improve speed of Location.get_airmass (#528)

* PERF speciify time index in location.get_airmass

* PERF speciify time index in ModelChain.prepare_inputs

* update whatsnew

Author: wholmgren

docs/sphinx/source/whatsnew/v0.6.0.rst

@@ -93,6 +93,9 @@ Enhancements
 * Add PVSyst thin-film recombination losses for CdTe and a:Si (:issue:`163`)
 * Python 3.7 officially supported. (:issue:`496`)
 * Improve performance of solarposition.ephemeris. (:issue:`512`)
+* Improve performance of Location.get_airmass. Most noticeable when
+  solar position is supplied, time index length is less than 10000, and
+  method is looped over. (:issue:`502`)
 
 
 Bug fixes

pvlib/location.py

@@ -271,7 +271,7 @@ def get_airmass(self, times=None, solar_position=None,
         airmass_absolute = atmosphere.get_absolute_airmass(airmass_relative,
                                                            pressure)
 
-        airmass = pd.DataFrame()
+        airmass = pd.DataFrame(index=solar_position.index)
         airmass['airmass_relative'] = airmass_relative
         airmass['airmass_absolute'] = airmass_absolute
 

pvlib/modelchain.py

@@ -736,7 +736,7 @@ def prepare_inputs(self, times=None, weather=None):
         if weather is not None:
             self.weather = weather
         if self.weather is None:
-            self.weather = pd.DataFrame()
+            self.weather = pd.DataFrame(index=times)
 
         if times is not None:
             self.times = times