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Re: Mt Wilson Record

Subject: Re: Mt Wilson Record
Date: 25 Jul 2005 13:00:29 -0700
Newsgroups: sci.astro.amateur
"decaf" said:

> In the South, it isn't anywhere NEAR 95% relative humidity
> WHEN it's 95 degrees F.

It's extremely unfortunate that relative humidity became a
staple of newscaster's jargon, and so got embedded in the
popular imagination. Logically, it's just as good to state
the temperature and the relative humidity as to state the
temperature and the dew point; given any two of these values,
you can easily compute the third. But most people have no
idea how sensitive relative humidity is to temperature,
and how much it varies over the course of a day.

Here in New England, on our most miserable days (which are
about average in a tropical environment), the dew point is
fairly stable at 75F, with minimum and maximum temperatures
of 80F and 100F. That means that relative humidity varies
between 85% in the morning and 45% at midday. Now 45%
relative humidity may sound modest, but when you're actually
experiencing it at 100F, it's thoroughly miserable.

So when somebody tells me that it's 95 degrees with 95%
humidity, I simply ignore the second statistic. I know it's
wrong, I don't want to bother exlaining why, and I'm sure
that the speaker never noted the dew point, which actually
would be meaningful. But if newscasters routinely reported
the dew point *instead* of the relative humidity, then that
value would become part of the common vocabulary, and we
would all be able to have meaningful conversations about
humidity -- which we can't as long as we stick to
relative humidity.

Getting back to astronomy, the same situation currently
pertains with respect to skyglow. People currently talk
about it in terms of naked-eye limiting magnitude, but
even with the most scrupulous observers, this varies so
much from one individual to another that it's essentially
meaningless for conversation between strangers. And things
are much worse with casual observers. I frequently read or
hear statements like "my limiting magnitude is 2 or 3."
That's like saying "the temperature outside is 120 or 180."
All I can glean from such statements is that it's really
bad -- which would be better stated without numbers.

The first night after I bought my stargazing glasses (see
the September S&T), I went outside my city apartment and
noted stars in the Hyades. I wasn't dark adapted, and the
night was good but not exceptional, yet I noted two stars
a full 1/4 magnitude fainter than I had ever seen before
with vision that was nominally perfectly corrected.
Conversely, my wife's limiting magnitude is at least
1/2 magnitude less faint than mine even when I'm not
wearing my stargazing glasses. But 1/2 magnitude of
limiting stellar magnitude corresponds to nearly 1
magnitude of skyglow, which is the difference between
seeing just the cores of a few galaxies in a telescope
and seeing extended disks for many galaxies.

Meaningful conversations about light pollution will begin
when we switch to some objective measure like magnitudes
per square arcsecond.

    - Tony Flanders

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