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diff --git a/noao/lib/linelists/airvac.cl b/noao/lib/linelists/airvac.cl
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+# AIRVAC -- Convert dispersion coordinates to air or vacuum values.
+# The index of refraction formulas used are from Allen's Astrophysical
+# Quantities (1973).
+#
+# This task takes in a file of wavelengths in Angstroms and converts between
+# air and vacuum to an output file of wavelengths in Angstroms.  It will
+# pass any comments and data after the first wavelength column to the
+# output.
+#
+# To install and use copy the script to your IRAF home directory with the
+# name airvac.cl.  Then interactively (for use during the current IRAF session)
+# or in your loginuser.cl (for use during future IRAF sessions) enter
+# "task airvac=home$airvac.cl".
+
+procedure airvac (input, output)
+
+file	input		    {prompt="Input file of wavelengths in Angstroms"}
+file	output		    {prompt="Output file of wavelengths in Angstroms"}
+string	type = "air2vac"    {prompt="Conversion (air2vac|vac2air)",
+			 	enum = "air2vac|vac2air"}
+real	t = 15.		    {prompt="Temperature (C)"}
+real	p = 760.	    {prompt="Pressure (mmHg)"}
+real	f = 4.		    {prompt="Water vapour pressure (mmHg)"}
+
+struct	*inlist
+
+begin
+	file	in, out
+	struct	label
+	int	n
+	real	a, b, w
+
+	in = input
+	out = output
+
+	printf ("# airvac (%s, %s, type=%s, t=%.6g, p=%.6g, f=%.6g)\n",
+	    in, out, type, t, p, f, > out)
+
+	inlist = input
+	while (fscan (inlist, line) != EOF) {
+	    n = fscan (line, w, label)
+	    if (n == 0) {
+		print (line, >> out)
+		next
+	    }
+
+	    b = (10000. / w) ** 2
+	    a = 64.328 + 29498.1 / (146 - b) + 255.4 / (41 - b)
+	    a = a * p * (1 + (1.049 - 0.0157 * t) * 1e-6 * p) /
+		(720.883 * (1 + 0.003661 * t))
+	    a = a - (0.0624 - 0.000680 * b) / (1 + 0.003661 * t) * f
+	    a = 1 + a / 1e6
+	    if (type == "air2vac")
+		w = a * w
+	    else if (type == "vac2air")
+		w = w / a
+
+	    print (w, label, >> out)
+	}
+	inlist = ""
+end