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The thermal structure of the standard atmosphere is like this:

Temperature stratify the atmosphere in layers of different density, this acts diverting the beam of light from celestial bodies.

An approximation to this fact is the well known formula for refraction:

Ro = 0.0167 / (tan (H + 7.31) / (H + 4.4) )

f = 0.28 * P/ (T+ 273)

R = f * Ro

H : apparent altitude, (sextant altitude corrected for index error and dip)

P: pressure [mb] or [hPa]

T: Temperature of the air [ºC]

But for low angles is more complicated and under abnormal situations unpredictable.

Also the meteorological phenomenon can disturb the standard refraction.

Andrés Ruiz

Navigational Algorithms

http://www.geocities.com/andresruizgonzalez