ROBERT SIEGEL, HOST:

From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

AUDIE CORNISH, HOST:

And I'm Audie Cornish. Hurricane Sandy has earned the nickname Frankenstorm for a reason. An odd confluence of weather events have helped the storm grow and the result is a frightening mix: high winds, storm surge, snow and driving rain. But what is the science behind Sandy? A cold weather system mixing with a tropical system to become extratropical or post-tropical, depending on who you ask.

To translate, I'm joined by Perry Samson, professor of atmospheric science at the University of Michigan. Welcome, Mr. Samson.

PERRY SAMSON: Thank you.

CORNISH: So what makes this storm unique?

SAMSON: We have, as you've mentioned, a combination of events going on. First, the tropical storm, which is moving up the East Coast following the Gulf Stream. And tropical storms are driven by the availability of warm water and moisture. These cause the clouds, and these then clouds form into the storms that make a hurricane.

Now, that storm is following the Gulf Stream, as it would normally do, as it moves towards the north and east. But the odd thing that's happening now is that the upper level winds are grabbing that storm and pulling it back towards the coastline. And that's where the tropical storm then gets embodied into these - the jet stream and these waves in the upper atmosphere to produce an extratropical storm.

CORNISH: Explain the term extratropical.

SAMSON: These are the kinds of storms that we normally get in the United States, and they're produced by variations in the jet stream in the upper atmosphere. Without going into it, these variations cause the winds to rise in the atmosphere and cause low pressure. These are fairly well understood and studied by many.

The combination of this kind of storm with a hurricane embedded in it, I don't remember a storm like this in the last 30 or 40 years. It's going to produce tremendous amounts of precipitation across Pennsylvania and New York and New Jersey. In addition, this has the bad luck of coming onshore at high tide and with a full moon.

So we're concerned and interested in how this is going to affect storm surge, how the topography of the Long Island Sound and how the topography of New Jersey and New York might confound this. These are questions that are hard to answer.

CORNISH: I've also heard that Sandy has extremely low pressure. Could you explain what that means and what role pressure plays in a hurricane's strength?

SAMSON: Central pressure in Hurricane Sandy right now is somewhere around 950 millibars. And typically, the pressure at sea level is somewhere around 1,012 millibars. The pressure in the center doesn't by itself cause the storminess. It's the change of pressure between the outside the storm and inside the storm. This pressure gradient, the stronger it gets, the stronger the winds become. So it's the difference in pressure from inside to outside the storm.

CORNISH: Looking at this as an academic, are there any aspects of the storm or the way it's being described that you think are maybe hyperbolic, r, you know, people kind of getting overexcited?

SAMSON: I have a trouble with the word Frankenstorm myself, but that's just me. But beyond that, this particular storm is going to offer a huge opportunity for our science to study what happens when a tropical storm and a extratropical storm come together, because these are very rare events. And the fact that this is happening and hitting an area of high population gives great concern about the current situation but also a great opportunity in the future to know more about the dynamics of our atmosphere.

CORNISH: Perry Samson, thank you so much for talking with us.

SAMSON: It's been my pleasure.

CORNISH: Perry Samson is a professor of atmospheric science at the University of Michigan.

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