Q Is this an unusual year?

A Over the past couple of decades, an average of about 1,200 tornadoes touched down each year in the continental United States. This year, about 1,076 had been counted even before this week.

Q How bad is this year's toll?

A The death toll, now at more than 480, is the highest since 1953, when an outbreak of twisters across the Midwest and the Northeast killed 519. This year's toll is all the more remarkable considering that early warning systems are in place throughout tornado country.

Q What caused the tornadoes?

A Warm, moist air from the Gulf of Mexico moved northward until it encountered cold air brought south by the jet stream.

Q Is climate change to blame?

A Probably not. The number of violent tornadoes has been declining in the U.S., even as temperatures have increased, making it likely that this year's outbreak is simply a remarkable natural event.