set default magnus coefficients per conversation in #1744

Author: Kurt Rhee

pvlib/spectrum/magnus_tetens.py

@@ -1,18 +1,25 @@
 import numpy as np
 
 
-def magnus_tetens_aekr(temperature, dewpoint):
+def magnus_tetens_aekr(temperature, dewpoint, A=6.112, B=17.62, C=243.12):
     """
     Calculate relative humidity using Magnus equation with AEKR coefficients.
     This function was used by First Solar in creating their spectral model
     and is therefore relevant to the first solar spectral model in pvlib.
+    Default magnus equation coefficients are from [2].
 
     Parameters
     ----------
     temperature : pd.Series
         Air temperature in degrees Celsius
     dewpoint : pd.Series
         Dewpoint temperature in degrees Celsius
+    A: float
+        Magnus equation coefficient A
+    B: float
+        Magnus equation coefficient B
+    C: float
+        Magnus equation coefficient C
 
     Returns
     -------
@@ -27,23 +34,22 @@ def magnus_tetens_aekr(temperature, dewpoint):
     References
     ----------
     .. [1] https://www.osti.gov/servlets/purl/548871-PjpxAP/webviewable/
+    .. [2] https://www.schweizerbart.de//papers/metz/detail/3/89544/Advancements_in_the_field_of_hygrometry?af=crossref
     """
-    # Magnus equation coefficients (AEKR)
-    MAGNUS_A = 6.1094
-    MAGNUS_B = 17.625
-    MAGNUS_C = 243.04
 
     # Calculate vapor pressure (e) and saturation vapor pressure (es)
-    e = MAGNUS_A * np.exp((MAGNUS_B * temperature) / (MAGNUS_C + temperature))
-    es = MAGNUS_A * np.exp((MAGNUS_B * dewpoint) / (MAGNUS_C + dewpoint))
+    e = A * np.exp((B * temperature) / (C + temperature))
+    es = A * np.exp((B * dewpoint) / (C + dewpoint))
 
     # Calculate relative humidity as percentage
     relative_humidity = 100 * (es / e)
 
     return relative_humidity
 
 
-def reverse_magnus_tetens_aekr(temperature, relative_humidity):
+def reverse_magnus_tetens_aekr(
+    temperature, relative_humidity, B=17.62, C=243.12
+):
     """
     Calculate dewpoint temperature using Magnus equation with
     AEKR coefficients.  This is just a reversal of the calculation
@@ -71,22 +77,18 @@ def reverse_magnus_tetens_aekr(temperature, relative_humidity):
     ----------
     .. [1] https://www.osti.gov/servlets/purl/548871-PjpxAP/webviewable/
     """
-    # Magnus equation coefficients (AEKR)
-    MAGNUS_B = 17.625
-    MAGNUS_C = 243.04
-
     # Calculate the term inside the log
     # From RH = 100 * (es/e), we get es = (RH/100) * e
     # Substituting the Magnus equation and solving for dewpoint
 
-    # First calculate ln(es/MAGNUS_A)
+    # First calculate ln(es/A)
     ln_term = (
-        (MAGNUS_B * temperature) / (MAGNUS_C + temperature)
+        (B * temperature) / (C + temperature)
         + np.log(relative_humidity/100)
     )
 
     # Then solve for dewpoint
-    dewpoint = MAGNUS_C * ln_term / (MAGNUS_B - ln_term)
+    dewpoint = C * ln_term / (B - ln_term)
 
     return dewpoint