm like Friday's to effects of climate change remains very challenging. Less than 10% of severe thunderstorms produce tornoes, which makes drawing conclusions about climate change and processes leing up to m tricky, said Harold Brooks, a torno scientist at National Severe Storms Laboratory.

Scientists have observed changes taking place to basic ingredients of a thunderstorm, however, as planet warms. Gensini says in aggregate, extreme storms are becoming more common because we have a lot warmer air masses in cool season that can support se s of severe wear outbreaks.

US is likely to see more tornoes occur in winter, Brooks said, as national temperatures rise above long-term average. Fewer events will take place in summer, he said.

Furto of University of Oklahoma said torno alley, a term used to describe where many twisters hit US, has shifted eastward into Mississippi River Valley. That shift is because of increases in temperature, moisture and shear.

Bottom line: people in Mississippi River Valley and Ohio River Valley are becoming increasingly vulnerable to more tornic activity with time, he said. (AP)