function[dp]=rh2dp(at,rh)
% rh2dp dewpoint from RH and air temp
% function[dp]=rh2dp(at,rh)
% From NDBC's  calc_dew_point_proc.c
%     %   Calculate and return  dewpoint from
%   relative humidity (%) and air temperature (degC)
%
%   CALLING ARGUMENTS/DESCRIPTIONS:
%      at   air temperature in Celcius
%      rh   relative humidity in Percent
%
%   OUTPUT VARIABLES:
%      dp  Dewpoint in Celsius
% 

% from Nan Galbraith

rh(find (rh < 1.0)) = 1.0;      % /* 0 causes log error */
at_k = at + 273.15;             % to kelvin
VPsat = exp((-5438/at_k) + 21.72);
VPabs = VPsat * (rh * .01);
DewPointk = -5438/(log(VPabs) - 21.72);
dp = DewPointk - 273.15;