REFRAC

Name

refrac

Purpose

Apply atmospheric refraction to a ``true'' zenith angle

Description

This calculation is based on a few different sources. First, it is
assumed that the index of refraction of air at the base of the atmosphere
can be calculated (see AIRINDEX). From the index of refraction, the
bending is computed from the formula on p.55 of the old Explanatory Supplment
to the Nautical Almanac. This formula has been modified by removing the
h/rho term. The explanatory supplement doesn't indicate that this is
legitimate but I've validated this computation against a more emperical
formalism from Eisele and Shannon (NRL memo 3058, May 1975). Eisele and
Shannon don't indicate the wavelength of light used but if I use 0.56 microns
and compare for the same input conditions (dry air only), the refraction
computed agrees to within 1 arcsec down to 51 degrees zenith angle and
is good to 10 arcsec down to 80 degrees.

Calling Sequence

zref = refrac(z,wave,pressure,temp,relhum)

Inputs

z - true zenith angle (as if there were no atmosphere), in radians
wave - wavelength of light, in microns
pressure - atmospheric pressure in mm of Hg
temp - atmospheric temperature in degrees C
relhum - Relative humidity (in percent)

Optional Input Parameters

Keyword Input Parameters

Outputs

returned is the refracted (or apparent) zenith distance.

Keyword Output Parameters

Common Blocks

Side Effects

Restrictions

Not accurate (nor useful) for z > 85 degrees.

Procedure

Modification History

97/03/03, Written by Marc W. Buie, Lowell Observatory