code: def dew_point(temp, hum):
      
      if temp is None or hum is None:
          return None
      a = 17.27
      b = 237.7
      gamma = ((a * temp) / (b + temp)) + math.log(float(hum) / 100.0)
      return (b * gamma) / (a - gamma)
docs: Compute dew point, using formula from
      http://en.wikipedia.org/wiki/Dew_point.