The deadliest US tornado season in 75 years has ripped babies from their mother's arms and transformed entire towns into Apocalyptic scenes of destruction as the toll hit 523 people Thursday.

And it isn't over yet.

While warmer summer weather should hopefully reduce their intensity, the peak tornado season runs through July and twisters can strike at any time.

The damage is as unimaginable as it is unpredictable.

Funnel clouds drop out of a darkened sky, tossing cars and mobile homes up into the air, pulling huge trees out of the ground and tearing buildings apart.

The smaller ones touch down so briefly that one side of a street is flattened while the other is largely unscathed.

The bigger ones stay on the ground for miles, destroying everything for blocks on either side of their random path.

Two bad days accounted for nearly all the deaths: an outbreak of dozens of tornados that killed 314 people in five states on April 27 and a massive twister that killed 138 in Joplin, Missouri on May 22.

It was the deadliest day and the deadliest single tornado strike since modern record keeping began in 1950. 2011 now ranks as the fifth deadliest year in US tornado history.

"We're still trying to wrap our heads around this one," said Greg Carbin, a meteorologist with the National Weather Service's storm center.

It's not clear whether climate change is playing a role, Carbin said.

Tornadoes are formed when two weather fronts of different temperatures create wind sheer.

The warmer temperatures caused by global warming should reduce wind sheer, but they have also led to more precipitation and could be breeding the thunderstorms that spawn twisters.

It is clear, however, that steady growth in the number of people living in "tornado alley" - the huge area between the Rocky and Appalachian Mountains - has led to higher tolls, particularly since so many are living in mobile homes and houses lacking storm cellars.

"We've spread out on the landscape," Carbin told AFP. "It's easier for a tornado to hit something in this day and age."

While warning systems have improved dramatically in recent years, sirens can be little help against a twister powerful enough to knock a nine story hospital off its foundation and reduce brick buildings to rubble.

That's what happened in Joplin, where a nearly mile-wide twister packing winds of more than 200 miles per hour cut a six-mile (nearly 10 kilometer) swath of destruction through the town of 50,000 people.

"I don't know if man could build something strong enough to handle what came through," Missouri Governor Jay Nixon said after surveying the damage.

President Barack Obama called it a "national tragedy" and recalled stories of heroism at a memorial service Sunday.

One such hero was pizza shop manager Christopher Lucas, a father of two, who ushered more than a dozen people into a walk-in freezer as the tornado approached.

The freezer door wouldn't close from the inside, so Lucas found rope and closed it from the outside.

"Christopher held it as long as he could. Until he was pulled away by the incredible force of the storm," Obama said.

"He died saving more than a dozen people in that freezer."

The damage was so extensive that it took officials 10 days to identify all the bodies and reunite hundreds of people separated from their loved ones.

For many, days of frantic searching ended in despair.

Like the family of 16-month Skyular Logsdon, who was pulled from his mother's arms after she was knocked unconscious when the twister ripped apart their home.

Another series of deadly twisters struck just two days after the Joplin tornado, killing 16 people in Kansas, Oklahoma and Arkansas.

A pregnant Catherine Hamil cowered in a bathtub with her three young children in Piedmont, Oklahoma that night.

When the storm passed, her 15-month-old son was dead, Hamil and her five-year-old daughter were in serious condition and her three-year-old son was gone.

It took two days to find his body.

Officials predict it will cost billions to repair the physical damage caused by the deadly twisters and months for life to return to normal.