26 February 2015

Question place 2015

Time to hang out the shingle again for questions.  What would you like to know about?

In the mean time -- See Dr. Kate Marvel's distressingly accurate description of the peer review process.  Fortunately it isn't always like that.  Unfortunately, it sometimes is, or at least is close.  While you're at it, add her to your regular reading.  See her also at @DrKateMarvel on twitter.

Also, If you need your fellow scientists to be dry & stern & aloof in order to take their work seriously, you are a terrible scientist. @AstroKatie  Scientists are usually passionate about their science.  How that gets expressed, varies.  Some like the dry+stern+aloof approach.  Some like the yippee! approach.  As she also said, versus the dull and inaccurate 'scientists mystified by X' headlines: All headlines about unexplained phenomena should read "Scientists Super Excited to Find New Juicy Juicy Mystery to Gleefully Obsess Over"

19 February 2015

Forecast Evaluation

Boy, blow one historic blizzard forecast and people get all cranky*.  Except, as H. Michael Mogil discusses, it was an almost perfect forecast.  For the specifics of that storm and its forecast, I refer you to Mogil's article.

I'm going to take up the more narrow topic of forecast evaluation.  (Disclosure: I do work for NOAA/NWS, but, as always, this blog presents my thoughts alone.  Not least here, because I agree more with Mogil than the head of the NWS, Louis Uccellinni, about this forecast.)  One school of forecast (or model) evaluation looks at computing large scale statistics.  The most famous one for global atmospheric models is the 5 day, 500 millibar (halfway up the atmosphere), wave number 1-20 (large scale patterns), anomaly correlation.  When people refer to the ECMWF model (or 'Euro') being better than the NWS's model (GFS), this is usually the number that is being compared.  But I don't live halfway up the atmosphere, nor do most of you.  We're somewhere near the bottom of the atmosphere.  And there is much more of interest than just average temperature through a layer of the atmosphere.  So there are many other scores (dozens of them) -- See http://www.emc.ncep.noaa.gov/gmb/STATS_vsdb/ for some examples and discussion of what the scores mean.

Most of those scores, though, don't get to my personal -- weather forecast consumer -- interest.  Namely, I'm trying to make a decision of some kind.  NYC, which heard a forecast of 24" (60 cm) but got 9" (22 cm), presumably made decisions that they wouldn't have if they'd heard the perfect forecast that hindsight now provides.  It's here, I think, that we get to the meat of forecast evaluation.  Had this same error been made over the ocean, rather than over the most populated city in the US, with the rest being as it happened, the NWS would be getting praised for their great forecast.  The important part was not difference between reality and forecast, but number of people who made the wrong (in hindsight) decisions.

So let's explore evaluating forecasts by way of our decisions.  I don't make decisions for major metropolitan areas, and not about street plowing and so forth, so will leave that aside.  One realm of weather-affected decisions is in my running.  Let's ignore summer decisions (I'd as soon avoid thinking about what summers are like here) and go with the path as temperatures drop.  Normal gear -- in pleasant weather conditions, is t-shirt and shorts.  Once it cools below 60 F (16 C), I pull on a pair of gloves for my run.

17 February 2015

Chandler and the Chandler Wobble

The fact that the earth wobbles was expected/predicted long before it was observed, which makes for a couple stories about the nature of science and the people who do it.  The story of the Chandler wobble starts up almost a century before Chandler was born.  In 1765, the Swiss mathematician Leonhard Euler, whose name appears throughout mathematics and physics, was examining the mathematics for conservation of angular momentum on a rotating spheroid -- i.e., something like the earth.  It turned out that such a body could have a wobble on top of its regular rotation.  Given what was known about the shape of the earth then, Euler predicted a period for his wobble of about 305 days.  (Modern information doesn't change this number much.)

The magnitude of such a wobble could also be estimated from the law of conservation of angular momentum, and was somewhere around 0.1 seconds of arc.  That made for a difficult observation in 1765, and it wasn't until 1841 that astronomers started trying to observe this 10 month (305 day) wobble.  Given data analysis methods of the day -- pencil and paper -- analyses were done looking to see if there was a signal with a 10 month period.  No such signal was found, even though several people looked.  Side note being that one of the astronomers who looked in to this was Friedrich Bessel, who was the first person to measure the parallax (thence distance) to a star.  Quality of observing skill was not an issue in his failure to detect the wobble. 

Enter, then, in the late 1880s the next start of our story, and some options of how to tell it.  I could tell the story about a 'lone genius, toiling in obscurity until his great moment'.  It would be doing some fair amount of violence to reality, but that hasn't stopped all story tellers.  Chandler was indeed not being paid to do science.  He made his living as a merchant.  But keep in mind, though, that in the 1800s, 'scientist' even as a label was fairly new, and very few people who were scientists, or rather, who were doing science, did it full time.  Many were men of independent means who used their free time to try to understand the world.

Chandler was one such, but his roots in astronomy extended far back -- to high school.

16 February 2015

New blog -- Philosophically Purple

Very different topics than here, very different writing style, but one similarity -- a love of the universe.  Secondary similarity is that the author is my sister. 

Philosophically Purple

From the close of her first note:

You will have to forgive what I can assure you will be phrasing and punctuating that lives outside the happy boundaries of the Chicago Manual of Style. I am aware of punctuation, I just reserve the right to use it — and word choice — as I see fit. Yep, control issues all over the place here.

My ponderings tend to run along the themes of living life well, kids, being a lifelong student, and friendship. If that has value to you, then come on over to this blog as often as you like. I’ll have a plate of metaphorical cookies at the ready.


09 February 2015

The earth wobbles

The earth wobbles about in its rotation.  This was predicted long before it was observed, which is a story itself that I'll tell later.  For now, consider the earth and its rotation.  The north pole of the earth points towards the north star, and rotates once per day.  Open your right hand.  Your thumb points north, and when you close your fingers, they are moving in the direction of the earth's rotation.  With your arm making a right angle at the elbow, hand aiming away from your torso, you have an x-y coordinate system.  When you rotate your forearm, that moves your thumb in the x direction (positive or negative), when swing your arm forward/backward, that's the y direction.

The thing is, the earth (your thumb) doesn't always point in exactly the same direction.  There's a small bit of variation.  That's the wobble.  Since astronomers make their observations from the earth, it's very important to know exactly where the earth is pointing at any instant.  This lead (over 100 years ago) to the foundation of the IERS -- International Earth Rotation and Reference Systems Service.  Daily data from 1 January 1962 to (very nearly) the present are available at http://datacenter.iers.org/eop/-/somos/5Rgv/getTX/213/eopc04_08.62-now
Two things stand out to me in looking at this: There's a very slow tendency to increase x and y over time (increasing movement of the north rotational pole away from the original 0 point), and the more dramatic periodic variation.  The business of having slowly varying amplitude (size of the up and down) for the fast variations suggests a 'beat' is going on.  Namely, there are two different periodic variations going on.  When they're both at maximum, you get a large amplitude.  When they're at minimum, you've got a small amplitude.