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From: Frank Reed (no email)
Date: Wed Oct 12 2005 - 19:18:29 EDT
Marcel you wrote:
"After downloading millions of balloon data of stations at different
latitudes, I calculated in a first test run refraction and dip for latitude
60N."
May I ask, who's your intended "market" or "user base" for your
calculations? I know you mentioned at one point that you were trying to calculate the
positions of stars etc. as accurately as possible. If this is strictly for
astronomical use, an interesting issue arises. Shouldn't you limit your balloon
data to days/nights when the sky was clear or mostly so? It's important because
certain types of temperature inversions arise because clouds are forming or
have formed at the altitude of the inversion. Also, there are big day/night
variations which may be much more important astronomically than some of the
latitudinal variation which has been the traditional "averaging" bin for these
sorts of data.
And:
" The results showed that the refraction and the dip vary with the
seasons and that the values are generally higher than the published values
which seem to have been calculated on the basis of a standard atmosphere.
The lowest (unrealistic?) values are those new ones published by USNO. The
results showed also that the Bowditch formula for calculating the dip (the
factor 1.76 in the metric version) should be at 60N during January around
1.65 and during July around 1.73 (the other months can be interpolated using
a cosine function). This might also be (one of) the reason(s) why Bill
encounters these differences with the Chicago buildings or for Asbjorn's
differences who is living somewhere around 60N. "
I don't think very much of it would come from differences in the *average*
lapse rate. It's really a very small difference. You have to be a hundred feet
above the ground before a 10% difference in the dip constant yields even a 1
minute of arc difference in the calculated dip. That said, we can expect
very large differences in the dip when there is a really large variance from the
standard atmospheric lapse rate (even at low observer heights above sea
level). For example, if the atmospheric lapse rate is -34.1deg Celsius per km (as
opposed to the average rates of -6.5 for moist air and -9.75 for dry air),
there is no refraction at all. That is, a pure geometric calculation of dip
will work and the equation sqrt(2*height/R_Earth) will match observations of
actual dip. One can go beyond this and calculate dip as a function of lapse
rate and temperature (dip DOES depend on temperature but only weakly).
By the way, when considering the refraction tables and dip tables published
in the Nautical Almanac, it's worth remembering that these are specifically
designed to be useful for observers AT SEA. If you look at weather balloon
sounding data from places like Bermuda, Jamaica, Pago Pago, etc, the patterns
are different from inland sites at similar latitudes. So a direct comparison
between the Nautical Almanac tables and your intended use may not work out very
well.
And:
"A main problem arose by realising that the
lapse rate distributions within a height layer are distributed
asymmetrically, meaning that taking the average or the median of these
values is not good enough. At the moment I try to derive a calculation
procedure in order to find an estimate for the most likely value (mode) of
lapse rate within a height layer. "
Why would you want that mode value? What I mean is, what purpose would that
serve (it doesn't necessarily have to serve any purpose at all --I'm just
curious to know how you would use this mode result)?
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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