# Program for obtaining dew point temperature from T, Tw, and P
# Written for Python version 2.7.13.
# Add parenthesis around the print statements at the bottom to get it to work for Python version 3.x.

import math

# First some useful functions are defined.

# Function to calculate the saturation vapor pressure over water in mb for temperature T in C.
# Relationship is from Bolton as quoted in Grant Petty's Atmospheric Thermo book.
def esatWater(T):
    return 6.112*math.exp(17.67*T/(243.5+T))
 
# Function to calculate relative humidity, RH, in %, from T and Tdew in C.
# This function calls the esatWater function.  
def RelativeHumidity(Tdew,T):
    return 100.0*esatWater(Tdew)/esatWater(T)
    
# Function to calculate Latent heat of water to vapor for T in C.
def LatentHeat(T):
    return 1000.0*(2500.8-2.36*T+0.0016*T*T-0.00006*T*T*T)
    
# print "Llv(10C)=",LatentHeat(10),"\n"

# Function to get Tdew and RH from To, Po, and Tw in C and mb.
def dewPoint(Po,To,Tw):
    Llv=LatentHeat(To)
    cp=1004. # J/kg K.
    eps=0.622 # mw H20/air
    eo=6.112 # mb H20 at saturation at 0 C.
    R=esatWater(Tw)/eo - Po*cp*(To-Tw)/(eps*eo*Llv)
    Tdew=243.5*math.log(R)/(17.67-math.log(R)) # Dewpoint T in C
    RH=RelativeHumidity(Tdew,To)
    return Tdew,RH

# Main program that calls the functions.
# Example calculation of Tdew given Po, To, and Tw.    
Po=870.0 # mb units
To=25.0  # C units
Tw=15.0  # C units
Tdew,RH = dewPoint(Po,To,Tw)  # C and % units.
print "INPUT VALUES:"
print "     Po",Po,"mb"
print "     To",To,"C"
print "     Tw",Tw,"C"
print "OUTPUT VALUES:"
print "     Tdew={0:1.2f}".format(Tdew),"C"
print "     RH={0:1.2f}".format(RH),"%\n"