I would simply avoid it by using year (spelled out) only in an informal
context (see the exception in the third paragraph, below). For "per year," I
would also spell it out, not just as "per year," but using narrative
variants such as "every year" or "each year," or better still, "annually."
It is not the purpose of SI to provide unit names and symbols for variables.
That would, in fact, be counterproductive. However, I don't think we should
be anal about it and rule out any discussion of those variables that are
popularly regarded as constants ("near constants," perhaps?). As 0.01° C is
an approximation anyway, I'm perfectly comfortable following that with "per
year" or "annually." Even easier for an audience would be "one hundredth of
a degree Celsius per year." Remembering something like that is somewhat
easier than saying to oneself, "Was that one or two zeroes he put after the
decimal point?"
For printed (or electronic) documentation, there's a very simple approach,
namely tables with column headings. "+0.01° C" would appear under the
heading "Average Annual Change." In fact, that would formalize the process,
without violating any SI rules. I would use such tables if I were doing a
PowerPoint, slide, or flipchart presentation.
The science of global warming is not exact (although it is terrifyingly
real). Nor is the "science" of weight gain or loss. People going to Weight
Watchers are interested in approximately how much they've lost in the
preceding week. They're also interested losing weight, consistently, week
after week. Expressing that symbolically, in terms of kg/Ms, is moderately
OK for a learned paper, but even there it's probably somewhat pretentious
and of doubtful utility. I'd rather see something like, "Participants
experienced an average weight loss of between two and three kilograms a
week."
Implementing SI widely and consistently is a good thing. However, let's not
treat it as a religion.
Bill
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf
Of J. Ward
Sent: Saturday, August 25, 2007 15:08
To: U.S. Metric Association
Subject: [USMA:39280] Re: Years (was energy units)
Hi Bill,
How would you propose dealing with this problem? The year is not a unit in
SI, but humans measure in years none-the-less. It is unavoidable that there
are numerous instances where the SI unit for time is extremely inconvenient
for a given application.
For example, imagine one is speaking in public or writing an article about
the health risks of obesity. Further, imagine one needs to discuss the rate
of weight-gain in adults. Speaking in terms of mass gain in kg/s or g/Ms
would bewilder the audience, even if they have a good understanding and
intuitive feel of kilograms and of seconds. On the other hand, talking
about adults gaining kg/year would be very clear. And indeed, in this sort
of context the uncertainty in definition of the year (about a part in a
thousand) would not be significant.
One would have a similar problem with climate change. I intuitively grasp
somewhat the impact of someone saying that average temperatures are rising
0.01° C/year. I immediately get a feel for how much warmer it will be in 10
years or 100 years at this rate. But if we talk about the temperature
rising in pK/s, or even µK/days, then the impact of the statement is
completely lost, and I for one would be reaching for a calculator.
Once again, I would be very interested to hear how you and the other experts
on this forum would deal with this issue.
J.
Bill Potts wrote:
>There's a problem with y for year.
>
>The unit y is neither an SI unit nor a unit approved for use with SI.
>The largest unit that is approved is d, for day.
>
>Bill Potts
>Roseville, CA
>http://metric1.org [SI Navigator]
>
>-----Original Message-----
>From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On
>Behalf Of J. Ward
>Sent: Thursday, August 23, 2007 19:32
>To: U.S. Metric Association
>Subject: [USMA:39273] Energy units
>
>I've written two prominent experts in energy requesting that they use
>SI units when talking about energy. One responded that he "entirely
agrees,"
>while the other says he's been going back and forth as to which units
>to use. The biggest difficulty is how to define energy, since, for
>example, the electricity produced by burning coal is the total thermal
>energy times the efficiency of converting the heat into electricity.
>Anyway, here is what I said in my letters.
>
>-J
>
>Dear ...,
>
>I would like to suggest a way to help people understand energy issues
>more clearly. It's very hard to understand energy issues when every
>source of energy uses its own units of measurement. It would be much
>more clear to use a single standard unit for power, preferably
>terawatts. For example, if you express oil production in TW (instead
>of barrels per day), coal production in TW (instead of tons / unit
>time), average solar production in TW (instead of kWh / day or kWh /
>year), natural gas in TW (instead of therms / unit time), etc. then it
>becomes very easy to compare all energy sources on a more-equal
>footing, and also to relate them to consumption. As an additional
>benefit, the watt is not only the world standard (SI) unit for power,
>it is also a unit that most people are familiar with from direct
>experience in their daily lives, and the "tera" prefix has become familiar
from computer data storage.
>
>Choosing a single unit for energy would be equally helpful. The most
>relevant time-scale for discussions of fossil fuels and climate change
>is the year. Thus, if one were already consistently using TW for
>power, then expressing energy in terawatt years would make comparisons
>between reserves, production, and consumption extremely quick and
>simple. For example, 100 TWy of reserves being consumed at a rate of 4
>TW implies a time-scale of 25 years.
>
>On the other hand, exajoules would be the relevant SI unit for energy.
>The UN World Energy Assessment uses exajoules to seamlessly compare
>different energy resources, then uses EJ / year to relate this back to
>average power to compare with consumption. Either way, I hope you will
>consider using TW and TWy or EJ and EJ/y to help people understand energy
issues more easily.
>
>Sincerely....
>
>
>
>
>
>
