Extreme Precipitation
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What is an extreme precipitation event and why are they occurring more frequently? Hi, I'm Bryan Yeaton and this is the Weather Notebook.

An extreme precipitation event is not a hyper-challenging water sport, but rather an amazingly wet storm, and for some reason they seem to be on the upswing.

Pinning down exactly what "extreme" means can be a little tricky since rainfall averages and intensities vary so much from place to place. For instance, a four inch rain event feels a lot more extreme in Las Vegas, Nevada, which averages only 3.4 inches of precipitation per year than in New Orleans, with an average of 60.3 inches. But, however you measure rainfall extremity, we're getting more of it than we used to.

Over the past century the US has seen about a 14 percent increase in very heavy rain events, and about a 10 percent boost in heavy events. " There's just no question that we've had a general shift to the wet", notes Stanley Chagnon, one of the deans of US climatology.

The big question, of course, is whether this upswing is due to global warming. And the tentative answer is: maybe. Since more water evaporates at higher temperatures and air can hold more moisture as it warms, a hotter world is likely to be a wetter world. The formula scientists have worked out is approximately 3 percent more rainfall for every degree Fahrenheit in temperature gain.

But that still doesn't account for why we're getting more of it in deluges. Someday, a research meteorologist is going to publish a paper with a title like "It Never Rains But It Pours: Precipitation Extremes In A Warming World." Until then, like ducks, we're just going to have to let it roll of our backs.

Thanks go out today to writer, David Laskin. The Weather Notebook is a production of the Mount Washington Observatory and is produced by Margaret Landsman.