compute heat index even without wind

closes #623

CHANGELOG.md

@@ -42,6 +42,7 @@ the social media posts for products that are more mudane (#604).
 - Fixed missing `ENH` and `MDT` from `spcpts.THRESHOLD_ORDER`.
 - Fixed jabber/twitter message generation for a VTEC product with multiple
 segments with the same vtec action (read tropical products).
+- Heat index is now computed without the presence of wind info (#623).
 - Improved logic behind `pyiem.plot.util.pretty_bins`, it no longer exactly
 returns the specified number of bins, but tries to do the right thing!
 - Increase remark trimming for LSR tweets for more length safety.

src/pyiem/observation.py

@@ -8,7 +8,7 @@
 try:
     from zoneinfo import ZoneInfo  # type: ignore
 except ImportError:
-    from backports.zoneinfo import ZoneInfo
+    from backports.zoneinfo import ZoneInfo  # type: ignore
 
 import numpy as np
 import pandas as pd
@@ -245,13 +245,14 @@ def calc(self):
         if self.data["feel"] is None and None not in [
             self.data["tmpf"],
             self.data["relh"],
-            self.data["sknt"],
         ]:
+            # sknt is not a hard requirement
+            sk = self.data["sknt"] if self.data["sknt"] is not None else np.nan
             self.data["feel"] = bounded(
                 mcalc.apparent_temperature(
                     self.data["tmpf"] * munits.degF,
                     self.data["relh"] * munits.percent,
-                    self.data["sknt"] * munits.knots,
+                    sk * munits.knots,
                     mask_undefined=False,  # less confusion this way
                 )
                 .to(munits.degF)

tests/test_observation.py

@@ -88,6 +88,21 @@ def test_calc():
     assert (ob.data["relh"] - 53.6) < 0.1
 
 
+def test_gh623_feelslike_without_wind():
+    """Test that we can compute feelslike without wind speed."""
+    ts = utc(2018)
+    ob = observation.Observation("FAKE", "FAKE", ts)
+    ob.data["tmpf"] = 89.0
+    ob.data["dwpf"] = 70.0
+    ob.calc()
+    assert (ob.data["feel"] - 94.3) < 0.1
+    ob = observation.Observation("FAKE", "FAKE", ts)
+    ob.data["tmpf"] = -19.0
+    ob.data["dwpf"] = -22.0
+    ob.calc()
+    assert ob.data["feel"] is None
+
+
 @pytest.fixture
 def iemob():
     """Database."""