pro airdensity,P,T,aden1,aden2
   
;-----------------------------------------------------------
; purpose:
;   compute air density
;
; input:
;    t     = temperature (K)
;    p     = pressure (mb)
;
; output:
;    aden1  = air density, kg / m3
;    aden2  = air density, # / cm3
;
; source:  Mark Hervig 
;-----------------------------------------------------------

   Rd    = 287.04       ; gas constant (J/kg/K)
   Na    = 6.02252e23   ; Avagadros #, molec/mol
   Md    = 28.964       ; molec wt dry air, g/mol
   
;- compute air density

   aden1  = 100.0 * p / (Rd * t)      ; air density, kg/m3 
   
   aden2  = 1e-3 * aden1 * Na / Md    ; air density, #/cm3
   
;- done

   return
   end