NAME:
	airindex
PURPOSE: (one line)
	Compute the real part of the refractive index of air.
DESCRIPTION:
	This function is based on the formulas in Filippenko, PASP, v. 94,
	pp. 715-721 (1982) for the index of refraction of air.  The conversion from
  relative humidity to vapor pressure is from the Handbook of Chemistry
  and Physics.
CATEGORY
  Miscellaneous
CALLING SEQUENCE:
	n = airindex(wave,pressure,temp,relhum)
INPUTS:
	wave     - wavelength of light, in microns
	pressure - atmospheric pressure in mm of Hg
	temp     - atmospheric temperature in degrees Celcius
	relhum   - Relative humidity (in percent)
OUTPUTS:
	return value is the index of refraction for the input conditions.
REVISION HISTORY:
	Written by Marc W. Buie, STScI, 1991/02/28
  1997/03/03, MWB, changed name from REFRAC, changed to relative humidity
  2005/08/23, MWB, fixed bug in one internal equation
  2019/10/22, MWB, everything now promoted to double