This Online Calculator allows to calculate Atmospheric Refraction using Bennett and Sæmundsson empirical formulas with pressure and temperature correction.
The calculator below for the Apparent Altitude H0 is based on a formula that was given by G. G. Bennett of the University of New South Wales. The formula gives the refraction in arcminutes. The calculator is accurate to 0.07 arcminute for all values of altitude H0. At 12° altitude the error is largest, it is 0.07 arcminute.
H0 = ° (Apparent altitude) T = ° C F P = mbar mmHg
Source: Astronomical Algorithms by Jean Meeus, page 102.
Another online calculator for atmospheric refraction is based on a formula that was developed by Sæmundsson, it is used for determining refraction R in arcminutes from true altitude H (in degrees).
H = ° (True altitude) T = ° C F P = mbar mmHg
This formula is consistent with Bennett’s formula to within 0.4".
The next calculator is based on the G. G. Bennett's formula and it is used in the U. S. Naval Observatory's Vector Astrometry Software.
H = ° (True altitude) T = ° C F P = mbar mmHg A = m (Height of observer in meters) Use T and P (Temperature and Pressure) Use A (Altitude). The result is correct for mean conditions (T = 10°C (50°F), P = 1010 mbar).
Source: Naval Observatory Vector Astrometry Software (NOVAS) C Edition, Version 3.1
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