Friday, November 24, 2006

Weather Science: on Nor'easters

I mentioned in my last post the "nor'easter" we had yesterday, and if you are not from the American Northeast, you may not be familiar with this term.

A Nor'easter is a substantial storm whose low-pressure center is just offshore, and whose prevailing winds come from the Northeast -- from the Atlantic Ocean. In my 17+ years in New York, I've been through many nor'easters, and they are characterized by moderate to high winds and LOTS of rain.

And cold. One must not forget that this a cold, soaking, dreary rain. This is an umbrella-slaying kindof storm, and I saw lots of dead umbrellas on the streets of Gotham yesterday.

I mentioned atmospheric pressure, and a change in this signalled the approaching storm. Over the past week at the diamond mine, we've been constructing and observing little homemade barometers (devices that measure atmospheric pressure). On Wednesday, we started to observe the telltale drop in pressure. Such drops in pressure often presage a coming front or storm.

Looking at my barometer (not homemade) here at the pad, I noticed that it continued to drop into Thursday. According to the National Weather Service archive it reached its low point at about 6:51(EST) yesterday afternoon (value = 1014.5 millibars). Now it is (officially) at 1020.5 millibars. Pressure usually rebounds after the leading edge of a storm passes through.

(As an aside: "standard" sea-level atmospheric pressure equals 1013.25 mb. Actual pressure varies widely above and below this average value over time and depending on location. The lowest recorded atmospheric pressure of 870 mb occurred in the Western Pacific during Typhoon Tip on October 12, 1979. The record high was 1086 mb, at Tosontsengel, Mongolia, on December 19, 2001.)

There is an interesting discussion of nor'easters on the wikipedia. There is some debate about the term and whether it is just some literary/media foppery or really a piece of New England nautical jargon.

In either case, the media always refers to them as "nor'easters" around here.

It says in the wikipedia article that these storms typically happen during the winter months or sometimes during the spring. Personal memory suggests to me that in New York, at least, they are most common in the month of December. So yesterday's was a bit early.

Today is beautiful: all that wet has blown back out to sea, and we're left with clear skies, a little more warmth and some dry air in which to commence our shopping madness.

Addendum: How to make a simple barometer
I got this "recipe" from the Pearson-Prentice Hall Science Explorer series, but made some slight modifications. It's very easy.

You'll need a balloon (not inflated), a jar (I used a 800 mL glass beaker), a drinking straw, some scotch tape and some cardboard. Oh, and something to write with. And scissors. And a little bit of modeling clay.

  1. Cut the neck off of the balloon.
  2. Fold back the edges of the balloon and carefully stretch it over the mouth of your jar (beaker). It should form a membrane like a drum over the mouth of the jar. A wide-mouthed jar is best. No air should be able to get in or out of the jar. (This is a species of aneroid barometer) The textbook said to use a rubber band to secure the balloon around the jar. I didn't find this necessary, so we skipped it.
  3. Tape one end of a drinking straw onto the center of the balloon membrane. (The text recommended glue - what a pain that is! Just use tape.)
  4. Using a strip of cardboard, create a scale in 1/2 cm increments along the edge, so that when the cardboard is stood up, the straw will point to this scale. I see this is rather hard to describe without pictures. Probably, it would be worthwhile to make the scale in 1 mm increments, if you have the patience.
  5. Fashion a little pointer out of modeling clay and stick it too the end of the straw that is to point to your scale.
  6. Pick a place for your barometer away from heat sources and secure both the cardboard scale and the jar so they don't move.

Obviously, this is very rudimentary. You won't get calibrated millibars or other units out of it, but if you've done a good job, you should see the pressure increase and decrease appropriately. It's really the change in pressure that is significant, not so much it's absolute value.

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