dewpoint.py

# Functions for calculating the dewpoint.
# $Id: dewpoint.py,v 1.1 2003/04/16 19:38:48 maciek Exp $

import math

def dewpoint(tempC, relhum):
        """ algorithm from Tom Whittaker 
tempC is the temperature in degrees Celsius,
relhum is the relative humidity as a percentage"""
        gasconst = 461.5
        latheat = 2500800.0

        dp = 1.0 / ( 1.0 / ( 273.15 + tempC ) - gasconst * math.log(  (0.0 + relhum) / 100 ) / \
             ( latheat - tempC * 2397.5 ))

        return min(dp - 273.15, tempC)

def awips_dewpoint(tempC, relhum):
        """ algorithm taken from http://meted.ucar.edu/awips/validate/dewpnt.htm 
tempC is the temperature in degrees Celsius,
relhum is the relative humidity as a percentage"""
        C15 = 26.66082
        C1  = 0.0091379024
        C2  = 6106.396
        C3  = 223.1986
        C4  = 0.0182758048

        t   = tempC + 273.15  # convert temperature to Kelvin
        rh  = relhum / 100    # convert relative humidity to ratio

        es = math.exp( C15 - C1 * t - C2 / t )   # saturation vapor pressure
        e  = rh * es
        b  = C15 - math.log(e)
        td = ( b - math.sqrt( b * b - C3 ) ) / C4

        return min(td - 273.15, tempC)