;+
; NAME:
;  ch4t
; PURPOSE: (one line)
;  Return vapor pressure of CH4 from Brown & Ziegler 1980
; CATEGORY:
;  Volatile Transport
; CALLING SEQUENCE:
;  t = ch4t(vp)
; INPUTS:
;  vp : vapor pressure in microbar
; KEYWORD INPUTS
;  None
; OUTPUTS:
;  t : temperature, K
; OPTIONAL OUTPUT
;  None
; RESTRICTIONS:
; PROCEDURE:
;  This function implements the tables of Brown and Zeigler 
; MODIFICATION HISTORY:
;  Written 2013 Jun 16, by Leslie Young, SwRI
;-
function ch4t, p

if isarray(p) then begin
   t = 0. * p
   for i = 0, n_elements(p)-1 do t[i] = ch4t(p[i])
   return, t
endif

; Return N2 temperature as a function of vapor pressure in microbar
a=[1.71271658e1, -1.11039009e3, -4.34060967e3, 1.03503576e5, -7.91001903e5]
a[0] = a[0] - alog(p/1.33322e3)
roots = fz_roots(a)
gd = where( imaginary(roots) eq 0 and real_part(roots) gt 0, ngd)
t = 1/real_part(roots[gd[0]])
return, t
end