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Geoffrey K, you wrote:
"Of course, due to the high temperatures (40 to 50 Centigrade) corrections
will have to me made to the standard Altitude Correction tables. But it
occurred to me that the Refraction Correction Tables for Non Standard
Conditions in the Nautical Almanac may not be appropriate in the middle of
the Sahara, where the hot sand will bake the air near the ground.
Does anyone have any data on altitude corrections for desert conditions?"
Sure. The main difference in the atmosphere over the Sahara as far as
refraction is concerned, apart from the temperature, is the lapse rate. In "normal"
parts of the world, the lapse rate may show slowly decreasing, stable, or
rising temperatures for the first few hundred meters of altitude and then
temperatures usually fall off along "moist adiabatic" curves which average about
-6.5 deg Celsius per kilometer. Over the oceans, it's not unreasonable to use
a moist adiabatic curve from sea level all the way up to the tropopause
(details above the tropopause and the exact height of the tropopause don't
matter). In deep desert areas, the air is extremely dry. So the air cools with
altitude at the "dry adiabatic rate" which is close to -9.75deg C/km. When you do
refraction integrations for these various lapse rate structures, one feature
that emerges over and over again is that the structure doesn't matter when
you do sights more than 3 or 4 degrees from the horizon.
Here's a refraction table prepared for ocean conditions for every half
degree of altitude from 0 to 5 degrees above the horizon:
33.8'
28.3
24.1
20.8
18.2
16.1
14.3
12.9
11.7
10.7
9.8
And here's a similar refraction table for desert conditions:
32.4'
27.5
23.6
20.5
18.0
15.9
14.3
12.9
11.7
10.7
9.8
As you can see, they match for altitudes of 3 degrees and higher. Both
tables are for standard temperature and pressure at sea level so you still would
need to apply corrections for non-standard T and P and for altitude above sea
level.
Of course, tables like these are only right when the actual conditions match
the input conditions. It's easy to imagine conditions in the desert, for
example, right after sunrise, when the temperature distribution would not match
the assumptions above at all.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
