Update snow events parameter docs for Townsend snow loss model (#1840)

* Update snow events definition for Townsend model

* Update whatsnew

* Add note on origin of new definition

---------

Co-authored-by: Areeba Turabi <[email protected]>

Author: aturabi

docs/sphinx/source/whatsnew/v0.10.2.rst

@@ -39,7 +39,9 @@ Documentation
   (:issue:`1741`, :pull:`1833`)
 * Fix error in :py:func:`pvlib.iotools.get_pvgis_hourly` documentation of `surface_azimuth`.
   (:issue:`1724`, :pull:`1838`)
-
+* Update definition of snow events parameter for :py:func:`pvlib.snow.loss_townsend`.
+  (:issue:`1839`, :pull:`1840`)
+  
 Requirements
 ~~~~~~~~~~~~
 
@@ -51,3 +53,4 @@ Contributors
 * Taos Transue (:ghuser:`reepoi`)
 * NativeSci (:ghuser:`nativesci`)
 * Lukas Grossar (:ghuser:`tongpu`)
+* Areeba Turabi (:ghuser:`aturabi`)

pvlib/snow.py

@@ -230,8 +230,10 @@ def loss_townsend(snow_total, snow_events, surface_tilt, relative_humidity,
         Snow received each month. Referred to as S in [1]_. [cm]
 
     snow_events : array-like
-        Number of snowfall events each month. May be int or float type for
-        the average events in a typical month. Referred to as N in [1]_.
+        Number of snowfall events each month. Snow events are defined as days
+        in the month that have snowfall greater than 1 inch. May be int or
+        float type for the average events in a typical month. Referred to as N
+        in [1]_.
 
     surface_tilt : float
         Tilt angle of the array. [deg]
@@ -274,7 +276,9 @@ def loss_townsend(snow_total, snow_events, surface_tilt, relative_humidity,
     axis to the module edge.
 
     The parameter `string_factor` is an enhancement added to the model after
-    publication of [1]_ per private communication with the model's author.
+    publication of [1]_ per private communication with the model's author. The
+    definition for snow events documented above is also based on private
+    communication with the model's author.
 
     References
     ----------