relhumid

  RELHUMID: finds relative humidity from wet/dry thermometer readings.
  rh=relhumid(Td,Tw,Pa,type) computes the relative humidity from
  wt and dry-bulb temperature measurements using the psychrometric eqn.
  and constants from Sargent (1980), Meteorol. Mag. 109, 238-246. The
  latter two inputs are optional.

  INPUTS : Td - dry bulb thermometer  [C]
           Tw - wet thermometer  [C]
           Pa - air pressure (optional)  [mb]
           type - 'assman' for Assman-type forced ventilation
                  'screen' for standard screen (natural ventilation)

  OUTPUT:  rh - relative humidity  [%]

0001 function rh=relhumid(Td,Tw,P,p_typ)
0002 %  RELHUMID: finds relative humidity from wet/dry thermometer readings.
0003 %  rh=relhumid(Td,Tw,Pa,type) computes the relative humidity from
0004 %  wt and dry-bulb temperature measurements using the psychrometric eqn.
0005 %  and constants from Sargent (1980), Meteorol. Mag. 109, 238-246. The
0006 %  latter two inputs are optional.
0007 %
0008 %  INPUTS : Td - dry bulb thermometer  [C]
0009 %           Tw - wet thermometer  [C]
0010 %           Pa - air pressure (optional)  [mb]
0011 %           type - 'assman' for Assman-type forced ventilation
0012 %                  'screen' for standard screen (natural ventilation)
0013 %
0014 %  OUTPUT:  rh - relative humidity  [%]
0015 
0016 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
0017 % 8/28/98: version 1.1 (contributed by RP)
0018 % 8/5/99: version 2.0
0019 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
0020 
0021 as_consts;
0022 
0023 if nargin==2,
0024  P=P_default;
0025  p_typ=psych_default;
0026 elseif nargin==3,
0027  if isstr(P),
0028    p_typ=P;
0029    P=P_default;
0030  else
0031    p_typ=psych_default;
0032  end;
0033 end;
0034 
0035 % psychrometric coefficient
0036 
0037 switch p_typ,
0038   case 'screen',
0039     A=0.000799;   % natural screens
0040   case 'assman',
0041     A = 0.000667; % Assmann-type with forced ventilation
0042   otherwise
0043     error(['unknown psychrometer type: ' p_typ]);
0044 end;
0045 
0046 % compute saturation vapour pressure for both temps.
0047 ed=satvap(Td,P);
0048 ewp=satvap(Tw,P);
0049 
0050 % The psychrometric eqn!
0051 e = ewp - A*P.*(Td-Tw);  % ambient vapour pressure
0052 
0053 rh= e./ed * 100;
0054