added a round-trip test for magnus tetens

Added a new test to show that you can calculate relative humidity from
dewpoint and then calculate dewpoint from relative humidity and it will
be the same as the original dewpoint values

Author: kurt-rhee

pvlib/tests/test_atmosphere.py

@@ -88,7 +88,34 @@ def test_gueymard94_pw():
     assert_allclose(pws, expected, atol=0.01)
 
 
-# Unit tests
+def test_tdew_to_rh_to_tdew():
+
+    # dewpoint temp calculated with who and aekr coefficients
+    dewpoint_original = pd.Series([
+        15.0, 20.0, 25.0, 12.0, 8.0
+    ])
+
+    temperature_ambient = pd.Series([20.0, 25.0, 30.0, 15.0, 10.0])
+
+    # Calculate relative humidity using pandas series as input
+    relative_humidity = atmosphere.rh_from_tdew(
+        temperature=temperature_ambient,
+        dewpoint=dewpoint_original
+    )
+
+    dewpoint_calculated = atmosphere.tdew_from_rh(
+        temperature=temperature_ambient,
+        relative_humidity=relative_humidity
+    )
+
+    # test
+    pd.testing.assert_series_equal(
+        dewpoint_original,
+        dewpoint_calculated,
+        check_names=False
+    )
+
+
 def test_rh_from_tdew():
 
     # dewpoint temp calculated with who and aekr coefficients
@@ -128,12 +155,6 @@ def test_rh_from_tdew():
         dewpoint=dewpoint.to_numpy()
     )
 
-    # Calculate relative humidity using float as input
-    rh_float = atmosphere.rh_from_tdew(
-        temperature=temperature_ambient.iloc[0],
-        dewpoint=dewpoint.iloc[0]
-    )
-
     # test
     pd.testing.assert_series_equal(
         rh_series,
@@ -152,6 +173,12 @@ def test_rh_from_tdew():
         relative_humidity_who.to_numpy(),
     )
 
+    # Calculate relative humidity using float as input
+    rh_float = atmosphere.rh_from_tdew(
+        temperature=temperature_ambient.iloc[0],
+        dewpoint=dewpoint.iloc[0]
+    )
+
     assert np.isclose(
         rh_float,
         relative_humidity_who.iloc[0]