Why has the weather gone cuckoo?

After historic flooding along the Mississippi River, record tornado sightings, and even a twister in Northeast Philadelphia last week, it is reasonable to ask if the atmosphere has gone out of its mind.

Has worldwide warming pushed it to the brink of riot? Or is its behavior more like that of a beloved relative who has been eccentric all along but we've started noticing?

To the chagrin of some scientists, variants of these questions are being disputed heatedly, polemically - and sometimes even entertainingly - in traditional media and on the Web.

As Richard B. Rood, an atmospheric scientist at the University of Michigan, has noted, the debates are driven by "many agendas," but way down on that list is the "rational pursuit" of answers.

Less visibly on the public radar, however, serious researchers have undertaken ambitious projects to come up with ways to quantify how subtle warming, and human contributions to it, might be affecting day-to-day weather.

"There's been a lot of thought put into that," Rood said last week. And given the events of the last year or so, those projects have taken on fresh urgency. Said Rood: "At this point, there's a lot of demand to quantify it."

Indisputably, this has been a period for wild weather. In the United States, last month's working tornado total, 875, was more than triple the old April record, 267, set in 1974. The Mississippi water levels have exceeded those of the historic 1927 floods.

What might warming have to do with all that?

Various studies have found that water vapor in the atmosphere is on the increase, and that extreme precipitation events have become more common. The World Meteorological Organization reported that the first decade of the 21st century was the wettest in the period of record, dating to 1850.

The Mississippi was engorged by snowmelt and heavy April rains near its junction with the Ohio River. The twisters were fueled by vapor-rich water off an abnormally warm Gulf of Mexico.

Case sealed? Not so fast.

Measurement, it turns out, is a complicated and messy business.

The National Climatic Data Center has been trying its hand at it, publishing a monthly U.S. Climate Extremes Index, which chronicles temperature, precipitation, and drought anomalies. It does show a general increase in extreme weather in the United States in the last 20 years, but the trends are not linear.