The eye of hurricane Matthew passes directly over a weather buoy | Hacker News Reader

The eye of hurricane Matthew passes directly over a weather buoy

ndbc.noaa.gov

384 pointsmatt2000151 comments

That's a beautiful graph but NOAA's insistence on non-metric units frustrates me endlessly. Knots for speed are bad enough but "inches" for pressure is multiple levels of wrong.

What's more wrong about inches than knots?

If taken literally the dimensionality is odd. Pressure is a force applied to an area (force/2-d), and that's just a length 1-d (as written)

Even if not taken literally, the particular unit of "inches" is largly meaningless in this case. All we know (assuming the obvious mechanism of measurement) is that the air pressure in <choice of force units> is linear to the reported value. The conversion factor is dependent on the instrument itself (surface area, internal resistance, etc) although there may be a standard that is known specialists.

Inches of mercury -- the pressure exerted by a column of liquid mercury that high.

It is a 1-D measurement. It's the pressure exerted by the atmosphere that would raise a column of mercury that 1-D length up a vacuum at 1g of gravity.

It doesn't matter that you're measuring it in an odd way - pressure still has units of force per unit area. 1 "inch Hg" is 3386 Nm⁻².

The fact that you can express length as "number of football fields" doesn't make length dimensionless.

Two square inches of mercury an inch tall weights twice as much as one square inch of mercury an inch tall. But there's twice as much area, so it's applying the same amount of force to each square inch.

The point is you care about the force, but don't care about the area, and a tube of mercury factors out the area.

Knots are one Nautical mile per hour, and 1 Nmi == 1 minute of latitude. Nautical miles are based on the shape of the earth and are thus independent from the rest of the US unit tragedy. And quite useful in navigation.

Well to be honest America got these units from the British. Not every tragedy is America's fault.

The tragedy is that they're still used now. Not that they were once used.

Yet, the British still use "stone" to measure their weight, then complain about universal standards not being used by the Americans.

Of course, the meter was originally based on the shape of the earth as well - the distance from the pole to the equator was defined as 10 Mm in 1793.

As far as the US unit tragedy goes, I think it's important to remember that SI units are full of compromises in exactly the same way that US units are, because they need to accommodate practical, human scale concerns. For example, degrees Celsius is a ridiculous concept from a scientific point of view (ask any beginning chemistry student who just messed up their test because they forgot to convert to Celsius to Kelvin before applying PV=nRT). If you believe the point of a unit system is to be useful for science, it's hard to defend a unit where 0 doesn't actually mean 0, since it introduces all sorts of confusion in the same way as traditional units do.

And why does the whole world still measure days with 24 hours in them, each with 60 minutes, each with 60 seconds? Because decimal time (although France tried to impose it during the transition to SI units) is just too darn inconvenient for humans to use. The failure of decimal time shows why humans prefer non-decimal units: because the resulting units are human-scale. Converting from seconds to hours or days is not as easy as multiplying by 1000, but kiloseconds is an inconvenient unit - too short for an hour, while 100 kiloseconds is significantly longer than a day. Somehow the metric world manages these conversions just fine (in the same way the US manages conversions between inches, feet and yards).

The traditional units evolved over hundreds of years, and I'm glad the US keeps them around - like nautical miles, they are quite useful.

> The failure of decimal time shows why humans prefer non-decimal units: because the resulting units are human-scale.

I really don't agree though. The failure of revolutionary France to switch to decimal time like it switched to decimal units, is that time was already well standardised back then, while a single standard of decimal units was a real improvement over the mess of local units that was in use before.

I think you're making the same mistake every argument against metric always makes: "inches are more convenient than centimeters, because otherwise how can we call 19" racks!". Well otherwise, racks would be 50 centimeters and not 19".

The same way, people would work just fine with 10-hour days, 100-minute hours and 100-second minutes. Maybe people would use half-hours more often than today to divide the day, but I can assure you that it would just work as conveniently.

There is nothing inherently natural in dividing the day in 24/60/60 parts, we're just used to it. Also, if you have ever worked with time keeping software (or worse, hardware) you would know the metric world actually doesn't manage these conversions just fine.

The reason decimal time did and always will fail is that no matter how you slice it, 365.24ish is not divisible by 10.

Moreover there is nothing inherently natural about 10 as a base. Like many of our modern conveniences it is entirely arbitrary and about the worst possible option.

Fin

> The reason decimal time did and always will fail is that no matter how you slice it, 365.24ish is not divisible by 10.

It's not divisible by 60 or 24 either, so I don't see the point. Decimal time only deals with units from the day down.

Decimal calendar deals with days, weeks, months and years. Its adoption went far better than decimal time.

> Moreover there is nothing inherently natural about 10 as a base.

No base is inherently natural, 60 certainly isn't. 10 just happens to be the one we use as a standard.

That is the point. There is no way to cleanly divide up the time taken by the earth's orbit by the time taken by the earth's rotation. They are simply incompatible. It is impossible to say objectively that any one scheme is better than another, with one exception: the current scheme is always better than all the others by virtue of being current.

And consider this: Leap seconds are a thing.

Your point about years is completely irrelevant to how you divide a day into hours, minutes, and seconds, which is what 'decimal time' talks about.

The advantage of decimalisation is the geometric expansion it engenders which cannot happen because the year is not 10, 100, 1000 or any other multiple of 10 days. It is not an integer multiple of anything days. It is evidence that no matter how much we try and break the universe up into nice coherent chunks, the universe has other ideas. Fighting reality will fail.

The advantage to breaking up the day into units less than 1 day is the factorisation that can be applied. 24 factors much better than 10. Pretty much everything factors much better than 10.

There is no advantage to decimalising the day except to give basement-dwellers a boner.

24 and 60 equally fail when you try to turn days into years. That doesn't mean you give up on picking a good system for fractions of days and multiples of years.

There is no[1] advantage to having a smattering of small factors. People make much too big a deal about dividing into thirds.

[1] "no" in the same way that decimalization has no advantage, in that the advantages of either system are small and unnecessary.

We didn't give up. We found one and we stuck to it. I don't care about factoring, that's a minor cool hack.

The point is simple: Adding or removing dots from the face of a clock is a complete waste of (heh) time.

If you people really need to wank over a calender, pick one with jugs in it or get rid of timezones and daylight savings. That would be actually helpful.

> Adding or removing dots from the face of a clock is a complete waste of (heh) time.

Nobody was advocating for doing that. You started an argument about doing so, in reply to a post merely saying that it would be "as convenient" as the current system, and that decimal time failed because we already had a unified time system.

> If you people really need to wank over a calender

Nobody had even mentioned calendars until you showed up. "you people" = "ChoHag".

I'm not sure if I'm missing something but wouldn't that be 36.524? (Metric user here, I honestly think I'm missing the point).

So a month will be 36.524ish days. Where in the metric regularity does 36.524ish fit? I can see that it's somewhere between the 10s and the 100s column...

A year isn't 100 days, it isn't 1000 days, and the whims of naked apes with a penchant for euclidean geometry isn't going to change that.

Ahh I see what your getting at, I was a little lost by end of the chain.

Well we could just convert a second, day, month, year into metric.

(Thanks for the answer btw, was curious :-) )

We could, but we gain nothing but more work.

We can't not consider current timekeeping. There is exactly 0 chance we can get everybody to change in the same instant and even if they did history is still there. Education, and thus civilisation's shared consciousness, will include the old timekeeping until the renaissance, industrial revolution, the inventions of computers and space travel and the near-ELE cataclysm of the final death knell of religion are all historical footnotes. Computers will need to understand the old timekeeping forever. We won't reduce the amount of bullshit humans deal with, we will have increased it. And with all that, the earth's orbit still won't be 1000 days.

And all for what? So I can count the hours in a day without getting my feet cold? Woo!

Well we use to use cubits and chains for measuring everything too. Science is slowly moving to si, we should probably all start cobaidering a proper system for time that works around metric.

I don't like the idea of explaining to my children that we use this system, because it's the way it has always been done. That's a horrible idea .

I look forward to teaching my child that we use this system and using its peculiarities to spawn discussions on number theory, astronomy, history, even art.

Among other things.

Well, the decimal calendar had 12 months of 30 days each, with either 5 or 6 extra days depending on leap years.

Months had three ten-day "weeks".

As you see, nobody is trying to fight physical reality, since day and year are hard lengths that you can't redefine.

But decimal time, decimal calendar and more generally decimal units are all about trying to eliminate as much as possible the historical peculiarities that are not grounded in physical reality:

The irregular length of months, the weird 24 hours in a day, and the base 60 which has been obsolete for millennia now. All of these do not give any advantage to the users of these units today.

Divisibility by 3, 6 or 12 are useless today because we now master decimal numbers easily. I know Imperial units users still often use fractions, but metric units users don't, and it just works. Not once in my life have I been thinking "oh, how I wish I could express 0.41 centimeters in fractions". In the same way, your now 1-hour task would take you something like half a decimal hour instead, or 50 decimal minutes, and you wouldn't even think about it.

> Well, the decimal calendar had 12 months of 30 days each, with either 5 or 6 extra days depending on leap years.

So not 10^x, 10^y and 10^z consistently without the constant need for special-case exceptions?

> Months had three ten-day "weeks".

So not 10^x-day weeks then?

> decimal [time-measuring] units are all about trying to eliminate as much as possible the historical peculiarities

Eliminate? I think you mean change.

> The irregular length of months, the weird 24 hours in a day, and the base 60 which has been obsolete for millennia now. All of these do not give any advantage to the users of these units today.

There is nothing more right about 24 than 10, unless you need to take your socks off to count.

There is nothing more right about 60 than 10, unless you need to take your friends' socks off to count.

10 gives advantages to people who count on their fingers. But, amusingly, less advantage to finger-counters than just about any other even-numbered base that isn't 2.

> Divisibility by 3, 6 or 12 are useless today because we now master decimal numbers easily.

I see you're a maths teacher! I, also, love the pure enthusiasm of the classroom. You can smell it a mile away.

I think that's enough from me. The rest is just imposition anyway.

The reason 12, 24, 60 are so useful is how many factors they have. You can divide the first two in half and thirds, and 60 additionally by 5ths and 10ths without resorting to fractions.

Being able to more simply divide by thirds and sixths might not be important most of the time, but it's not completely useless, and you can't do it in base-10.

Easy thirds are really unimportant in the grand scheme of clock uses. If we were used to decimal time, it would work just as well.

Actually while they're useful, they're not that useful.

The point is that being able to divide the day up into 10 chunks, which is just about the only thing we can't yet evenly divide it up into[+], is even less useful.

It's certainly less useful than the gargantuan international effort it would take to change one arbitrary system for another.

[+] And that's not even so bad - 2.4 or 2:24

iirc its inches of mercury, which isn't much more helpful. I've frequently seen it labeled as "in. hg"

Also used in certain fields as its metric equivalent, mmHg.

Which of course depends on your local gravitational field.

You can switch to the metric system on their website [1]. The graph can also be switched to metric by changing the "uom" query parameter from "E" to "M"

[1] http://www.ndbc.noaa.gov/station_page.php?station=42058&unit...

Here is the plot on m/s and mbar: http://www.ndbc.noaa.gov/show_plot.php?station=42058&meas=wd...

"Oh wow, air pressure fell to almost zero, a person would suffocate in ther... Wait a minute, the ground unit is 940! This is a 6% difference!"

Sometimes ground units reflect ground truths, even if the scale doesn't actually hit zero.

Yup, this is also why I like the metric version. With inches of quicksilver it's hard to know where zero is.

It would be better if NOAA used kilopascal (an actual SI unit) instead of millibar, but I guess the conversion is not too bad since 1000 mbar = 100 kPa.

The NOAA data isn't consumed solely by scientists though. I'm sure millibars were chosen as the metric measurement of pressure because people actually use them, such as mariners.

Just the other day a last minute task at work put me under 500 millibars of pressure.

I'll show myself out.

Try:

http://www.ndbc.noaa.gov/show_plot.php?station=42058&meas=wd...

FYI a Knot is standard for navigation world wide because the unit is standardized on earth's size. 1kt = 1 nautical mile per hour. 1 nautical mile = one minute of Earth's longitude. Learned this in sailing class :)

Wikipedia: "Worldwide, the knot is used in meteorology, and in maritime and air navigation—for example, a vessel travelling at 1 knot along a meridian travels approximately one minute of geographic latitude in one hour."

Latitude, not longitude - the length of one minute of longitude changes with your distance from the equator.

Also, I am not sure how that's relevant to NOAA?

NOAA is part of the Commerce Department and Oceanic is part of their name. An important consumer of the data are entities who navigate the sea and air.

Both constituencies use knots and nautical miles.

> An important consumer of the data are entities who navigate the sea and air.

You make it sound like birds and fish check in with the NOAA before planning migrations.

Edit: and that they're really picky about units of measurement.

Using "Knot" because it's tied to longitude and latitude is the exact point the metric system is designed to avoid: needless and inaccurate attachments between system of units and specific phenomena that actually complicate rather than simplify and hinder interoperation with every other system.

As you state:

1. a knot is only approximately equal to 1 min of latitude per hour (there are numerous cases where it's unusably wrong)

2. doesn't really help for longitudinal travel

3. maps are continuously projected to favor this mediocre form of navigation instead of something more accurate (see Mercator projection)

4. you could easily put kilometre lines on your charts instead of minutes of latitude and fix all the previous problems

That's going to work great when the earth goes back to being flat.

No Mercator projection is used because its the least weird conformal projection (you draw a straight line its really straight in the real world). If you are trying to plot a bearing at the scale of the planet you want to be using Mercator unless you are going over the pole :P That the same projection is then used for everything even when you don't need to preserve local angles isn't the fault of Mercator. That you think km lines on an atlas would be better than lat/long makes you crazy. A decimal day might be useful but a decimal year is not. A fractional decimal day is a completely useless reference of time for doing quantum mechanics, but perfectly reasonable for people planning their work. No one would get rid of the concept of "a day". A map that maybe only shows tens or hundreds of km might be fine but representing the whole world with an equal area map with all grid reference based on km isn't useful to many people who use maps a lot, especially as the world isn't describable in whole numbers of km! But does have absolute and continuous angles from the gravitational centre.

Lines on a Mercator map aren't straight on a globe. They are constant-compass bearing, but that's not the same as straight.

Also, as defined by the French academy of science ~1800, the meter is 1/10,000,000th the distance from the north pole to the equator, on the meridian through Paris.

Not sure what your metre comment is about, but it turned out that definition wasn't accurate enough to confidently state the speed of light, so the speed of light was set to 299792458 m/s and a metre is consequently the distance light travels in a 299792458th of a second.

(The second is in turn precisely defined by particle physics.)

It's a reference to Justin_K's earlier statement "a Knot is standard for navigation world wide because the unit is standardized on earth's size".

If that justifies knot then it also justifies kilometers, because it too was originally standardized on the earth's size.

Neither are exactly the size as originally defined.

A modern knot is 1852 meters. One minute of latitude is "about 1,855.325 m on the WGS 84 ellipsoid" says https://en.wikipedia.org/wiki/Nautical_mile. The difference is 0.16%.

A quarter of the Earth is:

  1855.325m * 60 seconds/degree * 360 degrees/circle /4 = 10018755.0
  = ~10018 km.

The difference is 0.19%.

The shortest distance between two points on sphere is along the great circle that contains the points, and are the closest thing to a "straight line" you can have on a sphere.

Straight lines on a Mercator projection aren't great circles, they're rhumb lines, and if anything they're a spiral on the sphere, not straight.

If you are holding a globe that a useful definition. However, that's not what navigators are doing. We can look at the stars and a compass both of which make rhumb lines far more useful.

Rhumb lines may be useful for tiny vessels with only a compass and a map on board, but any commercial vessel, whether it is on the water or in the air, will follow the great circle route. It's the shortest and fastest way to go and it uses the least amount of fuel. Following rhumb lines just because they happen to be straight lines on Mercator projected maps would be silly.

It's not the 1800's any more. We have computers and stuff, we don't have to rely on techniques developed for a time when your navigational tools consisted of a map, a compass and a sextant.

The passage of time doesn't diminish the usefulness of local navigation techniques...

a Mercator projection will give you a bearing to steer to arrive at your destination, which can be followed by eye, autopilot, etc etc.

Calculating a great circle arc and plotting it, following a bearing which may only change by a degree or two but isn't constant, is an unnecessary complication on short routes.

Even commercial vessels will use those old techniques if they don't need to consider the longer range effects. Error correction in the human process is always a consideration.

(Not even talking about data entry errors in GPS systems, but that Malaysia - Melbourne story was Funny.)

Computers don't care about your maps and will plot a great circle route just fine. However, in the event of system failure you want a real backup. Which is why the navy still teaches people how to use a sextant. http://www.npr.org/2016/02/22/467210492/u-s-navy-brings-back...

Note: The actual deviation from a great circle route is generally fairly minimal in practice.

PS: GPS is just another system that really can fail.

Unfortunately, though, the earth is not a cube.

The SI unit of length, the meter, was originally defined according to Earth measurements, as 1/10,000,000 of one half a meridian.

This was supplanted by reference bar measurements in 1889 and 1927, by a count of emitted light wavelengths in 1960, and as the distance traversed by light in a vacuum in a specified time interval in 1983.

(So space is measured by time.)

The metric system has its own foibles, one of which is being based on the number of digits a phenotypical abundant primate on a minor planet of a backwater solar system happens to have. It has certain conveniences, but isn't the hallmark of unquestionable rationality some would like to think it is.

The critical feature of SI is that it directly and explicitly incorporates a single positional numbering system used in scaling all quantities. Exact definition of quantities, while an important feature, has almost always been subordinate to that goal.

Arguendo, let's assume that you are right that the decimal numeral system arose because of the typical number of human fingers. Note, we do not have to agree for the sake of argument that the place-value system is intimately tied to decimal (and indeed in our history, the latter predates the former by several millennia).

That there are ten values per position is therefore parochial, but not as parochial as you think: almost every vertebrate has five digits per extremity (and essentially all of them embryologically[1]), and there are huge numbers of decapod arthropods extant and in the fossil record, and superorder Decapodiformes is an important group of cephalopods.

So if we use base ten because of the number of our fingers (this is far from proven, unfortunately; there are many examples of very different ways of counting and doing arithmetic on human fingers), it is reasonable to think that many plausible civilization-building non-primates would be liable to use similar representational systems. Unfortunately that conflicts with human history, in which there has been a wide variety of number representation...

[1] even in non-tetrapods: https://images.sciencedaily.com/2016/08/160817142550_1_540x3...

Some harder-core evodevo: http://www.scientificamerican.com/article/why-do-most-specie...

It makes no sense that our base has only 2 and 5 as factors. It is a stupid decision that can't be explained by rationality.

The base 12 system is way better. It has 2 3 4 6 as factors. It's so good we use it everywhere. 2x12 hours, 12 eggs, 12 doughnuts, 12 months, 60 minutes, 12 inches.

A base to be good needs many factors without being unwieldy. 10 is stupid as it skip the most useful ones 3 and 4. Nobody ever need to divide anything by 5.

The perfect measurement system would be SI with base 12.

Base 10 can be explained rationally. Look at your hands. See those pointed bits? Count them. Now do you understand why use base 10?

Look at your knuckles. You have 8 knuckles. Therefore, use base 8.

You can put each finger in an up, down, or medial state, except the two thumbs which give only up or down. That gives base 28.

You can count binary on your fingers as a positional number system. That's base 2.

Base 10 is not the only base that's usable on our dexterous digits.

> Look at your knuckles. You have 8 knuckles.

If you have all fingers on one hand, the thumb of the other hand, and the last joint of each of the remaining fingers removed, sure.

But 28 is the usual number of knuckles.

Why 12 and not 2, 1 or 1890? The choice of the number of values per position is entirely arbitrary, and almost the entire reason why you prefer twelve over ten is that twelve has more prime factors AND is close to ten. Six has the same number of prime factors as twelve; so does twenty-four.

"Nobody ever need to divide anything by 5"

So, what is left after you remove your "2 3 4 6 as factors" from your example up there of 60? If you work 35 or 40 hours in a week, how many hours should you expect to work on a work day?

Multiples of primes can be handy. 12 = 2 * 2 * 3, which gives subdivisions of 1/2, 1/3, 1/4, and 1/6.

Other possibly useful values might be 30 (235) or 210 (2357), which has factors of all the primes lower than 10.

The Babylonian system, based on 12, 30, 60, 120, 180, and 360 (223, 235, 2235, 2^3 * 3 * 5, and 2^3 * 3^2 * 5 respectively) have multiple useful subdivisions.

Base-ten decimal is fun, but produces non-finite values when fractional values are considered: 1/3 = 0.333..., 2/3 = 0.666..., etc. Systems in which those fractions are inherently supported, and for which by Benford's law you're likely to see the divisors 2 - 9 fairly frequently (https://en.m.wikipedia.org/wiki/Benford%27s_law), this can be convenient.

Particularly in an age of rulers, protractors, compases, and no digital electronic computors.

Isn't one of the main benefits of metric the ability to quick convert between the various scales? Converting between say 1.4m to mm is a lot easier than 4.5ft to inches. Or answering questions like how many square metres are in 5.2 hectares (vs square feet in acres). Arbitrary measurements of eggs and doughnuts (don't forget gallons) are not great examples - no one buys them for that precise amount, they buy them because that is what they are available in.

I've never understood why some Americans are so stubborn about the metric system. The rest of the world has essentially made the switch for various reasons but arguably the most advanced nation on earth can't?

If our number system had 12 digits, then you could apply powers of 12 by just moving the decimal point in exactly the same way.

Yes, but base 10 is the common language we use today. And mm to m, is a lot easier than in to ft...

In base 10 that's all true. The comment you replied to proposed using base 12 instead. If your arguments are based on the assumption that we can only use base 10, then you can't really make an argument against base 12. It seems your arguments are constructed on the assumptions that contradict the proposal.

Using order-of-magnitude prefixes to scale up and down units is orthogonal to base. Things just look strange if you refer to other bases in base 10.

Consider base-2 prefixes (now being known as kibi, mebi, gibi, etc)

    1 kibibyte = 1024 bytes

    1 mebibyte = 1048576 bytes

These don't look like nice round numbers because I am representing scaling of a base 2 number in base 10. It's more clear if I say:

    1 kibibyte = 10000000000

    1 mebibyte = 100000000000000000000

    1 gibibyte = 1000000000000000000000000000000

Each further prefix just adds 10 zeros to the binary value. Just like metric prefixes add zeros.

One kilometer would still equal 1000 meters. But that 1000 is now in base 12, not in base 10. So rather than 10^3 it's now 12^3. But that just seems strange because you grew up with base 10.

Eggs and doughnuts and many other things are measured in 12 because of its historical utility. It allows you to say things like "I'd like a quarter of dozen" and "a third of a dozen" which are not possible with 10.

Extending a bit further, if you want division by 2, 3, 4, and 5 you can go up to base 60. The Babylonians (among others) did this and nowadays we have 60 seconds in a minute, 60 minutes in an hour, 6 * 60 degrees in a circle, etc:

https://en.wikipedia.org/wiki/Sexagesimal

Use a calculator.

Specifically: use a units-aware calculator.

I've been doing a lot of calculations based on energy and related terms, and am finding GNU Units (and yes, distinctly specified as an alternative to, say, BSD Units, which has a small subset of features) a phenomenally useful tool.

It turns out that even SI units get confusing and non-base-ten-rational pretty quickly -- pretty much the moment you go from watts to watt-hours (and yes, there's an alternate unit of energy: Joules, but watt-hours turn out to be a really useful concept).

It turns out that units-based measurements are complicated, and keeping tabs manually is a PITA, and using a units-aware calculator is all kinds of awesome. I wrote a little paen to it a couple of years ago here: https://www.reddit.com/r/dredmorbius/comments/1x9u0f/gnu_uni...

What's powerful is that GNU Units doesn't just do straightforward converstions (e.g., what's 180 cm in inches), but can do calcuations of multiple quantities: energy per unit area times area to give energy for a given size of land, etc. Since you can specify the units of the result, there's also a built-in sanity check that your units correspond (a common source of error in physics calculations).

Where GNU Units really shines is in being extensible. You can define constants, values, or units that you want to have readily available. I've plugged in "hiroshima" (the equivalent energy content of the ~15 kt atomic bomb dropped over Hiroshima, Japan), as well as values for the land areas of earth, the various continents, and several nations and states, as well as other area and other metrics for planets and moons within the Solar System. This makes a few relative comparisons fairly straightforward.

I'm aware that metric is "more logical" by certain ways of thinking, but old-school units had their own logic:

Units of length were based off of body parts or could be rapidly assessed. An inch is roughly a thumb's width. A foot, obvious. A yard is roughly a man's pace. A cubit, the length from elbow to fingertips. A fathom, that width of a man's arms, outstretched, as you'd find them whilst measuring out a sounding line shipboard. An acre is the amount of land a man could plough in a day (the German for this recognises the origin explicitly: tagwerk, or "days work"). An acre-foot seems silly until you realise that knowing the area of a reservoir, you can calculate its capacity quickly by simply noting water depth in feet. A british thermal unit is handy if you're sourcing coal for a steam engine -- how much water are you going to heat and boil each day? A mile ("mille") was one thousand double paces, from a slightly shorter-than-typical-today Roman soldier. A furlong was, literally, "one furrow long".

Volume measures: "bushel" derives from "bosta", a handful. Cup is obvious, from a typical drinking vessel. "Gallon" originates from "galleta", latin for bucket or pail.

Measures of weight are ancient -- "pound" comes nearly directly from latin, "libra pondo", with the same meaning. Ton is derived from volume -- the weight of a tun cask. Grain is another obvious one.

What these have in common is that for a quick and dirty in-the-field (often literally) assessment, these measures offered an immediate and sensible-in-context quantification. Measuring land area in acres gave a sense of how much work was required for that land -- how much one man might farm in a year, or how many men and horses you'd need to work it. Egyptian stonemasons could measure out their work by arm, sailors water depths, soldiers distance, farmers grain, hydraulic engineers stored water, without any significant calculations. There's something to be said for that.

I'm not saying the units make more sense now. I am saying that failing to recognise the logic by which the units were derived misses some highly salient points.

> I'm not saying the units make more sense now. I am saying that failing to recognise the logic by which the units were derived misses some highly salient points.

I can recognise the logic in historical units of measurements, but I do think some of them are forced - "I want a third of a dozen" - wouldn't you just say you wanted four?.

"(So space is measured by time.)"

Using the SI units of metres and seconds applies a coordinate system on spacetime. One can take a coordinate interval using that, but careful observation shows that the coordinate system cannot be applied at all scales or agreed upon by all users.

In SI terms, the rate of the cycles of hyperfine structure transition frequency of Cs-133 atoms is strictly local to the atoms themselves, and non-local observers will see that rate only if their local Lorentz frame has the same velocity as the atom's, and they are on the same gravitational potential.

As we are not massless inhabitants of a Minkowski vacuum, errors accumulate with coordinate distance from the atom used as a source of frequency, and are observable with laboratory equipment at ranges of metres, and are already wholly non-negligible for practical applications (e.g. GNSS) at ranges comparable to the radius of the Earth and velocities comparable to the speed of sound at STP.

Sadly, SI predates modern conceptions of 3+1 spacetime and has not really adapted to what we know of our physical spacetime's properties.

I think that's more parochial than the base ten issue, personally. :-)

> a knot is only approximately equal to 1 min of latitude per hour (there are numerous cases where it's unusably wrong)

Whilst it isn't precisely equal it's certainly precise enough for navigational purposes. What are the numerous cases of which you speak (i.e. give me a sea navigation scenario where the current definition of a nautical mile is unusuably wrong, may be easier to come up with an air one but again I think you'll struggle).

Certainly as a sailor (who routinely navigates yachts off-shore) 1 nm = 1 knot = 1 minute of latitude is very useful, especially when combined with a mercator projection. Lines of constant bearing are straight which is a very useful property. For longer ocean passages where you may want to sail a great circle route to save distance other projections are available.

Yes you could use kilometre lines but what's your positioning system going to be? It won't map well to lat/long so you'll need to come up with an entirely new coordinate system.

Given basically all charts available (and the surveying data behind them) are done in a lat/long system (though many different datums exist) it's a huge job to switch to the new coordinate system for little benefit.

The current system of units also works very nicely for celestial navigation. Measure angle (A) of body above the horizon. (90 - A) * 60 = distance in nm between you and the point on the earth that body is directly overhead.

Have you actually got any experience of navigating by sea or air or are you just assuming the system of units used is ill thought out and useless and everyone's crying out to use metric instead?

> Yes you could use kilometre lines but what's your positioning system going to be? It won't map well to lat/long so you'll need to come up with an entirely new coordinate system.

Will you really? Why not use UTM [0] + UPS [1] or MGRS [2]?

[0] https://en.wikipedia.org/wiki/Universal_Transverse_Mercator_... [1] https://en.wikipedia.org/wiki/Universal_polar_stereographic_... [2] https://en.wikipedia.org/wiki/Military_grid_reference_system

Well indeed such coordinate systems exist but you've still got the cost of translating a large body of cartography into them.

Plus as pointed out by another commentor the number of KMs around the earth is not a round number, so you need to do some fudging to get a metricised spherical coordinate system (and you want a spherical coordinate system because the 60th of a degree = 1nm equivilance is very useful for simplfying the mathematics required for navigation).

The cost of translating is a mouse click [after a software pro writes the code behind the control].

Modern, computerized charts don't need to lie flat or use only one projection visualization. Furthermore, you can add up-to-date ocean current, jet stream, wind, wave, and weather information to a computerized chart, along with a vehicle performance model, and automatically optimize a route for fastest travel time or lowest fuel consumption.

Nowadays, the unit of distance on the printouts and reports is only needed for human readability and loss-of-power emergencies. Once you have planned out your route, you can scale your backup hard copy with whatever unit your navigator feels most comfortable with, but inside the computer, the software most definitely has a "native" measurement unit, and those are usually in the base SI unit. All other units are converted from that native representation as necessary.

You can navigate your vessel in any coordinate system you like, as long as the conversion functions are coded correctly. And once they are, all charts are translated into all coordinate systems. If you like constant-bearing lines on Mercator projections, just print it out that way. If you like following currents and great circles, just make sure your navigation computer has a backup generator.

The fudging you mention is 99.9% handled just by using the WGS84 ellipsoid. If you're doing any fancy rocket science, you also need a somewhat detailed gravitational model mapped on top of it. Those sorts of calculations are most easily done in an ECR cartesian coordinate system, which easily translates to ECEF cartesian, which somewhat less easily translates to geodetic coordinates--it is actually easier to translate that in the other direction. Unfortunately for that math, no one knows where their house is on the Earth in XYZ coordinates. The amount of error creeping into your double-precision floats is almost insignificant at the scale you need with one or two extra conversions.

The math only needs one arbitrarily chosen unit. I have never seen nautical miles chosen as the unit for length.

Surely in this context a minute is used as an angular measurment. I am not sure that km is directly comparable as it implies assumptions about projection.

Well, at least an inch isn't defined that way. It is exactly 2.54cm.

> FYI a Knot is standard for navigation world wide because the unit is standardized on earth's size.

Historically, a meter was too. A line of longitude is a little over 20,000km.

"1 nautical mile = one minute of Earth's longitude"

... along the Equator

and at the poles. It's latitude that gets compressed at the poles.

Going north means changing latitude to approach 90 degrees north latitude at the pole. Those degrees of latitude are evenly spaced in distance (s=rtheta, if theta is angle north of the equator).

Going east means changing longitude, rising from -180 degrees to 180 and then resetting. Those degrees of longitude correspond to less and less distance at the poles (s=rcos(theta)*phi, if theta is angle north of the equator and phi is angle around the axis).

Should get a refund then. A minute of longitude near a pole is inches. Its a minute of latitude for a reason.

For air navigation, there is the exception of Russia (and ex URSS countries) where metric system was the norm. Each time I have encountered meteorology software, they were using m.s-1. It was a big source of confusion.

One minute of Latitude, not longitude. (I teach the class...)

1 km is also one metrical minute (1/100 of grad, also "centigrade") of Earth's latitude.

One of the few things I remember from my 20-year-old meteorology degree is that 1 meter/second is very close to 2 knots. This conversion was also useful when I was in the Navy, especially for coming alongside piers or other ships.

Understanding the conversion is helped by using the same time unit:

1 knot (per hour): 1852 meters/hour

1 meter per second: 3600 meters/hour

So 2 knots is 3704 meters/h, or less than 3% more than 1 meter/s. Good rule of thumb.

I like metric as much as the next guy but you can pry knots from my cold, dead hands.

Sincerely,

An aviator and sailor

Your cold hands, Fahrenheit or Celsius? ;)

Kelvin

You may add feet for altitude (you fly in levels that are in hundreds of feet, like FL300 is 30000'. In meters you get to fly separations of 300m which can be much more error prone).

I've flown to Moscow dozens of times and the altitude in meters is much more confusing(even if you only use it now below the transition level, from 0 to 3000'), the wind speed in m/s you may get used to, but Knots is much better for all speed units (also the mach num aproximately corresponds with NM/min, example 0.80 Mach is 8 NM/min.)

Another pilot and weekend sailor.

This could just be down to politics. As a government agency that studies climate and environmental issues, their very existence already has plenty of ideological opposition. So why risk alienating the public further by using a non-traditional measurement system?

I think it's a nice quirk to be honest, keeps us on our toes doing more math (good for our brains). I think it's interesting that we champion diverse languages and cultures but balk when distinct measurement systems with their own quirks are used. Even though the metric system has plenty of brilliance to it, each has its own forms of human intuition backed in. And further, they're both completely arbitrary really.

Raw data is reported in different units and presentation in formats that the consumer might find useful - See http://www.ndbc.noaa.gov/measdes.shtml for explanation of the raw data.

Knots are much more useful for the primary consumers of this data: mariners. Knots are a global standard for measuring speed at sea. And virtually every weather bureau reports windspeeds in knots or Beaufort scale in their maritime reports with the notable exception being the French who insist using meters/second.

inHg measures are used in aviation btw, but yeah it's less ideal for a maritime report: mariners use millibars for pressure.

The good thing is that 1m/s is close enough to 2kts that once you have a mental map of "how much wind" is something in knots you can trivially apply it to m/s and vice versa.

inHg on the other hand always has me completely lost.

inHg is easy... 1 inHg is roughly 1 ftH2O.

Hope that helps ;)

Actually knots are derived from SI, being based on nautical miles (which are SI), which are 1/60th of a degree of latitude. So they're not completely useless or arbitrary like other imperial units.

Some other neat data from the same buoy.

Wave height http://www.ndbc.noaa.gov/show_plot.php?station=42058&meas=wv...

Wind direction http://www.ndbc.noaa.gov/show_plot.php?station=42058&meas=wd...

Air temperature http://www.ndbc.noaa.gov/show_plot.php?station=42058&meas=at...

Water surface temperature http://www.ndbc.noaa.gov/show_plot.php?station=42058&meas=wt...

Dewpoint http://www.ndbc.noaa.gov/show_plot.php?station=42058&meas=de...

I just tried the data format codes from the link someone posted earlier http://www.ndbc.noaa.gov/measdes.shtml

Cool stuff!

Being a former QuarterMaster in the Navy, all of this is so fascinating. 20 years ago this data was not easily accessible to the public so you had to wait for some sort of published journal to come out to see this information. I love that we all have access to it so easily.

Additionally, it is located in the Central Caribbean - 210 NM SSE of Kingston, Jamaica

http://www.ndbc.noaa.gov/station_page.php?station=42058

If anyone wants to see where it is, the hurricane's moved further up north away from the buoy's location.

https://earth.nullschool.net/#current/wind/surface/level/ort...

That's a great visualization. Reminded me of one posted to hacker news a few weeks ago that let's you cycle through visualizations of cloud cover, atmospheric pressure, precipitation etc in addition to wind:

https://www.windyty.com/?20.551,-74.487,5

I really love seeing all these conditions moving in concert at a global scale. As someone who's never experienced a hurricane, it makes the effects feel much less abstract.

Any idea what he used to make that? Super impressive

windytv is a fork of earth [0], which is open source [1] and made with D3.js [2], I'm guessing windytv is also using d3.

[0] https://news.ycombinator.com/item?id=12415777

[1] https://github.com/cambecc/earth

[2] https://earth.nullschool.net/about.html

Click the "Earth" link, there are much more options hidden.

What is this site about?

Current wind speed at surface level on an orthographic projection. You can basically read out the URL ;)

I was surprised some time ago when I found out that the modern wind speed measuring devices work without moving parts. I had expected them to all have these spinning ping pong ball halves.

To do it without moving parts, they are for using for example ultrasound. These devices have pairs on transducers, placed 10-20cm apart and the device then measures how long it takes for the sound to pass through that distance.

https://en.wikipedia.org/wiki/Anemometer#Ultrasonic_anemomet...

Does a speaker count as a moving part?

Well, if you want to be pedantic about it, hot-wire anemometers have absolutely no moving parts :-)

https://en.wikipedia.org/wiki/Anemometer#Hot-wire_anemometer...

Being in the eye of a hurricane is an odd feeling. Growing up I went through Hugo, and when the eye finally passed over, it went from chaos to an eerie dead calm. We went outside and to see the destruction, but knew there was not much we could do yet. A tree had clipped the corner of the house so my parents did what they could do to protect the exposed inside against the rain of back half of the storm. Beyond that, we just prepared hold tight for another few hours.

I lived near Charlotte during Hugo. The thing I remember most is going out afterwards and smelling all the pine sap (smells like the Pine Sol cleanser) from all the downed trees.

IIRC, the storm depressed the market for lumber for a year or more afterwards, as it was flooded with wood cut from the trees that were uprooted and then harvested.

I was in Charleston, SC. We literally had firewood for years afterwards. The storm itself was bad, but no power for almost 4 weeks was really rough.

One of the things I take away from this graph isn't just the peak wind speed, but the amount of time you'd be under hurricane-force winds.

A couple minutes of 90mph winds is one thing... hours of 90mph winds is entirely another thing.

Of course it depends on how fast the system is moving and how much area it covers.

I've been kept up all night with the noise caused by the high winds, particularly when it is gusting. The sound of the wind and all the banging and clattering of scrap being tossed around, doors and windows rattling and slamming, car alarms and so on.

The sound of the wind when it reaches its peak is equally awe inspiring and terrifying.

I've only experienced one eye and it is quite surreal. Screaming winds start to drop and it becomes eerily quiet and calm for (in this case) a couple of hours. Then the chaos and fury start to ramp up again.

It's not so much the wind as what is in the wind that really bowls you over.

That depends, at some point if the wind is fast enough and sustained for long enough, key parts of a building start to resemble wings.

This is it -- much like how getting caught in a tsunami is not very interesting itself, it is the debris (which includes vehicles and other large objects) that will kill you and otherwise cause devastation.

If I remember Ron White's line correctly, it's, "it's not that the wind is blowing; it's what the wind is blowing that matters."

I would be immensely curious to see the raw data given how the point spacing changes (assuming those aren't actual data measurements). The drop and recovery around the eye are so staggeringly smooth.

They are most likely smooth because there are so few data points. The red line has the peak, one data point halfway down, one at the bottom, one halfway up, and then the next peak. 'halfway' could be anywhere between 20% and 80% and it'd still look like a super smooth line.

Air pressure is less bad, but each data point is still moving a huge amount. If there was a lot of variance we'd have difficulties setting it at this resolution.

http://www.ndbc.noaa.gov/station_realtime.php?station=42058

http://www.ndbc.noaa.gov/data/hourly2/

http://www.ndbc.noaa.gov/data/hourly2/hour_01.cwind

I love the ndbc site, have been using it for 10+ years now

Thank you!

Wonder why we don't see winds at the reported speeds(130mph+) in the data?

Maybe it's a limit of the measuring device?

It could be a number of things:

- the max gust didn't occur at the buoy;

- the reported wind speed is a 10-min/1-hr mean or some other variation that wouldn't show the peak gust;

I really doubt that it's a limitation of the sensor.

Check out

http://www.nhc.noaa.gov/aboutwindprofile.shtml

In summary the peak is highly variable for every storm but is usually somewhere around 500-1000 feet above ground level.

Its not irrelevant that its so high off the ground ... a tree thats uprooted, picked up to 500 feet, accelerated to 150 MPH, and dropped on you might land on you in merely 100 MPH winds, but it'll still be flying at 150 MPH (maybe faster?)

Note there are plenty of broadcast transmitter towers where the top will get winds 50 MPH faster than the surface. 500 feet is only maybe 50 stories for a tall building. So there is some impact directly on the largest constructed things.

"How come the 150 MPH rated water tower survived but the 160 MPH radio antenna next to it collapsed?" well the water tower was in 130 MPH winds and the antenna was in 180 MPH winds at the same time so ...

Thank you -- I climb towers and skydive occasionally so I'm aware that winds aloft are often greater than surface winds.

But I was under the impression that the forecast wind speeds were surface winds, and assumed that surface meant ground level.

I shall research.

I love this graph

Call me a cynic, but my first thought on looking at this was "we can't do any better than a datum per hour?!" :) Must be a bandwidth constraint.

Call me an optimist, but the data may have been binned by hour.

I don't see why binning would be advantageous here. If it's noisy, apply a moving average. But binning it is just throwing (time) data away. Don't get me wrong, I think it's great we have access to this data at all. Just curious why it's not higher-resolution.

It could be power restraints -- the buoy is almost certainly solar powered and may not have enough reserve/design capacity to transmit at high resolutions over multiple days without sunlight.

Or it may have just been designed that way -- perhaps the cost of the necessary equipment to transmit/log at higher frequencies didn't justify the benefit. Perhaps once per hour is sufficient for the purposes the buoy was deployed for.

Yeah - power does seem a more likely constraint when you put it that way.

Photo of the buoy http://www.ndbc.noaa.gov/images/stations/42089.jpg

Looks like at most 50 watt solar panels, probably closer to 25 watts. It probably has 4 of them but only 2 can make any appreciable amount of power at a time. Not much power at all considering the instrumentation and lighting load(I believe most are lit at night for collision avoidance) and that it has to power the buoy and recharge the batteries during the day.

I considered why they wouldn't put larger panels/batteries -- cost, which, compared to what I would think is the overall cost, would be minimal.

Wind loading is another story -- It looks fairly streamlined. Larger panels would significantly increase the wind load, and they already have a fair bit of trouble keeping them attached in storms(though more likely due to the waves).

I have experience working with directional waverider buoys, and ultimately what is useful to me is a statistical representation of sea conditions for a given period (typically 3 hours). I could obtain the raw timeseries of accelerations from the buoy but this is really much less interesting than the rolling spectral statistics that are processed by and output by the buoy.

Then use a 3-hour moving average of the timeseries... This does not imply one data point per 3 hours, the rolling average timeseries interval can be (& generally is) far more granular than the window size of the average.

I think you're not understanding me, or I'm not understanding you...unless for some specific reason you want to know a wave elevation timeseries (which given the question "how do I describe the current sea conditons" you probably don't; also, noting that significant wave height is not the same as wave elevation), the statistical parameters which are derived from the timeseries data are more useful. These of course can be given on a rolling basis.

Yes, I think I misunderstood you originally - and instead of trying to explain it, I'll just say thanks for the thoughtful follow-up posts :)

To comment on the other measurements (temp, pressure, wind spd/dir): these are different from Hs in that Hs is by definition a statistical parameter; it is 4*stdev of the wave elevations or more practically the mean of the highest third of measured wave heights. You could have higher granularity of temperature, pressure, etc. which arguably more precisely tells you about those conditions at a given time, but a more granular time series of Hs does not necessarily define the sea state any better.

And coincidentally Hs corresponds well to visually estimated wave height.

At least with regard to significant wave height (rather, describing the sea conditions):

It's more useful to describe a "sea state" which is made up of infinitely many frequencies, directions, etc. than to have a timeseries of wave elevation at a given point. A snapshot of the overall sea conditions for a given time, if you will.

Sea states can be well described by a couple statistical parameters, namely significant wave height (Hs), peak wave period (Tp), and peak wave direction. There are other refining parameters such as the directional spread but Hs, Tp, and direction are quite informative.

Here's a wind speed plot with a data point every ten minutes: http://www.ndbc.noaa.gov/show_plot.php?station=42058&meas=sp...

I prefer the hourly binned one. It's much cleaner.

Just because the chosen visualisation doesn't have the accuracy you like doesn't mean that the data isn't there.

The hourly one is cleaner and more readable only because they've insisted on using a symbol for each datum which obscures the line itself... I'd like this one 6 times more than the original, otherwise.

Personally, I don't understand why anyone would want to look at timeseries data with anything less than the maximum possible information density & precision (accuracy is not the problem). Maybe if you're looking for cleanliness/aesthetic reasons, but that's not really the point of these charts. Obviously if they are noisy something needs to be done. But binning only really makes sense to me in histograms, not timeseries.

It's biuriful tears up

And now back to our regularly scheduled React component.

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